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Upon the constitution of binary alloys
Mar., I9O2,]
22t
Constitution o f B i n a r y Alloys.
Mining and Metallurgical Section. Staled 2Weeling, held Wednesday, November s3, I9oI.
UPON
THE C O N S T I T U T I O N
OF
BINARY
ALLOYS.
BY JOHN ALEXANDER MATHEWS, PH.D.
Continued from vol. diii, p. 14o.
DISCUSSION. MR. A. m. OUTERBRIDGE, JR--I have been m u c h intere s t e d in Dr. M a t h e w ' s paper, and I wish to ask w h e t h e r the s u b j e c t of " s e g r e g a t i o n " in b i n a r y alloys, such as silver and copper, has b e e n s t u d i e d b y t h e s e n e w m e t h o d s of scientific research. I am led to m a k e this inquiry, h a v i n g noticed a striking difference in the cooling curves s h o w n in the diag r a m s of two alloys, one only of w h i c h s e g r e g a t e s ; in [hese d i a g r a m s the cooling curve of an alloy of silver a n d copper a n d the cooling curve of an alloy of silver and gold are s h o w n in juxtaposition. T h e silver-copper curve is v e r y pronounced, and t e r m i n a t e s s h a r p l y at the point w h e r e the e u t e c t i c alloy s e p a r a t e s ; the silver-gold alloy, on the other hand, s h o w s a nearly s t r a i g h t line, and no eutectic. It is well k n o w n t h a t alloys of silver and copper s e g r e g a t e while alloys of silver and gold are stable. More than t w e n t y years ago, long before the m o d e r n science of m i c r o - m e t a l l u r g y h a d been developed, I had occasion to m a k e an i n v e s t i g a t i o n of the r a t h e r o b s c u r e p h e n o m e n o n k n o w n as s e g r e g a t i o n of the silver-copper alloys u s e d in coinage, and, incidentally, of t h e alloys of silver and gold, and copper and gold, in which s e g r e g a t i o n is a l m o s t nil. T h e s t a n d a r d alloy used for the silver coin of tile U n i t e d S t a t e s is c o m p o s e d of 900 parts of silver and IOO parts of copper, while the s t a n d a r d for the gold coins is 9oo p arts of gold and IOO parts of copper.* T h e gold-copper alloy, w h e n * A trace of silver is permitted in this alloy.
22~
Outerbridge :
[J. F. I.,
once formed by m e l t i n g a n d t h o r o u g h mixing, r e m a i n s practically h o m o g e n e o u s , so t h a t w h e n the m e t a l is cast into ingots and rolled into strips, from w h i c h the p l a n c h e t s or blanks are s t a m p e d out, all of the pieces are f o u n d to be of nearly u n i f o r m fineness. N o t so, however, in the case of the silver-copper alloy. No m a t t e r how c a r e f u l l y the metallic m i x t u r e m a y have been prepared, or how t h o r o u g h l y the molten m e t a l m a y have been stirred, it will be f o u n d t h a t segreg a t i o n will always occur, even to such an e x t e n t t h a t it is impossible to obtain a fair sample for assay by c u t t i n g off a little piece from an ingot, as is c o m m o n l y done w i t h the gold-copper alloy. T h e silver assay sample is, therefore, dipped out from the m o l t e n silver-copper alloy and p o u r e d in a fine s t r e a m into cold water, t h u s cooling the m e t a l i n s t a n t l y in the form of small shot or " g r a n u l e s "; in this w a y s e g r e g a t i o n in the sample is avoided, b u t it has never been overcome in the ingot. It is for this reason t h a t the coinage laws p e r m i t a m u c h g r e a t e r l i m i t of " t o l e r a n c e " or variation from true standard, in the case of silver coins, t h a n in the case of gold coins; nevertheless, I believe it has a c t u a l l y h a p p e n e d t h a t silver dollar coins struck from m e t a l which originally assayed of the proper fineness h a v e been c o n d e m n e d because the v a r i a t i o n in fineness due to segregation in the i n g o t exceeded the limits allowed by law. T h e true cause of s e g r e g a t i o n has never been satisfactorily explained so far as I know. It does not appear to be associated in a n y w a y w i t h the specific g r a v i t y of the different metals composing an alloy ; thus, for example, w i t h silver-copper alloys it has been f o u n d t h a t in certain proportions of silver and copper the silver s e g r e g a t e s towards the center of the bar, and in o t h e r proportions the r i c h e r alloy is f o u n d near the surface, while still a n o t h e r curious fact has been proven in this connection, namely, t h a t w h e n silver and copper are alloyed in the proportion of a b o u t three-fourths silver and one-fourth copper little or no segreg a t i o n occurs. In the case of silver-gold alloys there is, I believe, very little if a n y segregation, and it seemed to me t h a t there m a y be some connection between these p h e n o m e n a and those revealed by the d i a g r a m s on the screen to which I
Mar., I9o2.]
Constitutionof Binary Alloys.
223
have referred, s h o w i n g a p r o n o u n c e d eutectic alloy in the silver-copper m e t a l and t h e a b s e n c e of the euteetie in t h e silver-gold alloy. I t h e r e f o r e have t h o u g h t t h a t it m i g h t b e an i n t e r e s t i n g as well as a p p r o p r i a t e q u e s t i o n to ask Dr. M a t h e w s w h e t h e r the light of microscopic investigation, in eonnection with the s t u d y of the cooling of metals from a molten state, has been cast upon this i m p o r t a n t u n s o l v e d p r o b l e m in m e t a l l u r g y ? I m a y say, in conclusion, that some of the o b s e r v a t i o n s on alloys of the precious metals noted herein were m a d e b y me as long ago as I873, in the course of a s o m e w h a t prolonged investigation of a m e t h o d of q u a n t i t a t i v e spectroscopic analysis then recently proposed b y Sir N o r m a n L o c k y e r and the assayer of the R o y a l Mint, n o w Sir R o b e r t s A u s t e n . ~ T h e fact that silver-copper alloys s e g r e g a t e was k n o w n long before this time, b u t was r e g a r d e d as an unsolvable metallurgical riddle or paradox. I am inclined to believe t h a t a careful and t h o r o u g h i n v e s t i g a t i o n of the micro-structure of different portions of an ingot of silvercopper alloy, in which marked s e g r e g a t i o n has been s h o w n to exist b y the ordinary b u t v e r y accurate m e t h o d s of h u m i d a s s a y of silver, would be likely to increase our k n o w l e d g e of the u n d e r l y i n g cause of this s o m e w h a t m y s t e r i o u s phen o m e n o n k n o w n as segregation. MR. PAUL KREUZPOINTNER : - - T h e very instructive p a p e r read this e v e n i n g b y Professor M a t h e w s raises a good m a n y q u e s t i o n s in m y m i n d concerning the results of scientific research of the s t r u c t u r e of m e t a l s as to the relation of t h e s e results to applied, that is, practical metallurgy. However, before asking questions, I b e g to express g r e a t pleasure and satisfaction with the progress m a d e in the use of the microscope as applied to metallurgy. A n y one who k n o w s a n y t h i n g of the struggles, d i s a p p o i n t m e n t s and criticisms the pioneers of m e t a l l o g r a p h y were s u b j e c t to sixteen and t w e n t y years ago will appreciate the progress m a d e in this line of scientific work d u r i n g the last ten years, since when "Electrical Spectra of Metals."
Society, May, I874, Vol. XIV, p. I62.
Proceedings ~Imerican Philosophical
224
A~reuzgointner :
[J. F. I.,
the microscope has become an i n d i s p e n s a b l e i n s t r u m e n t in the work of i n v e s t i g a t i n g the properties of metals. B u t while I would be the last one to d e d u c t even an iota from the value of the work done in this line b y Sorbey, Martens, H e y n , O s m o n d a n d others, I b e g to r e m i n d you t h a t members of the M i n i n g and Metallurgieal Section of the F r a n k l i n I n s t i t u t e deserve credit and r e e o g n i t i o n for the pioneer work done in this country, and y o u r p r e s e n t president, Mr. Garrison, who took up the work of m e t a l l o g r a p h y t w e n t y y e a r s ago, p r e p a r i n g the soil for those who were to reap the fruit of his planting, knows w h a t diffieulties a pioneer in a n y line of work has to c o n t e n d with. As to some of the points raised by the lecturer it w o u l d be i n t e r e s t i n g to know w h y a metal, for i n s t a n c e steel, w h e n in a physical state where the eutectie is a l r e a d y more or less clearly formed, still appears to possess tolerably fair physical qualities a l t h o u g h a low elastieity. If I u n d e r s t a n d a r i g h t the f o r m a t i o n of the euteetie m e a n s a more or less complete s e g r e g a t i o n of the surplus of e l e m e n t s over and above t h a t p e r c e n t a g e of d e m e n t s which e v e n t u a l l y form the euteetie, and where the d e m e n t s composing the m e t a l and w h i c h are t h e n still in a fluid state will freeze s i m u l t a n e o u s l y . If s e g r e g a t i o n m e a n s a dissociation of elements; and dissociation contributes to the l o o s e n i n g or lessening of the cohesion by which the crystals are held t o g e t h e r a n d w h i c h is an essential in e o n t r i b u t i n g to the s t r e n g t h of a metal, t h e n w h y is it t h a t metals of such a large s e g r e g a t e d structure like illustration No. 3 (I believe, K), w h e r e the euteetic has f o r m e d or is forming, still show a h i g h degree of cohesive force in daily practice ? Is there a mastics f o r m e d d u r i n g s e g r e g a t i o n w h i e h acts as a binder, so to speak, or is the s t r e n g t h p r o d u c e d m e r e l y b y the m e c h a n i c a l i n t e r l o e k i n g of the crystals? By this I do not m e a n t h a t such m e t a l is equal in q u a l i t y to one of n o r m a l conditions, t h a t is, a m e t a l of fine s t r u c t u r e free from the d i s t u r b i n g influences of s e g r e g a t i o n as m u c h as is possible. T h e r e is a fertile field y e t for the scientific i n v e s t i g a t o r of the s t r u c t u r e of a m e t a l and the relation of t h a t s t r u c t u r e to the m a x i m u m usefulness and serviceability of t h a t m e t a l in daily practice
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Constitution o f Binary Aitoys.
225
in d e t e r m i n i n g the effects of h e a t on t h e s t r u c t u r e in an a s c e n d i n g ratio i n s t e a d of in a d e s c e n d i n g ratio, as is the case in d e t e r m i n i n g the cooling curve. W h a t I mean b y this is, w h a t is the n a t u r e of the various s t r u c t u r e s of a given m e t a l at various points of h e a t when we h e a t that m e t a l from the cold state to t h e point of fusion ? To Brinnel we o w e m u c h i n f o r m a t i o n in that line, b u t unless I am mistaken, no such systematic, t h o r o u g h researches h a v e been m a d e of the s t r u c t u r e s of metals w h e n heated up to the point of fusion as w h e n cooled down from the point of fusion and the c o r r e s p o n d i n g physical q u a l i t y at these different structures. I am well aware of the m a n y d e t a c h e d and s o m e t i m e s practically v a l u a b l e i n v e s t i g a t i o n s on the lines I refer to, b u t w h a t the practical metallurgist, the expert, w h o has n e i t h e r the time nor facilities to make cont i n u o u s i n v e s t i g a t i o n s needs, is a series of illustrations of the m i c r o s t r u c t u r e of the m o s t f r e q u e n t l y used structural m e t a l s at different degrees of h e a t i n g t h a t m e t a l and l e t t i n g it cool and the c o r r e s p o n d i n g physical s t r e n g t h at that heat. I m m e n s e q u a n t i t i e s of good metal have been spoilt in mill and shop on a c c o u n t of i m p r o p e r h e a t t r e a t m e n t , and while we know a little b e t t e r to-day there is still a g r e a t deal to be learned on the subject, and o u r technical universities, w h o are doing already such good work in physical metallurgy, can be of g r e a t benefit to the e n g i n e e r and o u r industries b y t h u s c o n t r i b u t i n g their mite to practical metallurgy. In a n s w e r to the q u e s t i o n b y the last speaker w h e t h e r crystals are necessarily always f o r m e d b y h e a t and are not likewise f o r m e d b y shock and vibrations, I w o u l d say that experience s e e m s to indicate t h a t metals, t h a t is iron and steel, do not crystallize u n d e r the influence of shock and v i b r a t i o n s as was c o m m o n l y s u p p o s e d formerly. B u t while metals do not crystallize due to shocks and v i b r a t i o n s t h e y u n d o u b t e d l y b e c o m e fatigued, and w h e n t h u s fatigued, iron and steel b r e a k more easily, and b e c a u s e t h e y b r e a k more easily w h e n fatigued, t h e y b r e a k more quickly u n d e r u n d u e strains, and b e c a u s e b r e a k i n g quickly t h e metal has no time to flow, its plasticity does not come into play, and if we succeed in b r e a k i n g a metal before flow VQL. CLIII.
No. 915.
15
226
Kreuzpointner :
[J. F. I.,
sets in, we g e t a transverse section of the i n d i v i d u a l fibers w h i c h will give the f r a c t u r e d surface a crystalline or granular appearance, which is then a s s u m e d to be a crystallization clue to shock and vibrations. T h u s , cause and effect are c o n f o u n d e d ; the crystalline fracture is a s s u m e d to be a cause, while the cause is the f a t i g u e of the metal, w h i c h breaks more easily and quickly because it is f a t i g u e d , and because it breaks more quickly the plastic p r o p e r t y of t h e metal does not come into play, flow will not take place, and because there is no flow the m e t a l will break crystalline, and what we take for crystallization due to shock is n o t h i n g b u t the transverse section of the i n d i v i d u a l crystals or fibers of the m e t a l t h u s broken. U n d e r favorable conditions we can break the finest staybolt iron, so t h a t it will show a so-called crystalline or, more properly expressed, a g r a n u l a r fracture. T h e condition is, t h a t the v i b r a t i o n s set up by the blow or blows in the pqrtion receiving the blow will not be t r a n s m i t t e d to t h a t portion from w h i c h the first portion is to be broken off, or, in o t h e r words, t h a t the portion held in some m a n n e r while one end or portion is b e i n g broken off, is held so rigidly t h a t the vibrations p r o d u c e d in the piece to be separated are not t r a n s m i s s i b l e to the piece held. T h e aim and object m u s t be to p r e v e n t flow of the metal, not to allow plasticity to come into play before f r a c t u r e takes place. If shock a n d vibrations p r o d u c e crystallization, t h e n iron car-axles o u g h t to crystallize sooner t h a n any other s t r u c t u r e ; b u t I h a v e come across iron car-axles w h i c h were sixteen, eighteen, t w e n t y and one was twenty-one years in service, and t h e y showed no sign of crystallization. On the o t h e r hand, I h a v e tested and e x a m i n e d 2-inch rods from old canal locks w h i c h n e v e r h a d been shocked by a n y m a n n e r of means, and y e t t h e y were l a r g e l y crystalline, the reason for this being, t h a t t h e iron was g r a n u l a r or cold-short w h e n it was new, as is often the case, and when such a g r a n u l a r portion or nest of crystals h a p p e n s to come in a location in the s t r u c t u r e which has to carry the load, t h a t s t r u c t u r e is liable to break u n d e r a lower stress t h a n was calculated for, because the s t r u c t u r e is w e a k e n e d
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Constitutionof Binary Alloys.
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by the g r a n u l a r spot of iron i m b e d d e d in the fibrous mass, and t h u s we h a v e a c o n t r i b u t a r y cause to produce f a t i g u e in the m e t a l sooner t h a n it o u g h t to become f a t i g u e d and, as said previously, the s t r u c t u r e will break more quickly because w e a k e n e d by a streak or nest of originally crystallineiron, and because b r e a k i n g quickly it will break w i t h o u t flowing. First-class metal, b r e a k i n g slowly by f a t i g u e or, as we call it, b r e a k i n g in detail, does not show a n y g r a n u l a r fract u r e at all, and I have e x a m i n e d such detail fractures w i t h the microscope a n u m b e r of times in iron and steel and never was able to detect a n y c h a n g e in the s t r u c t u r e of the m e t a l more t h a n an e i g h t h of an inch at the point of fracture, and t h e n the c h a n g e d m e t a l was of an a m o r p h o u s s t r u c t u r e and not crystalline, as it o u g h t to be if there is a n y t h i n g at all in the crystallization t h e o r y b y shock a n d vibrations. Steel b e i n g crystalline to begin with, we c a n n o t reasonably even speak of the crystallization of steel. In order for a m e t a l to crystallize it m u s t be a fluid solution like a saltsolution, for instance, or at least in a p a s t y state, and therefore the science of crystallization is opposed to the t h e o r y t h a t crystals can form in a cold piece of m e t a l of such a form and consistency as are f o r m e d in a solution according to the laws of crystallization. Crystals are the p r o d u c t of the solidification of chemical e l e m e n t s w h i c h are a t t r a c t e d toward each o t h e r while b e i n g held in a m o b i l e state i n t h e solution, according to the law of t h e i r affinities, w h a t e v e r t h a t law m a y be, and, moreover, t h e forces which t e n d to d e s t r o y a structure, or part of a structure, of w h a t e v e r kind, are directly opposed to t h e f o r m a t i o n of crystals, because strains and stress act upon a m e t a l in the n a t u r e of work, and therefore these d e s t r u c t i v e forces tend to break up the structure, to m a k e it smaller if large, or amorphous, silky, or velvety, if the s t r u c t u r e was small from the b e g i n n i n g . MR. ROBERT JOB: T h e q u e s t i o n of the last speaker, w h e t h e r iron becomes crystalline as a result of stresses in service, recalls a case which came u n d e r m y observation a n u m b e r of years ago. T h e end of a truss-rod upon a bridge
228
.fob :
[J. F. I.,
h a d broken, and the rod, which was of I~ inches rod iron, a b o u t ten feet long, h a d been t h r o w n upon the ground, and h a d broken into three pieces, s h o w i n g a coarsely g r a n u l a r fracture. T h e rod h a d been in position t h i r t y years, and the t h e o r y was a d v a n c e d t h a t g r a n u l a t i o n and b r i t t l e n e s s h a d r e s u l t e d o w i n g to c o n t i n u o u s vibrations in service. Thinki n g t h a t the conditions of the other rods m i g h t disprove this s t a t e m e n t , we h a d several r e m o v e d from the bridge from positions c o r r e s p o n d i n g exactly with t h a t occupied by the defective rod and s u b j e c t to exactly the same stresses for the same l e n g t h of time. U p o n fracture, we f o u n d a l o n g fibrous s t r u c t u r e in each case, w i t h tensile s t r e n g t h of a b o u t 48,o00 p o u n d s per square inch, and an e l o n g a t i o n of 35 per cent. in a 2-inch section. In o t h e r words, the vibrations of t h i r t y years' service h a d not caused a n y c h a n g e from the original fibrous structure. A n a l y s i s n e x t showed t h a t the composition of these different bars was closely alike, as were the e t c h e d sections, and it s e e m e d e v i d e n t t h a t t h e rods had been rolled from the same stock, and p r o b a b l y at the same time. T h u s it was d e a r l y s h o w n t h a t the coarse g r a n u l a t i o n h a d not r e s u l t e d from the stresses of service, b u t t h a t it h a d been present in the iron at the start, this condition b e i n g the result of defective h e a t - t r e a t m e n t in the m a n u f a c t u r e . T h e whole q u e s t i o n of h e a t - t r e a t m e n t of m e t a l s is today receiving g r e a t attention, and as a practical result, g r e a t i m p r o v e m e n t in the q u a l i t y of m a n y p r o d u c t s has been attained. As a case in point, we m a y cite the effect of h e a t - t r e a t m e n t upon the q u a l i t y of steel rails, as s h o w n by an i n v e s t i g a t i o n w h i c h has been in progress with us for some time. It was f o u n d t h a t a rail of a given composition w h e n finished at a yellow-heat was e x c e e d i n g l y brittle, a n d broke like glass u n d e r a single blow in the drop test. A raft of identically the same composition, b u t finished at a red color, w i t h s t o o d w i t h o u t f r a c t u r e e i g h t blows of a 2,ooo p o u n d w e i g h t falling a distance of 2o feet, the supports b e i n g 4 feet apart. Sections u n d e r the microscope showed in the f o r m e r case a very coarse g r a n u l a r structure, and in the l a t t e r a
Mar., I9O2.]
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much finer interlocking appearance, which amply accounted for the increased strength observed under the drop test. In service, similar differences would be observed as to wear, since it has been repeatedly proven by different observers that rapidity of wear varies--other thifigs being e q u a l - with the size of the grains of the metal. Thus, we have found that this mere difference in finishing, temperature has transformed a fragile u n t r u s t w o r t h y rail into one of vastly increased toughness and ability to resist wear. DR. JOSEPH W. RICHARDS (Lehigh U n i v e r s i t y ) b e g g e d leave to call attention to the information which might be gotten respecting the constitution of alloys by a consideration of the variation in other physical properties than the melting points. The specific gravity of alloys compared with their specific gravity calculated from that of their constituents, showed usually a contraction taking place during an alloying, sometimes an expansion. Dr. Mathews had shown that gold-silver alloys were peculiar in that their melting points varied regularly and uniformly from that of silver to that of gold. These very alloys are also peculiar in their specific gravities, which vary regularly from that of silver to that of gold without any expansion or contraction taking place at any point. The speaker had verified this fact m a n y times by finding the specific gravities of goldsilver alloys exactly the same as that calculated from their composition. This absence of expansion or contraction in alloying seems to be, therefore, characteristic of alloys which are simply homogeneous mixtures and which form no eutectic alloy. Dr. Mathews had shown that 0"5 per cent. of aluminium in tin depressed the melting point of the latter , but over this amount the melting point rose, I per cent. having the same melting point as pure tin. While the speaker had not measured the specific gravities of these alloys, he felt confident in predicting that similar phenomena would be found in this respect, that 0"5 per cent. of aluminium in tin would probably be found to increase the specific gravity of the latter, instead of decreasing it; that above this amount would lose its effect, and that the alloy with I per cent. would probably be found to be of the same
230
Notes and Comments.
[J. F. I.,
specific gravity as pure tin. It is k n o w n that one-half per cent. of a l u m i n i u m in iron increases its specific gravity. One per cent. of a n t i m o n y in lead makes an alloy heavier than lead ; 2 per cent. is of the same specific gravity as lead, and higher percentages are lighter. It is very likely that these alloys will s h o w an a n a l o g o u s behavior in their m e l t i n g points. Alloys are best studied by m a k i n g a complete correlation of as m a n y physical properties as can be observed, and such s t u d y is not only of h i g h theoretical interest but of i m m e d i a t e practical v a l u e in the arts.
M O N O C H R O M A T I C C O A T I N G S ON G E R M A N S I L V E R AND PLATINUM. ~ DR. R. BOET~G~R. After i n n u m e r a b l e experiments success has been achieved in producing t h e finest monochromatic shades on p l a t i n u m and G e r m a n silver by electrolytic decomposition of organic m a n g a n o u s salts, t h e metal to be treated b e i n g connected with t h e positive electrode. It has formerly been supposed t h a t it was principally t h e form of t h e negative pole t h a t influenced the nature of t h e Nobili figures. A l t h o u g h this c a n n o t be denied a l t o g e t h e r - - f o r with a sharply pointed p l a t i n u m wire only ring-shaped figures can be o b t a i n e d - - y e t the rule is subject to modification with regard to certain m a n g a n o u s salts. W i t h solutions of m a n g a n o u s hippurate, acetate or suecinate, even when a very t h i n platinum-wire serves as the negative pole, colored rings are n e v e r formed, b u t a m o n o c h r o m a t i c coating, on the positive electrode. This has only been observed with these special salts. It f u r t h e r appears t h a t metals which form h i g h e r oxides, such as lead aud manganese, are the best in solution for producing Nobili figures. I n using t h e special m a n g a n e s e salts for o b t a i n i n g t h e m o n o c h r o m a t i c coating, the shape of the negative pole appears to be immaterial ; but it is advisable to secure uniformity of t h e coating on the positive element, t h a t t h e negative element should take t h e form of a disk of platinum. N o t h i n g decided can be said about t h e s t r e n g t h of t h e manganese solutions, as t h e best s t r e n g t h depends upon the power of t h e current, and must be determined b y each e x p e r i m e n t e r for his own special case. T h e weaker t h e current t h e stronger t h e m a n g a n e s e solution must be. T h e color produced changes greatly and quickly, so t h a t it is essential instantly to b r e a k t h e current w h e n the desired color has been produced. Golden yellow, green, a n d purple are obtained with particular brilliancy. T h e metal should be removed from t h e b a t h as soon as the c u r r e n t is stopped, rinsed with distilled water, and carefully dried with soft b l o t t i n g paper. If m a n g a n o u s chloride or lead acetate is used, tile colors appear in rings instead of as a uniform layer and of all t h e colors of the rainbow, in the softest shades, the p r e d o m i n a n t hues b e i n g green, golden yellow and blue, each whole system of rings surrounded by a yellow z o n e . - - D e r Melallarbeiler.
22t
Constitution o f B i n a r y Alloys.
Mining and Metallurgical Section. Staled 2Weeling, held Wednesday, November s3, I9oI.
UPON
THE C O N S T I T U T I O N
OF
BINARY
ALLOYS.
BY JOHN ALEXANDER MATHEWS, PH.D.
Continued from vol. diii, p. 14o.
DISCUSSION. MR. A. m. OUTERBRIDGE, JR--I have been m u c h intere s t e d in Dr. M a t h e w ' s paper, and I wish to ask w h e t h e r the s u b j e c t of " s e g r e g a t i o n " in b i n a r y alloys, such as silver and copper, has b e e n s t u d i e d b y t h e s e n e w m e t h o d s of scientific research. I am led to m a k e this inquiry, h a v i n g noticed a striking difference in the cooling curves s h o w n in the diag r a m s of two alloys, one only of w h i c h s e g r e g a t e s ; in [hese d i a g r a m s the cooling curve of an alloy of silver a n d copper a n d the cooling curve of an alloy of silver and gold are s h o w n in juxtaposition. T h e silver-copper curve is v e r y pronounced, and t e r m i n a t e s s h a r p l y at the point w h e r e the e u t e c t i c alloy s e p a r a t e s ; the silver-gold alloy, on the other hand, s h o w s a nearly s t r a i g h t line, and no eutectic. It is well k n o w n t h a t alloys of silver and copper s e g r e g a t e while alloys of silver and gold are stable. More than t w e n t y years ago, long before the m o d e r n science of m i c r o - m e t a l l u r g y h a d been developed, I had occasion to m a k e an i n v e s t i g a t i o n of the r a t h e r o b s c u r e p h e n o m e n o n k n o w n as s e g r e g a t i o n of the silver-copper alloys u s e d in coinage, and, incidentally, of t h e alloys of silver and gold, and copper and gold, in which s e g r e g a t i o n is a l m o s t nil. T h e s t a n d a r d alloy used for the silver coin of tile U n i t e d S t a t e s is c o m p o s e d of 900 parts of silver and IOO parts of copper, while the s t a n d a r d for the gold coins is 9oo p arts of gold and IOO parts of copper.* T h e gold-copper alloy, w h e n * A trace of silver is permitted in this alloy.
22~
Outerbridge :
[J. F. I.,
once formed by m e l t i n g a n d t h o r o u g h mixing, r e m a i n s practically h o m o g e n e o u s , so t h a t w h e n the m e t a l is cast into ingots and rolled into strips, from w h i c h the p l a n c h e t s or blanks are s t a m p e d out, all of the pieces are f o u n d to be of nearly u n i f o r m fineness. N o t so, however, in the case of the silver-copper alloy. No m a t t e r how c a r e f u l l y the metallic m i x t u r e m a y have been prepared, or how t h o r o u g h l y the molten m e t a l m a y have been stirred, it will be f o u n d t h a t segreg a t i o n will always occur, even to such an e x t e n t t h a t it is impossible to obtain a fair sample for assay by c u t t i n g off a little piece from an ingot, as is c o m m o n l y done w i t h the gold-copper alloy. T h e silver assay sample is, therefore, dipped out from the m o l t e n silver-copper alloy and p o u r e d in a fine s t r e a m into cold water, t h u s cooling the m e t a l i n s t a n t l y in the form of small shot or " g r a n u l e s "; in this w a y s e g r e g a t i o n in the sample is avoided, b u t it has never been overcome in the ingot. It is for this reason t h a t the coinage laws p e r m i t a m u c h g r e a t e r l i m i t of " t o l e r a n c e " or variation from true standard, in the case of silver coins, t h a n in the case of gold coins; nevertheless, I believe it has a c t u a l l y h a p p e n e d t h a t silver dollar coins struck from m e t a l which originally assayed of the proper fineness h a v e been c o n d e m n e d because the v a r i a t i o n in fineness due to segregation in the i n g o t exceeded the limits allowed by law. T h e true cause of s e g r e g a t i o n has never been satisfactorily explained so far as I know. It does not appear to be associated in a n y w a y w i t h the specific g r a v i t y of the different metals composing an alloy ; thus, for example, w i t h silver-copper alloys it has been f o u n d t h a t in certain proportions of silver and copper the silver s e g r e g a t e s towards the center of the bar, and in o t h e r proportions the r i c h e r alloy is f o u n d near the surface, while still a n o t h e r curious fact has been proven in this connection, namely, t h a t w h e n silver and copper are alloyed in the proportion of a b o u t three-fourths silver and one-fourth copper little or no segreg a t i o n occurs. In the case of silver-gold alloys there is, I believe, very little if a n y segregation, and it seemed to me t h a t there m a y be some connection between these p h e n o m e n a and those revealed by the d i a g r a m s on the screen to which I
Mar., I9o2.]
Constitutionof Binary Alloys.
223
have referred, s h o w i n g a p r o n o u n c e d eutectic alloy in the silver-copper m e t a l and t h e a b s e n c e of the euteetie in t h e silver-gold alloy. I t h e r e f o r e have t h o u g h t t h a t it m i g h t b e an i n t e r e s t i n g as well as a p p r o p r i a t e q u e s t i o n to ask Dr. M a t h e w s w h e t h e r the light of microscopic investigation, in eonnection with the s t u d y of the cooling of metals from a molten state, has been cast upon this i m p o r t a n t u n s o l v e d p r o b l e m in m e t a l l u r g y ? I m a y say, in conclusion, that some of the o b s e r v a t i o n s on alloys of the precious metals noted herein were m a d e b y me as long ago as I873, in the course of a s o m e w h a t prolonged investigation of a m e t h o d of q u a n t i t a t i v e spectroscopic analysis then recently proposed b y Sir N o r m a n L o c k y e r and the assayer of the R o y a l Mint, n o w Sir R o b e r t s A u s t e n . ~ T h e fact that silver-copper alloys s e g r e g a t e was k n o w n long before this time, b u t was r e g a r d e d as an unsolvable metallurgical riddle or paradox. I am inclined to believe t h a t a careful and t h o r o u g h i n v e s t i g a t i o n of the micro-structure of different portions of an ingot of silvercopper alloy, in which marked s e g r e g a t i o n has been s h o w n to exist b y the ordinary b u t v e r y accurate m e t h o d s of h u m i d a s s a y of silver, would be likely to increase our k n o w l e d g e of the u n d e r l y i n g cause of this s o m e w h a t m y s t e r i o u s phen o m e n o n k n o w n as segregation. MR. PAUL KREUZPOINTNER : - - T h e very instructive p a p e r read this e v e n i n g b y Professor M a t h e w s raises a good m a n y q u e s t i o n s in m y m i n d concerning the results of scientific research of the s t r u c t u r e of m e t a l s as to the relation of t h e s e results to applied, that is, practical metallurgy. However, before asking questions, I b e g to express g r e a t pleasure and satisfaction with the progress m a d e in the use of the microscope as applied to metallurgy. A n y one who k n o w s a n y t h i n g of the struggles, d i s a p p o i n t m e n t s and criticisms the pioneers of m e t a l l o g r a p h y were s u b j e c t to sixteen and t w e n t y years ago will appreciate the progress m a d e in this line of scientific work d u r i n g the last ten years, since when "Electrical Spectra of Metals."
Society, May, I874, Vol. XIV, p. I62.
Proceedings ~Imerican Philosophical
224
A~reuzgointner :
[J. F. I.,
the microscope has become an i n d i s p e n s a b l e i n s t r u m e n t in the work of i n v e s t i g a t i n g the properties of metals. B u t while I would be the last one to d e d u c t even an iota from the value of the work done in this line b y Sorbey, Martens, H e y n , O s m o n d a n d others, I b e g to r e m i n d you t h a t members of the M i n i n g and Metallurgieal Section of the F r a n k l i n I n s t i t u t e deserve credit and r e e o g n i t i o n for the pioneer work done in this country, and y o u r p r e s e n t president, Mr. Garrison, who took up the work of m e t a l l o g r a p h y t w e n t y y e a r s ago, p r e p a r i n g the soil for those who were to reap the fruit of his planting, knows w h a t diffieulties a pioneer in a n y line of work has to c o n t e n d with. As to some of the points raised by the lecturer it w o u l d be i n t e r e s t i n g to know w h y a metal, for i n s t a n c e steel, w h e n in a physical state where the eutectie is a l r e a d y more or less clearly formed, still appears to possess tolerably fair physical qualities a l t h o u g h a low elastieity. If I u n d e r s t a n d a r i g h t the f o r m a t i o n of the euteetie m e a n s a more or less complete s e g r e g a t i o n of the surplus of e l e m e n t s over and above t h a t p e r c e n t a g e of d e m e n t s which e v e n t u a l l y form the euteetie, and where the d e m e n t s composing the m e t a l and w h i c h are t h e n still in a fluid state will freeze s i m u l t a n e o u s l y . If s e g r e g a t i o n m e a n s a dissociation of elements; and dissociation contributes to the l o o s e n i n g or lessening of the cohesion by which the crystals are held t o g e t h e r a n d w h i c h is an essential in e o n t r i b u t i n g to the s t r e n g t h of a metal, t h e n w h y is it t h a t metals of such a large s e g r e g a t e d structure like illustration No. 3 (I believe, K), w h e r e the euteetic has f o r m e d or is forming, still show a h i g h degree of cohesive force in daily practice ? Is there a mastics f o r m e d d u r i n g s e g r e g a t i o n w h i e h acts as a binder, so to speak, or is the s t r e n g t h p r o d u c e d m e r e l y b y the m e c h a n i c a l i n t e r l o e k i n g of the crystals? By this I do not m e a n t h a t such m e t a l is equal in q u a l i t y to one of n o r m a l conditions, t h a t is, a m e t a l of fine s t r u c t u r e free from the d i s t u r b i n g influences of s e g r e g a t i o n as m u c h as is possible. T h e r e is a fertile field y e t for the scientific i n v e s t i g a t o r of the s t r u c t u r e of a m e t a l and the relation of t h a t s t r u c t u r e to the m a x i m u m usefulness and serviceability of t h a t m e t a l in daily practice
Mar., I9o2.~
Constitution o f Binary Aitoys.
225
in d e t e r m i n i n g the effects of h e a t on t h e s t r u c t u r e in an a s c e n d i n g ratio i n s t e a d of in a d e s c e n d i n g ratio, as is the case in d e t e r m i n i n g the cooling curve. W h a t I mean b y this is, w h a t is the n a t u r e of the various s t r u c t u r e s of a given m e t a l at various points of h e a t when we h e a t that m e t a l from the cold state to t h e point of fusion ? To Brinnel we o w e m u c h i n f o r m a t i o n in that line, b u t unless I am mistaken, no such systematic, t h o r o u g h researches h a v e been m a d e of the s t r u c t u r e s of metals w h e n heated up to the point of fusion as w h e n cooled down from the point of fusion and the c o r r e s p o n d i n g physical q u a l i t y at these different structures. I am well aware of the m a n y d e t a c h e d and s o m e t i m e s practically v a l u a b l e i n v e s t i g a t i o n s on the lines I refer to, b u t w h a t the practical metallurgist, the expert, w h o has n e i t h e r the time nor facilities to make cont i n u o u s i n v e s t i g a t i o n s needs, is a series of illustrations of the m i c r o s t r u c t u r e of the m o s t f r e q u e n t l y used structural m e t a l s at different degrees of h e a t i n g t h a t m e t a l and l e t t i n g it cool and the c o r r e s p o n d i n g physical s t r e n g t h at that heat. I m m e n s e q u a n t i t i e s of good metal have been spoilt in mill and shop on a c c o u n t of i m p r o p e r h e a t t r e a t m e n t , and while we know a little b e t t e r to-day there is still a g r e a t deal to be learned on the subject, and o u r technical universities, w h o are doing already such good work in physical metallurgy, can be of g r e a t benefit to the e n g i n e e r and o u r industries b y t h u s c o n t r i b u t i n g their mite to practical metallurgy. In a n s w e r to the q u e s t i o n b y the last speaker w h e t h e r crystals are necessarily always f o r m e d b y h e a t and are not likewise f o r m e d b y shock and vibrations, I w o u l d say that experience s e e m s to indicate t h a t metals, t h a t is iron and steel, do not crystallize u n d e r the influence of shock and v i b r a t i o n s as was c o m m o n l y s u p p o s e d formerly. B u t while metals do not crystallize due to shocks and v i b r a t i o n s t h e y u n d o u b t e d l y b e c o m e fatigued, and w h e n t h u s fatigued, iron and steel b r e a k more easily, and b e c a u s e t h e y b r e a k more easily w h e n fatigued, t h e y b r e a k more quickly u n d e r u n d u e strains, and b e c a u s e b r e a k i n g quickly t h e metal has no time to flow, its plasticity does not come into play, and if we succeed in b r e a k i n g a metal before flow VQL. CLIII.
No. 915.
15
226
Kreuzpointner :
[J. F. I.,
sets in, we g e t a transverse section of the i n d i v i d u a l fibers w h i c h will give the f r a c t u r e d surface a crystalline or granular appearance, which is then a s s u m e d to be a crystallization clue to shock and vibrations. T h u s , cause and effect are c o n f o u n d e d ; the crystalline fracture is a s s u m e d to be a cause, while the cause is the f a t i g u e of the metal, w h i c h breaks more easily and quickly because it is f a t i g u e d , and because it breaks more quickly the plastic p r o p e r t y of t h e metal does not come into play, flow will not take place, and because there is no flow the m e t a l will break crystalline, and what we take for crystallization due to shock is n o t h i n g b u t the transverse section of the i n d i v i d u a l crystals or fibers of the m e t a l t h u s broken. U n d e r favorable conditions we can break the finest staybolt iron, so t h a t it will show a so-called crystalline or, more properly expressed, a g r a n u l a r fracture. T h e condition is, t h a t the v i b r a t i o n s set up by the blow or blows in the pqrtion receiving the blow will not be t r a n s m i t t e d to t h a t portion from w h i c h the first portion is to be broken off, or, in o t h e r words, t h a t the portion held in some m a n n e r while one end or portion is b e i n g broken off, is held so rigidly t h a t the vibrations p r o d u c e d in the piece to be separated are not t r a n s m i s s i b l e to the piece held. T h e aim and object m u s t be to p r e v e n t flow of the metal, not to allow plasticity to come into play before f r a c t u r e takes place. If shock a n d vibrations p r o d u c e crystallization, t h e n iron car-axles o u g h t to crystallize sooner t h a n any other s t r u c t u r e ; b u t I h a v e come across iron car-axles w h i c h were sixteen, eighteen, t w e n t y and one was twenty-one years in service, and t h e y showed no sign of crystallization. On the o t h e r hand, I h a v e tested and e x a m i n e d 2-inch rods from old canal locks w h i c h n e v e r h a d been shocked by a n y m a n n e r of means, and y e t t h e y were l a r g e l y crystalline, the reason for this being, t h a t t h e iron was g r a n u l a r or cold-short w h e n it was new, as is often the case, and when such a g r a n u l a r portion or nest of crystals h a p p e n s to come in a location in the s t r u c t u r e which has to carry the load, t h a t s t r u c t u r e is liable to break u n d e r a lower stress t h a n was calculated for, because the s t r u c t u r e is w e a k e n e d
Mar., I 9 o 2 . ]
Constitutionof Binary Alloys.
227
by the g r a n u l a r spot of iron i m b e d d e d in the fibrous mass, and t h u s we h a v e a c o n t r i b u t a r y cause to produce f a t i g u e in the m e t a l sooner t h a n it o u g h t to become f a t i g u e d and, as said previously, the s t r u c t u r e will break more quickly because w e a k e n e d by a streak or nest of originally crystallineiron, and because b r e a k i n g quickly it will break w i t h o u t flowing. First-class metal, b r e a k i n g slowly by f a t i g u e or, as we call it, b r e a k i n g in detail, does not show a n y g r a n u l a r fract u r e at all, and I have e x a m i n e d such detail fractures w i t h the microscope a n u m b e r of times in iron and steel and never was able to detect a n y c h a n g e in the s t r u c t u r e of the m e t a l more t h a n an e i g h t h of an inch at the point of fracture, and t h e n the c h a n g e d m e t a l was of an a m o r p h o u s s t r u c t u r e and not crystalline, as it o u g h t to be if there is a n y t h i n g at all in the crystallization t h e o r y b y shock a n d vibrations. Steel b e i n g crystalline to begin with, we c a n n o t reasonably even speak of the crystallization of steel. In order for a m e t a l to crystallize it m u s t be a fluid solution like a saltsolution, for instance, or at least in a p a s t y state, and therefore the science of crystallization is opposed to the t h e o r y t h a t crystals can form in a cold piece of m e t a l of such a form and consistency as are f o r m e d in a solution according to the laws of crystallization. Crystals are the p r o d u c t of the solidification of chemical e l e m e n t s w h i c h are a t t r a c t e d toward each o t h e r while b e i n g held in a m o b i l e state i n t h e solution, according to the law of t h e i r affinities, w h a t e v e r t h a t law m a y be, and, moreover, t h e forces which t e n d to d e s t r o y a structure, or part of a structure, of w h a t e v e r kind, are directly opposed to t h e f o r m a t i o n of crystals, because strains and stress act upon a m e t a l in the n a t u r e of work, and therefore these d e s t r u c t i v e forces tend to break up the structure, to m a k e it smaller if large, or amorphous, silky, or velvety, if the s t r u c t u r e was small from the b e g i n n i n g . MR. ROBERT JOB: T h e q u e s t i o n of the last speaker, w h e t h e r iron becomes crystalline as a result of stresses in service, recalls a case which came u n d e r m y observation a n u m b e r of years ago. T h e end of a truss-rod upon a bridge
228
.fob :
[J. F. I.,
h a d broken, and the rod, which was of I~ inches rod iron, a b o u t ten feet long, h a d been t h r o w n upon the ground, and h a d broken into three pieces, s h o w i n g a coarsely g r a n u l a r fracture. T h e rod h a d been in position t h i r t y years, and the t h e o r y was a d v a n c e d t h a t g r a n u l a t i o n and b r i t t l e n e s s h a d r e s u l t e d o w i n g to c o n t i n u o u s vibrations in service. Thinki n g t h a t the conditions of the other rods m i g h t disprove this s t a t e m e n t , we h a d several r e m o v e d from the bridge from positions c o r r e s p o n d i n g exactly with t h a t occupied by the defective rod and s u b j e c t to exactly the same stresses for the same l e n g t h of time. U p o n fracture, we f o u n d a l o n g fibrous s t r u c t u r e in each case, w i t h tensile s t r e n g t h of a b o u t 48,o00 p o u n d s per square inch, and an e l o n g a t i o n of 35 per cent. in a 2-inch section. In o t h e r words, the vibrations of t h i r t y years' service h a d not caused a n y c h a n g e from the original fibrous structure. A n a l y s i s n e x t showed t h a t the composition of these different bars was closely alike, as were the e t c h e d sections, and it s e e m e d e v i d e n t t h a t t h e rods had been rolled from the same stock, and p r o b a b l y at the same time. T h u s it was d e a r l y s h o w n t h a t the coarse g r a n u l a t i o n h a d not r e s u l t e d from the stresses of service, b u t t h a t it h a d been present in the iron at the start, this condition b e i n g the result of defective h e a t - t r e a t m e n t in the m a n u f a c t u r e . T h e whole q u e s t i o n of h e a t - t r e a t m e n t of m e t a l s is today receiving g r e a t attention, and as a practical result, g r e a t i m p r o v e m e n t in the q u a l i t y of m a n y p r o d u c t s has been attained. As a case in point, we m a y cite the effect of h e a t - t r e a t m e n t upon the q u a l i t y of steel rails, as s h o w n by an i n v e s t i g a t i o n w h i c h has been in progress with us for some time. It was f o u n d t h a t a rail of a given composition w h e n finished at a yellow-heat was e x c e e d i n g l y brittle, a n d broke like glass u n d e r a single blow in the drop test. A raft of identically the same composition, b u t finished at a red color, w i t h s t o o d w i t h o u t f r a c t u r e e i g h t blows of a 2,ooo p o u n d w e i g h t falling a distance of 2o feet, the supports b e i n g 4 feet apart. Sections u n d e r the microscope showed in the f o r m e r case a very coarse g r a n u l a r structure, and in the l a t t e r a
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Constitution o): Binary Alloys.
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much finer interlocking appearance, which amply accounted for the increased strength observed under the drop test. In service, similar differences would be observed as to wear, since it has been repeatedly proven by different observers that rapidity of wear varies--other thifigs being e q u a l - with the size of the grains of the metal. Thus, we have found that this mere difference in finishing, temperature has transformed a fragile u n t r u s t w o r t h y rail into one of vastly increased toughness and ability to resist wear. DR. JOSEPH W. RICHARDS (Lehigh U n i v e r s i t y ) b e g g e d leave to call attention to the information which might be gotten respecting the constitution of alloys by a consideration of the variation in other physical properties than the melting points. The specific gravity of alloys compared with their specific gravity calculated from that of their constituents, showed usually a contraction taking place during an alloying, sometimes an expansion. Dr. Mathews had shown that gold-silver alloys were peculiar in that their melting points varied regularly and uniformly from that of silver to that of gold. These very alloys are also peculiar in their specific gravities, which vary regularly from that of silver to that of gold without any expansion or contraction taking place at any point. The speaker had verified this fact m a n y times by finding the specific gravities of goldsilver alloys exactly the same as that calculated from their composition. This absence of expansion or contraction in alloying seems to be, therefore, characteristic of alloys which are simply homogeneous mixtures and which form no eutectic alloy. Dr. Mathews had shown that 0"5 per cent. of aluminium in tin depressed the melting point of the latter , but over this amount the melting point rose, I per cent. having the same melting point as pure tin. While the speaker had not measured the specific gravities of these alloys, he felt confident in predicting that similar phenomena would be found in this respect, that 0"5 per cent. of aluminium in tin would probably be found to increase the specific gravity of the latter, instead of decreasing it; that above this amount would lose its effect, and that the alloy with I per cent. would probably be found to be of the same
230
Notes and Comments.
[J. F. I.,
specific gravity as pure tin. It is k n o w n that one-half per cent. of a l u m i n i u m in iron increases its specific gravity. One per cent. of a n t i m o n y in lead makes an alloy heavier than lead ; 2 per cent. is of the same specific gravity as lead, and higher percentages are lighter. It is very likely that these alloys will s h o w an a n a l o g o u s behavior in their m e l t i n g points. Alloys are best studied by m a k i n g a complete correlation of as m a n y physical properties as can be observed, and such s t u d y is not only of h i g h theoretical interest but of i m m e d i a t e practical v a l u e in the arts.
M O N O C H R O M A T I C C O A T I N G S ON G E R M A N S I L V E R AND PLATINUM. ~ DR. R. BOET~G~R. After i n n u m e r a b l e experiments success has been achieved in producing t h e finest monochromatic shades on p l a t i n u m and G e r m a n silver by electrolytic decomposition of organic m a n g a n o u s salts, t h e metal to be treated b e i n g connected with t h e positive electrode. It has formerly been supposed t h a t it was principally t h e form of t h e negative pole t h a t influenced the nature of t h e Nobili figures. A l t h o u g h this c a n n o t be denied a l t o g e t h e r - - f o r with a sharply pointed p l a t i n u m wire only ring-shaped figures can be o b t a i n e d - - y e t the rule is subject to modification with regard to certain m a n g a n o u s salts. W i t h solutions of m a n g a n o u s hippurate, acetate or suecinate, even when a very t h i n platinum-wire serves as the negative pole, colored rings are n e v e r formed, b u t a m o n o c h r o m a t i c coating, on the positive electrode. This has only been observed with these special salts. It f u r t h e r appears t h a t metals which form h i g h e r oxides, such as lead aud manganese, are the best in solution for producing Nobili figures. I n using t h e special m a n g a n e s e salts for o b t a i n i n g t h e m o n o c h r o m a t i c coating, the shape of the negative pole appears to be immaterial ; but it is advisable to secure uniformity of t h e coating on the positive element, t h a t t h e negative element should take t h e form of a disk of platinum. N o t h i n g decided can be said about t h e s t r e n g t h of t h e manganese solutions, as t h e best s t r e n g t h depends upon the power of t h e current, and must be determined b y each e x p e r i m e n t e r for his own special case. T h e weaker t h e current t h e stronger t h e m a n g a n e s e solution must be. T h e color produced changes greatly and quickly, so t h a t it is essential instantly to b r e a k t h e current w h e n the desired color has been produced. Golden yellow, green, a n d purple are obtained with particular brilliancy. T h e metal should be removed from t h e b a t h as soon as the c u r r e n t is stopped, rinsed with distilled water, and carefully dried with soft b l o t t i n g paper. If m a n g a n o u s chloride or lead acetate is used, tile colors appear in rings instead of as a uniform layer and of all t h e colors of the rainbow, in the softest shades, the p r e d o m i n a n t hues b e i n g green, golden yellow and blue, each whole system of rings surrounded by a yellow z o n e . - - D e r Melallarbeiler.