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Silvering mirrors
253 Translated for the Journal of the Franklin Institute.
Silvering Mirrors. Process of MM. BROSSETTE~ CO. Patented in :France, 1855.--Report of the Committee of the Societz/for the .Encouragement of National Industry of ~ranee. After the surface of the glass to be silvered has been cleaned with Spanish whiting and water, it is rubbed over with jeweller's putty (oxide of tin), with some ammonial nitrate of silver added to it ; this is the same salt which is afterwards used in the silvering. Before proceeding to silver, the glass as it lies on its supports, is washed with a caoutchouc roller soaked in distilled water, and then placed truly horizontally on a cast iron table covered with oil-cloth~ which forms the upper part of a rectangular trough filled with water whose temperature is raised to 60 ° Cent. (140 ° :Fah.) by means of steam. When this has been done, there is poured upon the glass a solution 100 parts by weight of nitrate of silver dissolved in 500 parts of distilled water and 60 parts of liquid ammonia of sp. gray. 0"87 or 0"88 ; to this is added drop by drop after filtration 7½ parts of tartaric 'reid first dissolved in 30 parts of distilled water. This liquid (solution :No. 1) is retained on the surface of the glass by simple capillarity. It is left about 15 minutes, care being taken that every part which is to he silvered shall be .well moistened with it, and at the end of this time the glass is inclined and the liquid allowed to run off, mixed with a considerable quantity of non-adherent silver, into grooves arranged around the table ; it is then washed, and after being replaced in its former position, the second coating of silvering is applied (solution :No. 2), which differs from the first only in containing a double quantity of tartaric acid. This must remain on for 25 minutes, after which, the glass after being washed with distilled water is laid in a very oblique position, and then covered with a coating of red lead and oil, which dries rapidly. The first coating itself produces a coating which reflects perfectly, but which, although adhering strongly, would not be solid enough to receive the coating of paint; if the glass in this condition be held up to the light, many defects in the coating will be seen, which disappear after the second coating. According to the experiments of the committee, the first coating represents about 12 grammes of silver to the square metre (17"2 grs. per sq. foot), and the two coatings together 29 grammes (41"57 grains per sq. foot). There would perhaps be an advantage in prolonging the contact of the liquids with the glass, as experiment shows that they continue to deposit their silver for several hours after their decantation ; yet the liquid always retains at least one-half of the silver in solution. This process (the committee says) gives satisfactory results both for plane and curved mirrors ; and for the ordinary mirrors found in commerce, the expense does not exceed that of the ordinary method, over which it has the advantage of not being endangered by being put in any position either while carrying or when hung. It also appears to have the advantage of not being affected by moisture or exposure to Vow. XLI.--TamD S~aIEs.--No. 4.--AeniL, 1861.
22
254
Mechanics, _Physics, and Chemistry.
sun-llght. But they are not proof against vapors containing sulphuretted hydrogen; these, especially when aided by moisture, are not completely guarded against by red lead.--Bulletin de la Societ~ d'.Encouragement pour l'!ndustrie 2Vationale, May, 1860. For the Journal of the Franklin Institute.
UTon the Practical Relative Economy of using Steam with Different _Measures of.Expansion. By ALBAN C. STIMERS, Chief Engineer, U. S. :Navy. The most simple and obvious mode of using steam to obtain power in a steam engine, is to permit it to flow freely from the boiler into the cylinder during the entire stroke of the piston; and this was the plan adopted in the earliest engines. The ingenious and philosophical mind of Watt, however, upon the announcement by Mariotte that the volume of the fixed gases, when maintained at a constant temperature and unaffected by the greater or less proximity of their molecules, was inversely as their pressure, or, conversely, that the pressure was inversely as their volume, soon made the application to steam ; and, assuming this law and this appli. cation of it to be correct, it is easily shown that great gains in economical effect are produced by suppressing the flow of steam into the cylinder before the piston has completed its stroke, and permitting it to expand during the remainder. This was done by Watt, and apparatus for effecting this suppression at any desired point in the stroke, forms one of his many patents; but, notwithstanding the fact that both his mechanism and his patent covered the whole ground of the expansion question, that is, enabled him to cut off at any point in the stroke, his engines, which were generally paid for by a portion of the fuel he saved over that used by those displaced, were arranged, after a few trials, for suppressing the steam at about three-fourths the stroke of the piston. Any one familiar with Watt's history must have observed how uniformly he put every important conception tending to improve the steam engine to the test of a practical experiment; and though we have no account of an especial set of experiments having been tried by him to test the exact value of the expansion principle, there is but little doubt that his very accurate practice would soon determine whether the practical result was equal to the theoretical prediction, and that, when he found it was not, he determined by a complete set of experiments the most favorable degree of expansion and its actual value in the practical steam engine. The fact that he published no account of such experiments is no proof against their having been made, as he had every incentive as a business man to permit his rivals to follow the natural proneness of mankind for settling all such questions by mathematical demonstration rather than by carefully-conducted experiments, which require time, money, ingenuity, patience, and a much greater knowledge of the physical laws, for drawing correct inferences from the experimental data than is needed in any cal-
Silvering Mirrors. Process of MM. BROSSETTE~ CO. Patented in :France, 1855.--Report of the Committee of the Societz/for the .Encouragement of National Industry of ~ranee. After the surface of the glass to be silvered has been cleaned with Spanish whiting and water, it is rubbed over with jeweller's putty (oxide of tin), with some ammonial nitrate of silver added to it ; this is the same salt which is afterwards used in the silvering. Before proceeding to silver, the glass as it lies on its supports, is washed with a caoutchouc roller soaked in distilled water, and then placed truly horizontally on a cast iron table covered with oil-cloth~ which forms the upper part of a rectangular trough filled with water whose temperature is raised to 60 ° Cent. (140 ° :Fah.) by means of steam. When this has been done, there is poured upon the glass a solution 100 parts by weight of nitrate of silver dissolved in 500 parts of distilled water and 60 parts of liquid ammonia of sp. gray. 0"87 or 0"88 ; to this is added drop by drop after filtration 7½ parts of tartaric 'reid first dissolved in 30 parts of distilled water. This liquid (solution :No. 1) is retained on the surface of the glass by simple capillarity. It is left about 15 minutes, care being taken that every part which is to he silvered shall be .well moistened with it, and at the end of this time the glass is inclined and the liquid allowed to run off, mixed with a considerable quantity of non-adherent silver, into grooves arranged around the table ; it is then washed, and after being replaced in its former position, the second coating of silvering is applied (solution :No. 2), which differs from the first only in containing a double quantity of tartaric acid. This must remain on for 25 minutes, after which, the glass after being washed with distilled water is laid in a very oblique position, and then covered with a coating of red lead and oil, which dries rapidly. The first coating itself produces a coating which reflects perfectly, but which, although adhering strongly, would not be solid enough to receive the coating of paint; if the glass in this condition be held up to the light, many defects in the coating will be seen, which disappear after the second coating. According to the experiments of the committee, the first coating represents about 12 grammes of silver to the square metre (17"2 grs. per sq. foot), and the two coatings together 29 grammes (41"57 grains per sq. foot). There would perhaps be an advantage in prolonging the contact of the liquids with the glass, as experiment shows that they continue to deposit their silver for several hours after their decantation ; yet the liquid always retains at least one-half of the silver in solution. This process (the committee says) gives satisfactory results both for plane and curved mirrors ; and for the ordinary mirrors found in commerce, the expense does not exceed that of the ordinary method, over which it has the advantage of not being endangered by being put in any position either while carrying or when hung. It also appears to have the advantage of not being affected by moisture or exposure to Vow. XLI.--TamD S~aIEs.--No. 4.--AeniL, 1861.
22
254
Mechanics, _Physics, and Chemistry.
sun-llght. But they are not proof against vapors containing sulphuretted hydrogen; these, especially when aided by moisture, are not completely guarded against by red lead.--Bulletin de la Societ~ d'.Encouragement pour l'!ndustrie 2Vationale, May, 1860. For the Journal of the Franklin Institute.
UTon the Practical Relative Economy of using Steam with Different _Measures of.Expansion. By ALBAN C. STIMERS, Chief Engineer, U. S. :Navy. The most simple and obvious mode of using steam to obtain power in a steam engine, is to permit it to flow freely from the boiler into the cylinder during the entire stroke of the piston; and this was the plan adopted in the earliest engines. The ingenious and philosophical mind of Watt, however, upon the announcement by Mariotte that the volume of the fixed gases, when maintained at a constant temperature and unaffected by the greater or less proximity of their molecules, was inversely as their pressure, or, conversely, that the pressure was inversely as their volume, soon made the application to steam ; and, assuming this law and this appli. cation of it to be correct, it is easily shown that great gains in economical effect are produced by suppressing the flow of steam into the cylinder before the piston has completed its stroke, and permitting it to expand during the remainder. This was done by Watt, and apparatus for effecting this suppression at any desired point in the stroke, forms one of his many patents; but, notwithstanding the fact that both his mechanism and his patent covered the whole ground of the expansion question, that is, enabled him to cut off at any point in the stroke, his engines, which were generally paid for by a portion of the fuel he saved over that used by those displaced, were arranged, after a few trials, for suppressing the steam at about three-fourths the stroke of the piston. Any one familiar with Watt's history must have observed how uniformly he put every important conception tending to improve the steam engine to the test of a practical experiment; and though we have no account of an especial set of experiments having been tried by him to test the exact value of the expansion principle, there is but little doubt that his very accurate practice would soon determine whether the practical result was equal to the theoretical prediction, and that, when he found it was not, he determined by a complete set of experiments the most favorable degree of expansion and its actual value in the practical steam engine. The fact that he published no account of such experiments is no proof against their having been made, as he had every incentive as a business man to permit his rivals to follow the natural proneness of mankind for settling all such questions by mathematical demonstration rather than by carefully-conducted experiments, which require time, money, ingenuity, patience, and a much greater knowledge of the physical laws, for drawing correct inferences from the experimental data than is needed in any cal-