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LECTURES ON VEGETABLE CHEMISTRY,
LONDON, SATURDAY, JUNE
[1827-8.
21.
impurities or foreign bodies LECTURES ON
VEGETABLE
CHEMISTRY, BY
at
the
Royal
Institution Britain.
of
Great
LECTURE XLV.
Gum, Sugar, Starch, and
Gluten. the last lecture, that the ultimate elements of vegetable substances were few. Now, when a vegetable substance consists of carbon, oxygen, and hydrogen, its decomposition by heat presents us with a variety of substances resulting from their union ; but if it should also happen to contain nitrogen, you will then have ammonia formed, and a of other products resulting from the variety union of nitrogen with the other substances I MENTIONED,
Gentlemen, in
named. In proceeding to the analysis of any vegetable body, the chemist makes some experiments to determine its general nature ; and we will suppose that it contains no nitrogen, but that, like most vegetable substances, it is composed of carbon, oxygen, and hydrogen ; his object is to determine the relative proportions in which these elements are combined. Now, if we take sugar, starch, gum-resin, camphor, tan, and 1 great variety of other vegetable products, we find that they are all composed essentialiy of carbon, oxygen, and hydrogen, and you wilt naturally infer that the determination of their relative proportions in their bodies, will throw an interesting and considerable light upon the nature of the compounds generally, and serve to show how far they approximate to each other. 1 shall I not t8..e up your time by detailing all that has been done upon this subject, but describe to you the mode generally adopted for the examination of vegetable subManeee.
adhering to
when heated alone, but which, when heated in contact with vegetable substances, imparts oxygen to their elements, in proportion as they are decomposed. So that if you take a tube of this kind, and introduce a quantity of peroxide of copper into it, with a given weight of sugar for instance, and apply heat at the further end of the tube, and gradually carry the heat down to the other extremity, a decomposition is effected; and it follows, that whatever the product may be, that is produced by the action of heat upon the copper, it will be found at the other end of the tube, either in the gaseous or liquid form. Here you see the analysis going on ; you see that as the tube becomes thoroughly heated, a gas is extricated, which will afterwards be found to be carbonic acid gas ; and to be certain of this, we may absorb it by a small quantity of potash, or soda, or ammonia, and its volume leads us soon to determine the amount of carbon present, and hence the index to the quan. tity of carbon which the vegetable matter contained. But the vegetable matter also contained hydrogen, and that, with the oxygen given out from the copper, will form water, which is to be absorbed by a portion of asbestos placed in the tube, and the amount of water absorbed may be learnt by its increase in weight, after the experiment. We know how much hydrogen is contained in a given weight of water, because nine
water contain one of hydrogen. Then the quantity of carbon being determined, and the quantity of hydrogen being determined, there will be a certain loss, which is to be ascribed to the oxygen. Now, although this shows you the form of the experiment, you must remember that it is one which requires great caution to obtain accurate results, and it is necessary to repeat the experiment, four or five times, on sub- the same substance, before you can depend
If you wish to analyse a vegetable stance, you must examine it to see that
No. 251.
are
the substance may be hygrometric ; and if that is the case, you must avoid the contact of air ; you must introduce it into a brass tube in a given state of dryness, and then mix it thoroughly with a certain quantity of or
peroxide of copper,which gives out no oxygen
PROFESSOR BRANDE.
Delivered
it,
no
parts of
with
accuracy tinn-n
the results
354
Supposing that you had operated onin the vegetable acids; and then thet- are fifteen grains of sugar, and that you had other vegetable substances, in which hydroobtained a certain measure of carbonic acid predominates, as in the resins, war, gas. equivalent to six grains by weight ofoil, and combustible bodies. The examinacarbon, and that you had also obtained nine tion of the proximate principles of vegetagrains of water, which would contain one bles would afford ample scope for a fi.J grain of hydrogen, you would say that there course of lectures, so that we can only Ieis contained in fifteen grains of sugar six state it very generally, not go into the decarbon, one hydrogen, and a certain loss tails of abstract chemistry. The separation of the proximate principks equivalent to eight, which you ascribe to oxygen. Now, suppose we take sugar, of vegetables is generally effected bv cer. starch, gum, and wood, they contain no tain solvents ; if we take a piece of bark, for excess of oxygen, they are compounds ofexample, we first steep it in cold, then in carbon and water; and when I say they hot water, and then in alcohol. If you then contain no excess of oxygen, I use the term distil it with water, it may afford an essen. to imply that oxygen and hydrogen are in tial oil, or the oil which it contains may be the same relative proportions as in water. expressed ; and in this way, by the action Now you will easily remark, how closely of solvents, we separate sub. these four substances correspond in their stances into a certain number of proximate ultimate composition, and therefore you will principles. It does not much matter how not be surprised why it often happens, that we arrive at these ; but I shall endeavour by artificial means, we are capable of con- to throw them into a class founded upon this verting one of these substances into the composition, according as they may contaitt other, by changing the relative proportions hydrogen and oxygen, in the same proporof their element; that we can, for example, I tion as water, .according as they may canconvert starch and gum into sugar, sugar tain either in excess; and lastly, such as into gum, and gum into starch ; and these may contain nitrogen. Gum is a specimen of the combination of are processes daily going on in nature, in the growth and ripening of fruits, and so elements of the first class ; it exudes from on, but more especially the passing of the acacia and other trees, and you find that starch and gum into sugar. Now suppose there are great varieties of gum which differ that any nitrogen had been separated from a little in their chemical properties. Some the substances you were examining, that chemists have divided the gums into two would throw great difficulty in the way of kinds; the first into the gums, properly so the analysis, but to these nitrous compounds called ; the second, such as are comparawe shall advert afterwards. If nitrogen be tively insoluble in water, as tragacanth and present, and if the operation of analysis be cherry-tree gum, and to these they have successfully conducted, it might also be given the name cerasin. collected in the tube ; but as nitrogen is The general character of gum is that it not soluble in potash, you throw a little is soluble in cold and warm water, forming potash into the tube, by which you have a viscid solution, known by the name of carbonate of potash formed, and the nitro- mucilage. As far as gum-arabic is concerned, gen left free, and then you calculate the it is distinguished by not being very prone to decomposition, if we except its becomweights, and make out the proportions. I have told you, that this process is one ing slightly sour. and bv its being insoluble of difficult performance, and the chemist, in alcohol. So, that, ifyou add a solution who is in the habit of doing it, will tell of gum to alcohol, the gum is precipitated;-, you, that he is frequently obliged to repeat or if you put gum into alcohol, it is not the experiment before he can agree as to taken up by it. It is soluble in water the results ; and I mention this to caution therefore, and insoluble in alcohol, and tlm you against receiving these results as the is a property by which gum is recognised. real truth ; you must merely consider them Now, another character of gum is, thatit as approximations to it, and wait until it has is soluble in alkalies, and solutions of the reached a more ripened state. As far as earths, and that it is precipitated regards carbon, hydrogen, and oxygen, the by the acids and by the sabacetate of lead; experiments made serve to convince us, that and it is found that the precipitate which down is a compound of gum and the they form the elements of substances of these bodies : of that there cannot be much oxide of lead, called by Berzelius a gummate and he has ingeniously arailed himdoubt. Here is a table of vegetable substances, self of the opportunity afforded by this in which hydrogen and oxygen are mixed compound, and some others, to determine in the same proportions as in water, neither the equivalent number of these reaeuble being in excess, as in sugar, starch, gum, bodies. [The formation of this compound You ob. &:c. ; then we come to a class of vegetable was then shown by bodies in which oxygen predominates, as serve that instantly there is a coagulum
gen
vegetable
i
alkaline falls of lead,
experiment.]
355 which is insoluble, composed of bases are more white than the apices of the gum and oxide of lead ; and, in some cases, cones, they are generally.cut off ; and when this is worth remembering, especially by it is an object to obtain very fine sugar, the medical men. who are not unfrequently or- loaves, as they are called, are made of a ’ dering the solution of the sugar of lead in smaller size. so and their and on, mucilaginous solutions, If you wish to crystallise sugar, you disobject must be frustrated by the subacetate solve it, and pass strings the solulosing its activity. When gum is exposed tion, and the crystals are then deposited upon and the to beat it undergoes decomposition, strings in the shape of hexagonal crystals, products are formed which I adverted to the irregularly terminated. The loaves are afother morning, especially vinegar, and I terwards dried, and so far the process is mention this particularly, because at one complete. Now by the application of the time this vinegar was considered to be of a required heat to the sugar in this process, a peculiar kind, and was called pyro-mucous great loss generally ensues; the blood caracid, but it is the same as other vinegars. ries away a large quantity, and so forth, and Sugar exists in great quantities in the sap therefore it has been proposed to boil the suof vegetables, in ripe fruits, and in the roots gar in vacuo, by which it is made to boil at a of certain plants ; but the great source of temperature of 80° or 100°, instead of at the sugar, as far as we are concerned, is the ordinary temperature under the atmospheric sugar-cane; and the manner of obtaining pressure ; in this way the syrup may be the sugar is sufficiently simple. and do not run the .risk It is generally imported into this country evaporated, the you nor suffer that loss of burning sugar, in the raw state, called mllsco1)(1(lo sugar, and as the other mode. The matthe refiner chooses that which has a bright terbyhas been thrown down colouring albumen, by hard grain, and is not guided by the colour. I and the treacle obtained by passing the If he finds it sandy and hard, he prefers it the sugar in the cones, and to the soft and finer sugar, and the East syrup through so on. These are some of the principal India sugar is, on that account, unfit for reof the process, which are interesting fining; that brought from the West Indies steps to the chemist. being the best. The mode of refining sugar You may obtain sugar in considerable is briefly this, and if you consider sugar as II crystallised and uncrystallised compound, quantity from the maple, and from the juice vou have its two extremes of purity and of the beet-root. Figs, grapes, and honey, you have it crystallised in sugar- contain a peculiar species of sugar, which candy, and uncrystallised in molasses, and is easily distinguished by the taste. As to these two exist combined in the common white candy, or pure sugar, it is soluble in raw sugars, and the process of refining is its own weight of water at 60°, and the soto separate them. This process is generally lution, if concentrated, is not very prone to commenced by filling large boilers with a change, but if diluted, it begins to ferment, mixture of lime-water and sugar, with cer- and to form vinegar and acetic products. tain quantities of bullock’s blood ; this was The acids act upon sugar in a peculiar the old mode of proceeding, but lately a way ; nitric acid converts it into oxalic patent has been taken out for an improve- acid, and sulphuric acid causes it to throw ment upon it. The object of the process is down charcoal, and to produce water and two-fold, the blood, coagulating by heat, I acetous acid. Lime-water dissolves sugar, forms a scum upon the surface, and carries and so do the alkalies, gradually converting away a great quantity of impurity from the it into a gummy matter. Oxide of lead and sagar of various kinds, which are then sugar combine to form what Berzelius has skimmed off ; the use of the lime-water called a sacchante of lead. Manna is another iato render the treacle very soluble, so that kind of sugar, furnished by a species of it may not afterwards interfere with the ash in Italy ; it yields oxalic and saccholacseparation of the crystallised sugar. This tic acids, especially when treated with nimixture is boiled until, by taking up a drop tric acid. Now sugar is soluble in cold and of it between the finger and thumb, it can warm water, and is separable by the same be drawn into threads, and it is then taken manner as gum ; so that if you have gum out into vessels, where it is stirred about and sugar mixed, you may separate them with wooden instruments until it becomes, by alkalies ; the separation is usually imlike the original raw sugar, in a granulated perfect, but the sweet taste of sugar ena. state. This sugar is afterwards put into bles you to recognise it. conical moulds, made partly of clay, and Starch exists in a great number of vegetawater is poured upon their bases, and as the water trickles through the sugar it carries bles, and contributes very much to their with it the uncrystallised part, and the nutritive qualities. Sugar and gum are very rones become gradually whiter and whiter nutritious substances, but starch more so as the treacle trickles through. As the than either.
produced,
through
impurity ;
356 To obtain starch from the substances conat this table, in which starch, gm, and the process is to wash them in sugar, as far as their ultimate elements are are compared, you will see hot cold water, and as it is insoluble in cold water, the starch is washed out of the ve- little they differ from each other; and you getables, and afterwards dried. The com- can imagine that a slight change in their mon wheat starch is obtained by first bruis- ultimate elements, will convert the one sub. ing the wheat in a mill, and then steeping stance into another. If you take March, it in water; after a time the water begias and add a little sulphuric acid to it, it is to get sour; it is then drawn off, and the converted into sugar ; gum, when heated ap starch is collected and dried. In drying, to a certain point, under peculiar circumit splits into small columnar masses, and it stances, seems to pass into a substance is usually coloured by the manufacturer somewhat resembling sugar; and starch, with a little indigo. You may procure when heated up to about 700°, very readily starch from various substances ; arrowroot passes into a state of gum. Ilere are some is the starch of the maranta arundinacea; specimens of starch which have been thus and the pith of certain palms abounds in roasted; it is converted into a substance starch ; salop, tapioca, and cassava, are called British gitm ;it does not precipitate obtained from that source. Potato starch is iodine ; it is soluble in water;and when also frequently used for culinary purposes. gum could not be obtained, roasted starch Now although I represent these to you as was substituted by the calico printers, who the main varieties of starch, there are slight use a great deal of it. chemical differences between them ; but I Gluten.-When you separate starch from think it would be an useless sort of refinea substance remains called gluten, flour, ment in chemistry, to endeavour to desig- which resembles in some particulars animal nate these starches by different names. The matter; and here you have a vegetable body, starch being separated from the gummy and you perceive, well calculated for huron other parts of the vegetable, and dried, is food ; and wheaten bread, you know, is wel fit for use, and its chemical characters are fitted for affording nutriment, children living peculiar and well defined. It is insoluble principally upon it. Gluten exists in the in cold water, as its mode of preparation greatest abundance in wheat flour, and the teaches ; it is soluble in warm water at a tem- wheat of the south of Europe contains a peratur-s of about 160°, and forms asolution, larger quantity of gluten than that grown in which is slightly viscid. If you dissolve it in the north. It also exists in barley and oat. a smaller quantity of water, especially at a meal, and, being mixed with starch, gives a higher temperature, you get a kind of jelly, great tenacity to bread made with these which afterwards is not very soluble in materials. When yeast is added to a mixwater; on the contrary, you may diffuse it I, ture of wheaten flour and water at a certhrough water, and it will not dissolve with- tain temperature, the carbonic acid of the out great difficulty. This circumstance is yeast is diffused throughout the mass of well known to the brewer; if he lets the dough, and is detained there by the gluten, water in upon the malt too hot, it forms a set, and thus we get a light bread. Wheat grown and spoils his mash. Starch is also insolu- in this contains from 18 to 24 per country ble in alcohol. When, therefore, you have cent. of and, as I before said, it may gluten, a vegetable substance containing starch, be obtained from wheat flour by washing out gum, and sugar, you separate the gum by the starch. It is of a grey colour, and, eold water, the sugar by alkalies, and the dried, it becomes brittle, and of a starch by hot water ; but suppose you have colour. It is nearly insoluble in darker a vegetable substance before you, and you water, and when submitted to a destruccannot tell whether it contains starch or tive distillation, it gives out ammonia, thus not, you then make an infusion of it in wa- resembling animal products in containing ter at i 60°, and then drop into it a little solution of iodine. Now iodine has the sin- nitrogen. gular property of combining with starch, ’ In the next Lecture we shall proceed to and of producing a blue compound ; and it is curious that no other vegetable matter at the consideration of the remainingb proxiall effects iodine. [This was shown by ex- mate parts of vegetables, extractive mattannin, and so on, and give you a short periment. This compound has been called ter, an iodide of starch. There are other charac- account of the process called tanning. teristics of starch, but it is not necessary to point them out, as these, I have mentioned, will generally suffice.
taining it,
) look
concerned,-
when
Starch is converted into sugar by a variety operations, and in malting you see this ,-oin- 0 on to a considerable extent. If you
of
,
[1827-8.
21.
impurities or foreign bodies LECTURES ON
VEGETABLE
CHEMISTRY, BY
at
the
Royal
Institution Britain.
of
Great
LECTURE XLV.
Gum, Sugar, Starch, and
Gluten. the last lecture, that the ultimate elements of vegetable substances were few. Now, when a vegetable substance consists of carbon, oxygen, and hydrogen, its decomposition by heat presents us with a variety of substances resulting from their union ; but if it should also happen to contain nitrogen, you will then have ammonia formed, and a of other products resulting from the variety union of nitrogen with the other substances I MENTIONED,
Gentlemen, in
named. In proceeding to the analysis of any vegetable body, the chemist makes some experiments to determine its general nature ; and we will suppose that it contains no nitrogen, but that, like most vegetable substances, it is composed of carbon, oxygen, and hydrogen ; his object is to determine the relative proportions in which these elements are combined. Now, if we take sugar, starch, gum-resin, camphor, tan, and 1 great variety of other vegetable products, we find that they are all composed essentialiy of carbon, oxygen, and hydrogen, and you wilt naturally infer that the determination of their relative proportions in their bodies, will throw an interesting and considerable light upon the nature of the compounds generally, and serve to show how far they approximate to each other. 1 shall I not t8..e up your time by detailing all that has been done upon this subject, but describe to you the mode generally adopted for the examination of vegetable subManeee.
adhering to
when heated alone, but which, when heated in contact with vegetable substances, imparts oxygen to their elements, in proportion as they are decomposed. So that if you take a tube of this kind, and introduce a quantity of peroxide of copper into it, with a given weight of sugar for instance, and apply heat at the further end of the tube, and gradually carry the heat down to the other extremity, a decomposition is effected; and it follows, that whatever the product may be, that is produced by the action of heat upon the copper, it will be found at the other end of the tube, either in the gaseous or liquid form. Here you see the analysis going on ; you see that as the tube becomes thoroughly heated, a gas is extricated, which will afterwards be found to be carbonic acid gas ; and to be certain of this, we may absorb it by a small quantity of potash, or soda, or ammonia, and its volume leads us soon to determine the amount of carbon present, and hence the index to the quan. tity of carbon which the vegetable matter contained. But the vegetable matter also contained hydrogen, and that, with the oxygen given out from the copper, will form water, which is to be absorbed by a portion of asbestos placed in the tube, and the amount of water absorbed may be learnt by its increase in weight, after the experiment. We know how much hydrogen is contained in a given weight of water, because nine
water contain one of hydrogen. Then the quantity of carbon being determined, and the quantity of hydrogen being determined, there will be a certain loss, which is to be ascribed to the oxygen. Now, although this shows you the form of the experiment, you must remember that it is one which requires great caution to obtain accurate results, and it is necessary to repeat the experiment, four or five times, on sub- the same substance, before you can depend
If you wish to analyse a vegetable stance, you must examine it to see that
No. 251.
are
the substance may be hygrometric ; and if that is the case, you must avoid the contact of air ; you must introduce it into a brass tube in a given state of dryness, and then mix it thoroughly with a certain quantity of or
peroxide of copper,which gives out no oxygen
PROFESSOR BRANDE.
Delivered
it,
no
parts of
with
accuracy tinn-n
the results
354
Supposing that you had operated onin the vegetable acids; and then thet- are fifteen grains of sugar, and that you had other vegetable substances, in which hydroobtained a certain measure of carbonic acid predominates, as in the resins, war, gas. equivalent to six grains by weight ofoil, and combustible bodies. The examinacarbon, and that you had also obtained nine tion of the proximate principles of vegetagrains of water, which would contain one bles would afford ample scope for a fi.J grain of hydrogen, you would say that there course of lectures, so that we can only Ieis contained in fifteen grains of sugar six state it very generally, not go into the decarbon, one hydrogen, and a certain loss tails of abstract chemistry. The separation of the proximate principks equivalent to eight, which you ascribe to oxygen. Now, suppose we take sugar, of vegetables is generally effected bv cer. starch, gum, and wood, they contain no tain solvents ; if we take a piece of bark, for excess of oxygen, they are compounds ofexample, we first steep it in cold, then in carbon and water; and when I say they hot water, and then in alcohol. If you then contain no excess of oxygen, I use the term distil it with water, it may afford an essen. to imply that oxygen and hydrogen are in tial oil, or the oil which it contains may be the same relative proportions as in water. expressed ; and in this way, by the action Now you will easily remark, how closely of solvents, we separate sub. these four substances correspond in their stances into a certain number of proximate ultimate composition, and therefore you will principles. It does not much matter how not be surprised why it often happens, that we arrive at these ; but I shall endeavour by artificial means, we are capable of con- to throw them into a class founded upon this verting one of these substances into the composition, according as they may contaitt other, by changing the relative proportions hydrogen and oxygen, in the same proporof their element; that we can, for example, I tion as water, .according as they may canconvert starch and gum into sugar, sugar tain either in excess; and lastly, such as into gum, and gum into starch ; and these may contain nitrogen. Gum is a specimen of the combination of are processes daily going on in nature, in the growth and ripening of fruits, and so elements of the first class ; it exudes from on, but more especially the passing of the acacia and other trees, and you find that starch and gum into sugar. Now suppose there are great varieties of gum which differ that any nitrogen had been separated from a little in their chemical properties. Some the substances you were examining, that chemists have divided the gums into two would throw great difficulty in the way of kinds; the first into the gums, properly so the analysis, but to these nitrous compounds called ; the second, such as are comparawe shall advert afterwards. If nitrogen be tively insoluble in water, as tragacanth and present, and if the operation of analysis be cherry-tree gum, and to these they have successfully conducted, it might also be given the name cerasin. collected in the tube ; but as nitrogen is The general character of gum is that it not soluble in potash, you throw a little is soluble in cold and warm water, forming potash into the tube, by which you have a viscid solution, known by the name of carbonate of potash formed, and the nitro- mucilage. As far as gum-arabic is concerned, gen left free, and then you calculate the it is distinguished by not being very prone to decomposition, if we except its becomweights, and make out the proportions. I have told you, that this process is one ing slightly sour. and bv its being insoluble of difficult performance, and the chemist, in alcohol. So, that, ifyou add a solution who is in the habit of doing it, will tell of gum to alcohol, the gum is precipitated;-, you, that he is frequently obliged to repeat or if you put gum into alcohol, it is not the experiment before he can agree as to taken up by it. It is soluble in water the results ; and I mention this to caution therefore, and insoluble in alcohol, and tlm you against receiving these results as the is a property by which gum is recognised. real truth ; you must merely consider them Now, another character of gum is, thatit as approximations to it, and wait until it has is soluble in alkalies, and solutions of the reached a more ripened state. As far as earths, and that it is precipitated regards carbon, hydrogen, and oxygen, the by the acids and by the sabacetate of lead; experiments made serve to convince us, that and it is found that the precipitate which down is a compound of gum and the they form the elements of substances of these bodies : of that there cannot be much oxide of lead, called by Berzelius a gummate and he has ingeniously arailed himdoubt. Here is a table of vegetable substances, self of the opportunity afforded by this in which hydrogen and oxygen are mixed compound, and some others, to determine in the same proportions as in water, neither the equivalent number of these reaeuble being in excess, as in sugar, starch, gum, bodies. [The formation of this compound You ob. &:c. ; then we come to a class of vegetable was then shown by bodies in which oxygen predominates, as serve that instantly there is a coagulum
gen
vegetable
i
alkaline falls of lead,
experiment.]
355 which is insoluble, composed of bases are more white than the apices of the gum and oxide of lead ; and, in some cases, cones, they are generally.cut off ; and when this is worth remembering, especially by it is an object to obtain very fine sugar, the medical men. who are not unfrequently or- loaves, as they are called, are made of a ’ dering the solution of the sugar of lead in smaller size. so and their and on, mucilaginous solutions, If you wish to crystallise sugar, you disobject must be frustrated by the subacetate solve it, and pass strings the solulosing its activity. When gum is exposed tion, and the crystals are then deposited upon and the to beat it undergoes decomposition, strings in the shape of hexagonal crystals, products are formed which I adverted to the irregularly terminated. The loaves are afother morning, especially vinegar, and I terwards dried, and so far the process is mention this particularly, because at one complete. Now by the application of the time this vinegar was considered to be of a required heat to the sugar in this process, a peculiar kind, and was called pyro-mucous great loss generally ensues; the blood caracid, but it is the same as other vinegars. ries away a large quantity, and so forth, and Sugar exists in great quantities in the sap therefore it has been proposed to boil the suof vegetables, in ripe fruits, and in the roots gar in vacuo, by which it is made to boil at a of certain plants ; but the great source of temperature of 80° or 100°, instead of at the sugar, as far as we are concerned, is the ordinary temperature under the atmospheric sugar-cane; and the manner of obtaining pressure ; in this way the syrup may be the sugar is sufficiently simple. and do not run the .risk It is generally imported into this country evaporated, the you nor suffer that loss of burning sugar, in the raw state, called mllsco1)(1(lo sugar, and as the other mode. The matthe refiner chooses that which has a bright terbyhas been thrown down colouring albumen, by hard grain, and is not guided by the colour. I and the treacle obtained by passing the If he finds it sandy and hard, he prefers it the sugar in the cones, and to the soft and finer sugar, and the East syrup through so on. These are some of the principal India sugar is, on that account, unfit for reof the process, which are interesting fining; that brought from the West Indies steps to the chemist. being the best. The mode of refining sugar You may obtain sugar in considerable is briefly this, and if you consider sugar as II crystallised and uncrystallised compound, quantity from the maple, and from the juice vou have its two extremes of purity and of the beet-root. Figs, grapes, and honey, you have it crystallised in sugar- contain a peculiar species of sugar, which candy, and uncrystallised in molasses, and is easily distinguished by the taste. As to these two exist combined in the common white candy, or pure sugar, it is soluble in raw sugars, and the process of refining is its own weight of water at 60°, and the soto separate them. This process is generally lution, if concentrated, is not very prone to commenced by filling large boilers with a change, but if diluted, it begins to ferment, mixture of lime-water and sugar, with cer- and to form vinegar and acetic products. tain quantities of bullock’s blood ; this was The acids act upon sugar in a peculiar the old mode of proceeding, but lately a way ; nitric acid converts it into oxalic patent has been taken out for an improve- acid, and sulphuric acid causes it to throw ment upon it. The object of the process is down charcoal, and to produce water and two-fold, the blood, coagulating by heat, I acetous acid. Lime-water dissolves sugar, forms a scum upon the surface, and carries and so do the alkalies, gradually converting away a great quantity of impurity from the it into a gummy matter. Oxide of lead and sagar of various kinds, which are then sugar combine to form what Berzelius has skimmed off ; the use of the lime-water called a sacchante of lead. Manna is another iato render the treacle very soluble, so that kind of sugar, furnished by a species of it may not afterwards interfere with the ash in Italy ; it yields oxalic and saccholacseparation of the crystallised sugar. This tic acids, especially when treated with nimixture is boiled until, by taking up a drop tric acid. Now sugar is soluble in cold and of it between the finger and thumb, it can warm water, and is separable by the same be drawn into threads, and it is then taken manner as gum ; so that if you have gum out into vessels, where it is stirred about and sugar mixed, you may separate them with wooden instruments until it becomes, by alkalies ; the separation is usually imlike the original raw sugar, in a granulated perfect, but the sweet taste of sugar ena. state. This sugar is afterwards put into bles you to recognise it. conical moulds, made partly of clay, and Starch exists in a great number of vegetawater is poured upon their bases, and as the water trickles through the sugar it carries bles, and contributes very much to their with it the uncrystallised part, and the nutritive qualities. Sugar and gum are very rones become gradually whiter and whiter nutritious substances, but starch more so as the treacle trickles through. As the than either.
produced,
through
impurity ;
356 To obtain starch from the substances conat this table, in which starch, gm, and the process is to wash them in sugar, as far as their ultimate elements are are compared, you will see hot cold water, and as it is insoluble in cold water, the starch is washed out of the ve- little they differ from each other; and you getables, and afterwards dried. The com- can imagine that a slight change in their mon wheat starch is obtained by first bruis- ultimate elements, will convert the one sub. ing the wheat in a mill, and then steeping stance into another. If you take March, it in water; after a time the water begias and add a little sulphuric acid to it, it is to get sour; it is then drawn off, and the converted into sugar ; gum, when heated ap starch is collected and dried. In drying, to a certain point, under peculiar circumit splits into small columnar masses, and it stances, seems to pass into a substance is usually coloured by the manufacturer somewhat resembling sugar; and starch, with a little indigo. You may procure when heated up to about 700°, very readily starch from various substances ; arrowroot passes into a state of gum. Ilere are some is the starch of the maranta arundinacea; specimens of starch which have been thus and the pith of certain palms abounds in roasted; it is converted into a substance starch ; salop, tapioca, and cassava, are called British gitm ;it does not precipitate obtained from that source. Potato starch is iodine ; it is soluble in water;and when also frequently used for culinary purposes. gum could not be obtained, roasted starch Now although I represent these to you as was substituted by the calico printers, who the main varieties of starch, there are slight use a great deal of it. chemical differences between them ; but I Gluten.-When you separate starch from think it would be an useless sort of refinea substance remains called gluten, flour, ment in chemistry, to endeavour to desig- which resembles in some particulars animal nate these starches by different names. The matter; and here you have a vegetable body, starch being separated from the gummy and you perceive, well calculated for huron other parts of the vegetable, and dried, is food ; and wheaten bread, you know, is wel fit for use, and its chemical characters are fitted for affording nutriment, children living peculiar and well defined. It is insoluble principally upon it. Gluten exists in the in cold water, as its mode of preparation greatest abundance in wheat flour, and the teaches ; it is soluble in warm water at a tem- wheat of the south of Europe contains a peratur-s of about 160°, and forms asolution, larger quantity of gluten than that grown in which is slightly viscid. If you dissolve it in the north. It also exists in barley and oat. a smaller quantity of water, especially at a meal, and, being mixed with starch, gives a higher temperature, you get a kind of jelly, great tenacity to bread made with these which afterwards is not very soluble in materials. When yeast is added to a mixwater; on the contrary, you may diffuse it I, ture of wheaten flour and water at a certhrough water, and it will not dissolve with- tain temperature, the carbonic acid of the out great difficulty. This circumstance is yeast is diffused throughout the mass of well known to the brewer; if he lets the dough, and is detained there by the gluten, water in upon the malt too hot, it forms a set, and thus we get a light bread. Wheat grown and spoils his mash. Starch is also insolu- in this contains from 18 to 24 per country ble in alcohol. When, therefore, you have cent. of and, as I before said, it may gluten, a vegetable substance containing starch, be obtained from wheat flour by washing out gum, and sugar, you separate the gum by the starch. It is of a grey colour, and, eold water, the sugar by alkalies, and the dried, it becomes brittle, and of a starch by hot water ; but suppose you have colour. It is nearly insoluble in darker a vegetable substance before you, and you water, and when submitted to a destruccannot tell whether it contains starch or tive distillation, it gives out ammonia, thus not, you then make an infusion of it in wa- resembling animal products in containing ter at i 60°, and then drop into it a little solution of iodine. Now iodine has the sin- nitrogen. gular property of combining with starch, ’ In the next Lecture we shall proceed to and of producing a blue compound ; and it is curious that no other vegetable matter at the consideration of the remainingb proxiall effects iodine. [This was shown by ex- mate parts of vegetables, extractive mattannin, and so on, and give you a short periment. This compound has been called ter, an iodide of starch. There are other charac- account of the process called tanning. teristics of starch, but it is not necessary to point them out, as these, I have mentioned, will generally suffice.
taining it,
) look
concerned,-
when
Starch is converted into sugar by a variety operations, and in malting you see this ,-oin- 0 on to a considerable extent. If you
of
,