Pagina-afbeeldingen
PDF
ePub

3.

2. Piletiers: Iragments. A. Magann de Londres puur Mars 1749. 5. tatalogue of lewerblications. 6. Uluccisal Magarive for May 1810,

ulaganini. 2. thin Atagarene 1.1.

[ocr errors]
[ocr errors]
[ocr errors][ocr errors][ocr errors]

THE LANCET.

VOL. II.)

LONDON, SATURDAY, JUNE 21.

(1827-8.

BY

impurities or foreign bodies are adhering to LECTURES ON VEGETABLE and if that is the case, you must avoid the

it, or the substance may be hygrometric ; CHEMISTRY,

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 PROFESSOR BRANDE. peroxide of copper, which gives out nooxygen

when heated alone, but which, when heated Delivered at the Royal Institution of Great in contact with vegetable substances, imBrituin.

parts oxygen to their elements, in proportion

as they are decomposed. So that if you LECTURE XLV.

take a tube of this kind, and introduce a Gum, Sugar, Starch, and Gluten.

quantity of peroxide of copper into it, with

a given weight of sugar for instance, and I MENTIONED, Gentlemen, in the last apply heat at the further end of the tube, lecture, that the ultimate elements of vege- and gradually carry the heat down to the table substances were few. Now, when a other extremity, a decomposition is effected; vegetable substance consists of carbon, and it follows, that whatever the product oxygen, and hydrogen, its decomposition may be, that is produced by the action of by heat presents us with a variety of sub- heat upon the copper, it will be found at the stances resulting from their union ; but if it other end of the tube, either in the gaseous should also happen to contain nitrogen, you or liquid form. Here you see the analysis will then have ammonia formed, and a going on; you see that as the tube becomes variety of other products resulting from the thoroughly heated, a gas is extricated, union of nitrogen with the other substances which will afterwards be found to be carLamed.

bonic acid gas ; and to be certain of this, we In proceeding to the analysis of any may absorb it by a small quantity of potash, vegetable body, the chemist makes some or soda, or ammonia, and its volume leads experiments to determine its general nature; us soon to determine the amount of carbon and we will suppose that it contains no present, and hence the index to the quannitrogen, but that, like most vegetable sub-tity of carbon which the vegetable matter stances, it is composed of carbon, oxygen, contained. But the vegetable matter also and hydrogen ; his object is to determine contained hydrogen, and that, with the the relative proportions in which these ele oxygen given out from the copper, will form ments are combined. Now, if we take water, which is to be absorbed by a portion sugar, starch, gum-resin, camphor, tan, and of asbestos placed in the tube, and the a great variety of other vegetable products, amount of water absorbed may be learnt by we find that they åre alt composed essen- its increase in weight, after the experiment. tially of carbon, oxygen, and hydrogen, and we know how much hydrogen is contained you will naturally infer that the determina in a given weight of water, because nine tion of their relative proportions in their parts of water contain one of hydrogen. bodies, will throw an interesting and con. Then the quantity of carbon being detersiderable light upon the nature of the com- mined, and the quantity of hydrogen being pounds generally, and serve to show how determined, there will be a certain loss, far they approximate to each other. I shall which is to be ascribed to the oxygen. not take up your time by detailing all that Now, although this shows you the form of has been done upon this subject, but de the experiment, you must remember that it scribe to you the mode generally adopted is one which requires great caution to obfor the examination of vegetable sub tain accurate results, and it is necessary to

repeat the experiment, four or five times, on If you wish to analyse a vegetable sab- the same substance, before you can depend stance, you must examine it to see that no with accuracy upon the results.

No. 251.

stances.

[ocr errors]

Supposing that you had operated on in the vegetable acids; and then there are fifteen grains of sugar, and that you had other vegetable substances, in which hydroobtained a certain measure of carbonic acid gen predominates, as in the resins, wax, gas, equivalent to six grains by weight of oil, and combustible bodies. The examinacarbon, and that you had also obtained nine tion of the proximate principles of vegeta. grains of water, which would contain one bles would afford ample scope for a full grain of hydrogen, you would say that there course of lectures, so that we can only here is 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. equivalent to eight, which you ascribe to The separation of the proximate principles oxygen. Now, suppose we take sugar, of vegetables is generally effected by cer» starch, gum, and wood, they contain no tain solvents; if we take a piece of bark, for excess of oxygen, they are compounds of example, we first steep it in cold, then in çarbon 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 vegetable subthese 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 contain other, by changing the relative proportions hydrogen and oxygen, in the same proporof their element; that we can, for example, tion as water, according as they may conconvert 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.. are processes daily going on in nature, in Gum is a specimen of the combination of 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 compara. we 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. weights, and make out the proportions. to decomposition, if we except its becom

I have told you, that this process is one ing slightly sour, and by its being insoluble of difficult performance, and the chemist, in alcohol. So, that, if you add a solution who is in the habit of doing it, will tell of gum to alcohol, the gum is precipitated ;s 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 this 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, that it 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 alkaline earths, and that it is precipitated regards carbon, hydrogen, and oxygen, the by the acids and by the subacetate of lead ; experiments made serve to convince us, that and it is found that the precipitate which they form the elements of substances of falls down is a compound of gum and the these bodies : 'of that there cannot be much oxide of lead, called by Berzelius a gummate doubt.

of lead, and he has ingeniously availed him. 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 vegetable being in excess, as in sugar, starch, gum, bodies. [The formation of this compound

then we come to a class of vegetable was then shown by experiment.] You ob. ies in which exygen predominates, as serve that instantly there is a coagulum.

[ocr errors][ocr errors][merged small]

SO on.

prodaced, 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.
mucilaginous solutions, and so on, and their
object must be frustrated by the subacetate solve it

, and pass strings through the solu

If you wish to crystallise sugar, you dislosing its activity. When gum is exposed tion, and the crystals are then deposited upon to heat it undergoes decomposition, and the 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 time this vinegar was considered to be of a

complete. Now by the application of the peculiar kind, and was called pyro-mucous great loss generally ensues; the blood car,

required heat to the sugar in this process, a acid, but it is the same as other vinegars. · Sugár exists in great quantities in the sap therefore it has been proposed to boil the su

ries away a large quantity, and so forth, and of vegetables, in ripe fruits, and in the roots of certain plants ; but the great source of gar in vacuo, by which it is made to boil at a

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. It is generally imported into this country of burning the sugar, nor suffer that loss

evaporated, and you do not run the risk in the raw state, called muscovado sugar, and as by the other mode. The colouring matthe refiner chooses that which has a bright ter has been thrown down by albumen, hard grain, and is not guided by the colour. and the treacle obtained by passing the If he finds it sandy and hard, he prefers it to the soft and finer sugár, and the East syrup through the sugar in the cones, and

These are some of the principal India sugar is, on that account, unfit for rehuing; that brought from the West Indies steps of the process, which are interesting

to the chemist. being the best. The mode of refining sugar is briefly this, and if you consider sugar as You may obtain sugar in considerable å crystallised and uncrystalliséd compound, quantity from the maple, and from the juice you have its two extremes of purity and of the beet-root. Figs, grapes, and honey, impurity; 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, misture 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, 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 tlie it into a gummy matter. Oside of lead and sugar of varivus kinds, which are then sugar combine to form what Berzelius has skimmed off; the use of the lime-water called a saccharite of lead. Manna is another is to 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 saccholac, separation of the crystallised sugar. This tic acids, especially when treated with ni, mixture 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 im. like the original raw sugar, in a granulated perfect, but the sweet taste of sugar enastate. This sugar is afterwards put into bles you to recognise it. conical moulds, made partly of clay, and water is poured upon their bases, and as the Starch exists in a great number of vegetawater 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 cones become gradually whiter and whiter nutritious substances, but starch more so as the treacle trickles through. As the than either.

2 A 2

[ocr errors]
« VorigeDoorgaan »