VOL. II.] THE LANCET. LONDON, SATURDAY, JUNE 21. [1827-8. 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, BY PROFESSOR BRANDE. 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 peroxide of copper,which gives out no oxygen when heated alone, but which, when heated Delivered at the Royal Institution of Great in contact with vegetable substances, im Britain. LECTURE XLV. parts 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 Gum, Sugar, Starch, and Gluten. 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 should also happen to contain nitrogen, you will then have ammonia formed, and a variety of other products resulting from the union of nitrogen with the other substances named. 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 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 are all 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 sub-the same substance, before you can depend stance, you must examine it to see that no with accuracy upon the results. stances. No. 251. 2 A in the vegetable acids; and then there are other vegetable substances, in which hydrogen predominates, as in the resins, wax, oil, and combustible bodies. The examination of the proximate principles of vegetables would afford ample scope for a full course of lectures, so that we can only here state it very generally, not go into the details of abstract chemistry. The separation of the proximate principles of vegetables is generally effected by cer tain solvents; if we take a piece of bark, før example, we first steep it in cold, then in hot water, and then in alcohol. If you then distil it with water, it may afford an essential oil, or the oil which it contains may be expressed; and in this way, by the action of solvents, we separate vegetable substances into a certain number of proximate principles. It does not much matter how we arrive at these; but I shall endeavour to throw them into a class founded upon this composition, according as they may contain hydrogen and oxygen, in the same proportion as water, according as they may contain either in excess; and lastly, such as may contain nitrogen. Supposing that you had operated on fifteen grains of sugar, and that you had obtained a certain measure of carbonic acid gas, equivalent to six grains by weight of carbon, and that you had also obtained nine grains of water, which would contain one grain of hydrogen, you would say that there is contained in fifteen grains of sugar six carbon, one hydrogen, and a certain loss equivalent to eight, which you ascribe to oxygen. Now, suppose we take sugar, starch, gum, and wood, they contain no excess of oxygen, they are compounds of carbon and water; and when I say they contain no excess of oxygen, I use the term to imply that oxygen and hydrogen are in the same relative proportions as in water. Now you will easily remark, how closely these four substances correspond in their ultimate composition, and therefore you will not be surprised why it often happens, that by artificial means, we are capable of converting one of these substances into the other, by changing the relative proportions of their element; that we can, for example, convert starch and gum into sugar, sugar into gum, and gum into starch; and these 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 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 weights, and make out the proportions. to decomposition, if we except its becomI 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 ;; 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 himHere 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 s in which oxygen predominates, as serve that instantly there is a coagulum produced, 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 losing its activity. When gum is exposed to heat it undergoes decomposition, and the products are formed which I adverted to the other morning, especially vinegar, and I mention this particularly, because at one time this vinegar was considered to be of a peculiar kind, and was called pyro-mucous acid, but it is the same as other vinegars. Sugar exists in great quantities in the sap of vegetables, in ripe fruits, and in the roots of certain plants; but the great source of sugar, as far as we are concerned, is the sugar-cane; and the manner of obtaining the sugar is sufficiently simple. to the chemist. You may obtain sugar in considerable quantity from the maple, and from the juice of the beet-root. Figs, grapes, and honey, contain a peculiar species of sugar, which As to is easily distinguished by the taste. white candy, or pure sugar, it is soluble in its own weight of water at 60°, and the solution, if concentrated, is not very prone to change, but if diluted, it begins to ferment, and to form vinegar and acetic products. solve it, and pass strings through the soluIf you wish to crystallise sugar, you distion, and the crystals are then deposited upon strings in the shape of hexagonal crystals, irregularly terminated. The loaves are afterwards dried, and so far the process is required heat to the sugar in this process, a complete. Now by the application of the great loss generally ensues; the blood cartherefore it has been proposed to boil the suries away a large quantity, and so forth, and gar in vacuo, by which it is made to boil at á temperature of 80° or 100°, instead of at the pressure; in this way the syrup may be ordinary temperature under the atmospheric 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 the refiner chooses that which has a bright ter has been thrown down by albumen, as by the other mode. The colouring mathard 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 sugar, and the East Syrup through the sugar in the cones, and so on. These are some of the principal India sugar is, on that account, unfit for refining that brought from the West Indies steps of the process, which are interesting being the best. The mode of refining sugar is briefly this, and if you consider sugar as a crystallised and uncrystallised compound, you have its two extremes of purity and impurity; you have it crystallised in sugar candy, and uncrystallised in molasses, and these two exist combined in the common raw sugars, and the process of refining is to separate them. This process is generally commenced by filling large boilers with a mixture of lime-water and sugar, with certain 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 the it into a gummy matter. Oxide of lead and sugar of various 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 saccholacseparation of the crystallised sugar. This mixture is boiled until, by taking up a drop of it between the finger and thumb, it can be drawn into threads, and it is then taken out into vessels, where it is stirred about with wooden instruments until it becomes, like the original raw sugar, in a granulated state. This sugar is afterwards put into 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. tic acids, especially when treated with nitric acid. Now sugar is soluble in cold and warm water, and is separable by the same manner as gum; so that if you have gum and sugar mixed, you may separate them by alkalies; the separation is usually imperfect, but the sweet taste of sugar enables you to recognise it. 2 A 2 look at this table, in which starch, gum, and sugar, as far as their ultimate elements are concerned, are compared, you will see how little they differ from each other; and you can imagine that a slight change in their ultimate elements, will convert the one substance into another. If you take starch, and add a little sulphuric acid to it, it is converted into sugar; gum, when heated up to a certain point, under peculiar circumstances, seems to pass into a substance To obtain starch from the substances containing it, the process is to wash them in cold water, and as it is insoluble in cold water, the starch is washed out of the vegetables, and afterwards dried. The common wheat starch is obtained by first bruising the wheat in a mill, and then steeping it in water; after a time the water begins to get sour; it is then drawn off, and the starch is collected and dried. In drying, it splits into small columnar masses, and it is usually coloured by the manufacturer somewhat resembling sugar; and starch, with a little indigo. You may procure starch from various substances; arrowroot is the starch of the maranta arundinacea; and the pith of certain palms abounds in starch; salop, tapioca, and cassava, are obtained from that source. Potato starch is also frequently used for culinary purposes. when heated up to about 700°, very readily passes into a state of gum. Here are some specimens of starch which have been thus roasted; it is converted into a substance called British gum; it does not precipitate iodine; it is soluble in water; and when gum could not be obtained, roasted starch was substituted by the calico printers, who use a great deal of it. Now although I represent these to you as the main varieties of starch, there are slight chemical differences between them; but I think it would be an useless sort of refine flour, a substance remains called gluten, Gluten. When you separate starch from 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 other parts of the vegetable, and dried, is food; and wheaten bread, you know, is well you perceive, well calculated for human 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 perature of about 160°, and forms asolution, which is slightly viscid. If you dissolve it in a smaller quantity of water, especially at a higher temperature, you get a kind of jelly, which afterwards is not very soluble in water; on the contrary, you may diffuse it through water, and it will not dissolve without great difficulty. This circumstance is well known to the brewer; if he lets the water in upon the malt too hot, it forms a set, and spoils his mash. Starch is also insoluble in alcohol. When, therefore, you have a vegetable substance containing starch, gum, and sugar, you separate the gum by cold water, the sugar by alkalies, and the starch by hot water; but suppose you have a vegetable substance before you, and you cannot tell whether it contains starch or not, you then make an infusion of it in wa ter at 160°, and then drop into it a little larger quantity of gluten than that grown in the north. It also exists in barley and oatmeal, and, being mixed with starch, gives a great tenacity to bread made with these materials. When yeast is added to a mixture of wheaten flour and water at a certain temperature, the carbonic acid of the yeast is diffused throughout the mass of dough, and is detained there by the gluten, and thus we get a light bread. Wheat grown in this country contains from 18 to 24 per cent. of gluten, and, as I before said, it may be obtained from wheat flour by washing out the starch. It is of a grey colour, and, when dried, it becomes brittle, and of a darker colour. It is nearly insoluble in water, and when submitted to a destructive distillation, it gives out ammonia, thus resembling animal products in containing nitrogen. solution of iodine. Now iodine has the singular property of combining with starch, and of producing a blue compound; and it In the next Lecture we shall proceed to is curious that no other vegetable matter at the consideration of the remaining proxiall effects iodine. [This was shown by ex-mate parts of vegetables, extractive matperiment.] This compound has been called ter, tannin, and so on, and give you a short 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. Starch is converted into sugar by a variety of operations, and in malting you see this going on to a considerable extent. If you FOREIGN DEPARTMENT. function of the membrana tympani, is to ON THE FUNCTIONS OF THE DIFFERENT Antenrieth and Kerner, according to which RY DR. CH. L. ESSER, THIS paper is the extract of a work to which the prize was adjudicated in 1825, by the Faculty of Medicine of the University of Bonn. The cartilage of the external ear appears to contribute very little to render the sounds more distinct, but it serves to increase their force; not only by reflecting a part of the vibrations into the meatus auditorius, especially those which fall into the concha, but also by means of the vibrations which the undulations produce, and which are transmitted to the membrane of the tympanum. It is not correct, therefore, to suppose, (as M. Itard does,) that the external ear is of no service to man in hearing. the membrane of the tympanum is considered as an assemblage of cords differently stretched, in proportion as the membrane is round or oblong, is shown to be without foundation. The distinction of the different sounds does not rest on a mechanical ar rangement of the ear, but on a psychologi cal cause. The Eustachian tube is the principal aux. iliary of the membrane of the tympanum, and performs four different functions; first, it allows the air contained in the cavity of the tympanum to be placed in equilibrio with the external air. If this equilibrium be disturbed, derangements in the functions of hearing take place, such as the tinkling and stopping up of the ear. If the quantity of air contained in the cavity be increased by deep expirations, great pressure The bones of the head do not contribute less is made on the membrana tympani, and on than the external ear to the propagation of the other parts of the cavity, especially the sounds, which does not take place solely fenestra rotunda; this pressure is caused through the medium of nerves, as some by the stopping up of the ear, which dimiauthors have supposed, (Treviranus, Swan, nishes in proportion as the equilibrium of &c.) for, in such a case, a watch applied on the air is re-established by means of the the cheek, ought to communicate sounds Eustachian tube. If the air in the cavity as clear through the medium of the facial of the tympanum be rarefied, and the Eusnerve, as if it were applied on the zygo-tachian tube shut from the effect of spasm, matic arch; but this is not the case the external air presses on the membrana The occipital bone is more adapted to the tympani, penetrates through its pores, and propagation of sounds, than the bones at the the tinkling is produced through this pas anterior part of the head; this is explained sage. The two phenomena disappear as by its connexions with the labyrinth, and soon as the equilibrium of the air is re-esta by the vicinity of the mastoid cells. The blished in the cavity of the tympanum, which use of these cells is not to impede the echo is effected by pushing the air towards the in the interior of the ear, as M. Treviranus Eustachian tube, the mouth and nose being supposes; this function belonging to the firmly closed; or by introducing the little Eustachian tube. finger very deeply into the meatus auditorius externus, and by gradually drawing it back, and pressing from below to above against the wall of this canal. Thus a vacuum is formed, the membrane of the tympanum is inclined towards the meatus auditorius externus, and the Eustachian tube gives passage to air from the back of the mouth. It is evident that this explana tion cannot apply to all the phenomena which occur in the organ of hearing. Cerebral congestions, or derangements of ner vous action, are the causes of more chronic sensations. In several animals, the bones which surround the labyrinth, and those of the head in general, present arrangements very favourable for the propagation of sounds, in this way compensating for the absence of the external ear. The meatus auditorius externus is evidently the part which contributes most to concentrate and transmit the sounds to the membrana tympani. The membrana tympani is put into vibration by the undulations which reach it; a point doubted by M. Itard, although all other authors are agreed on it; this is, however, not the only use of this membrane; for the vibrations of sound can arrive at the internal ear without the assistance of it, and even with greater force; another * Kastner's Archiv. fur die ges. Naturlehre; tom. xii. 1er. cah. 1827, p. 52. 2d. The second function of the Eustachian tube is to allow the air contained in the cavity to be put in vibration; which could not take place if it were closed. In deafness from the obliteration of the Eustachian tube, the perforation of the tympanum is a means of cure, by re-establishing the communication of the tympanum with |