d. The residue, after digestion with ether, is treated with boiling water; the solution thus obtained may contain urea, sugar, sugar of milk, extractive and saline matters,—in short, every thing except the albumen, which is com pletely dried and then weighed. When no easein is present, the uric acid, if any, will ccompany the albumen, and may be separated rom it in the manner directed for its separation rom casein.
e. The filtered liquid is evaporated to dry ness and treated with a mixture of one part of anhydrous ether with-two of absolute alcohol, by which urea, muriate of ammonia, lactates, the alcoholic extractive matters so called, and a small part of the sugar, are dissolved. The remainder of the sugar, sugar of milk, aqueous extractive matters, urates, sulphates, chlorides, and phosphates remain behind, forming re sidue (1).
f. The alcoholic solution is evaporated to dryness and weighed : the solid matter divided into two portions ; one is dissolved in water acidulated with nitric acid, and treated with nitrate of silver, by which the chlorine is sepa rated as chloride of silver, and hence the mu riate of ammonia is determined ; from the"other portion we determine the quantity of urea by oxalic acid with the usual precautions. Having thus determined the weight of the muriate of ammonia and of the urea, we infer the defici ency to consist of a little sugar, lactates, and alcoholic extract.
g. The residue (1), which contains sugar, su gar of milk, watery extract, and salts, is boiled with proof-spirit as long, as any thing is dis solved ; the solution is evaporated to dryness, and if grape sug-ar be present, half • the residue must be dissolved in water and fermented with yeast to determine the proportion of this sub stance, and its weight is deducted from the weight of the residue left on evaporating the spirituous solution; the other half residue is incinerated, and the quantity of saline matter ascertained ; by deducting the weight of the sugar and salts we then obtain that of the sugar of milk, together with the alcoholic extract, from which we possess no exact means of se parating it. It, however, very rarely happens that in the same fluid we meet with grape sugar and sugar of milk ; the absence of sugar will obviously much simplify the method of proceeding'.
h. The portion undissolved by proof-spirit is dried and weighed ; it is incinerated and again weighed; the difference betvveen the two weighings gives the quantity of watery extract.
This will be the general plan of operations if it be required to determine the quantity of each individual ingredient. From the number of operations required, and the destructible na ture of the ingredients, the result, as already mentioned, is not rigidly accurate. Frequently, however, it is merely necessary to ascertain the proportion in which one substance only is found; the presence or absence of others being all that it is desired to know concerning them.
We proceed now to the special consideration of the different animal principles.
Fibrin.—Although the identity, in chemical composition, of fibrin, albumen, and casein has lately been strongly insisted on by Liebig and his pupils ; yet, as in their physical properties at least, and in the offices they perform in the body, they differ considerably, it is frequently of great importance to determine the relative proportion of each in the fluids and secretions. These three principles occur both in the coagu lated and uncoarrulated state. When coagulated,
the separation of fibrin and albumen cannot be effected by any means with which we are ac quainted, and, indeed, the first authorities differ when they attempt to decide which of the two they have to deal with, if they occur in the coagulated state. When uncoagulated, their separation and quantitative determination may be effected with considerable accuracy. Pure fibrin, when moist, is white and some what elastic, is insoluble in water, alcohol, or ether. It is readily taken up by stro»g acetic acid, and from this solution it is precipitated by ferrocyanide of potassium (prussiate of pot ash); fibrin also dissolves in solution of pot ash ; if, when thus dissolved, it be heated gently and the liquid neutralized by an acid, a white flocculent precipitate occurs, which redissolves in excess of acid, and the solution emits an odour of sulphuretted hydrogen. Strong nitric acid turns fibrin yellow, forming a yellow so lution with gradual evolution of gas ; in con centrated hydrochloric acid it slowly dissolves with a rich violet colour. These are properties it possesses in common with albumen and ca sein ; but it is distinguished from them and from all other animal matters by its sponta neous coagulation when removed from the living body. This furnishes us with a certain test of its presence when in the liquid form, and enables us to separate it in good degree from other bodies; within its pores, however, is ob stinately retained a quantity ot the fluid from which it has just separated itself, together with most of the globules and particles suspended in the secretion. It becomes necessary, there fore, to wash out these ingredients, an opera tion rendered possible by the insolubility of the fibrin in cold water. The coag,ulum from a known quantity of fluid is cut into very thin shreds by a sharp knife, tied in a piece of linen, and a, gentle stream of water allowed to fall upon it; from time to time the clot is gently kneaded and the washing continued ; in the case of blood, till all traces of colouring matter are removed, or, where no colour is present, so long as may be deemed necessary ; the residue is then removed from the linen, ilried, digested in ether to remove adhering fatty matters, again dried and weighed. A portion is then incinerated, the weight of the fixed matters determined and deducted from the gross weight of the dried fibrin, by which WC obtain that of the organic matter.* Fatty multers.—Several peculiar oily sub stances occur in the fluids and solids of the animal body. Among the saponifiable fats chemists have distinctly ascertained the pre sence of inargarin, elain, and butyrin ; besides these we have cholesterin and serolin, which are not saponifiable by boiling in alkaline solu tions, and there are others containing phos phorus and sulphur, but their composition and properties are yet involved in Uncertainty. Our analytical processes for separating these bodies arc very imperfect ; the fats are all soluble in boiling alcohol, and still more freely in ether. Cholesterin and scrolin may be isolated from the other fats by boiling the residue, after eva poration of the ether, with solution of caustic potash, as they remain undissolved by this menstruum, whilst the margarin, elain, and butyrin form soaps which are dissolved by the water.