Fibrin and hydrochloric acid. — When treat ed with strong hydrochloric acid fibrin be comes gelatinous, and gradually dissolves, giving the solution a beautiful blue colour, which is characteristic of all the protein compounds : if . this solution be diluted with water, a white precipitate appears, which is a compound of hydrochloric acid and protein. When the acid is very dilute it has the property of gradually dissolving fibrin ; and as a trace of free hydro chloric acid is generally to be found in the stomach, it is probable that its solvent action tends to assist materially in the process of digestion. Bouchardat says that water con taining only one two-thousandth of its weight of hydrochloric acid causes moist fibrin to become gelatinous, and eventually to dissolve, leaving only a small quantity of insoluble mat ter, which he calls epidernzose : the soluble portion he has called albuminose, but Mulder considers it binoxide of protein, which asser tion, however, has recently been contradicted by Liebig.
Fibiin and potash. —Fibrin dissolves rea dily in a solution of potash, even when very dilute. If the solution be gently heated, the fibrin is gradually decomposed, the sulphur and phosphorus being removed, and protein remains combined with the potash, from which it may be separated by neutralizing with acetic acid. Ammonia behaves in a similar manner, but its action is much less rapid.
Fibrin readily dissolves in the gastric juice, which appears to owe its solvent action both to the organic principle pepsine, and also to a little free hydrochloric acid in the stomach, which is derived frorn common salt. Tlie same effect may' be produced artificially by an infu sion of the fourth stomach of the calf; to which a little hydrochloric acid has been added.
It is curious that the presence of certain salts, as nitrate of potash and sulphate of soda, prevents the coagulation of the fibrin of the blood ; and even when coagulated, provided it be still moist, it is again dissolved by some saline solutions, as, for instance, muriate of ammonia. Moreover, M. Denis has found that if moist fibrin be digested in a solution of nitrate of potash containing a little soda, at a temperature of about 100°, it becomes gradu ally converted into a substance in almost every respect identical with albumen, being soluble in water, and coagulable by heat. This change is most readily produced when the fibrin em ployed has been obtained from venouS blood, by allowing it to coagulate spontaneously ; while if it be separated by agitation, or if the blood be arterial, it scarcely experiences any alteration in the saline solution. Changes of
this kind, of the several modifications of pro tein into one another, are constantly occurring in the animal economy, and the great similarity of their composition must render such meta morphoses comparatively easy.
The composition of fibrin is C400 1-13{0 N6c, SP, or ten equivalents of protein united to one of sulphur and phosphorus. It also usually contains from 1.3 to 2.3 per cent. of inorganic matter, chiefly phosphate and sulphate of lime, and alkaline salts.
Albumen.— This important compound, so called from its constituting the solid matter of white of egg, exists in two conditions, perfectly distinct in physical properties from each other ; the one soluble and miscible with water in all proportions, as it is found in the serum and white of egg ; the other solid, and quite inso luble in water, as in white of egg after boiling. The solid form is also met with, in a some what modified condition, in the albuminous tissues of the body, as the brain, spinal cord, nerves, &c. The proportion of albumen con tained in some of the animal products may be seen in the following table.
Albumen, in a state of absolute purity, has been but imperfectly examined. It may be prepared by the following process, recently adopted by Wurtz. A quantity of white of egg is well beaten up with about twice its bulk of water, and strained through linen to separate the cellular membrane. A solution of subace tate of lead is cautiously added, which throws down a copious precipitate ; but care must be taken to avoid adding an excess of the preci pitant, which would partly redissolve it. The precipitate should be well washed, and while suspended in water a stream of carbonic acid passed through it : the liquid soon becomes frothy, owing to the decomposition of the albu minate of lead and liberation of free albumen, carbonate of lead being precipitated. The solu tion of albumen, after filtration, generally con tains a trace or oxide of lead, which may be separated by adding a few drops of solution of sulphuretted hydrogen, and warming the liquid till it just begins to coagulate, when the whole of the sulphuret oflead is entangled in the coa gulum: the liquid, which after another filtration is clear and transpareiat, should be cautiously evaporated at a temperature not exceeding 120°, when it leaves a residue of pure albumen.