Erythroprotid, when pure, is of a fine red colour ; it is soluble in boiling alcohol and in water, and is precipitated from its solutions, of a rose red colour, by many of the metallic salts, as those of silver, mercury, and lead : it is thrown down also by tannic acid. When a current of sulphuretted hydrogen is passed through its aqueous solution, it gradually be comes colourless ; but if the solution, thus treated, be kept in vacuo a short tirhe, the colour returns. The formula of erythroprotid is C,. H. NO,.
Protid (C, 3 H. NO4) may be separated from the impure alcoholic solution by diluting with water, and precipitating with subacetate of lead, which throws down protid, but not erythroprotid, which latter is also present in small quantity. The precipitate is washed with water, and decomposed by sulphuretted hydrogen ; the solution is filtered and evapo rated, after which the protid is left in a state of purity. It is of a pale yellow colour, amor phous, and, when dry, very brittle. It differs from erythroprotid in not being precipitated from its solutions by any of the metallic salts except basic acetate of lead; while erythro protid is not affected by that reagent : conse quently if the two exist together in solution, the erythroprotid may be thrown down- by the neutral acetate, and the protid by the basic salt.
Leucin, which gradually separates when the alcoholic solution is concentrated, is a crystal line substance closely resembling chloresterine in appearance : it consists of C, 2 H, 2 NO,.
It is tolerably soluble in water and alcohol, but quite insoluble in ether ; and when heated to about 340° it sublimes without decomposition. When treated with nitric acid, a crystalline nitroleucic acid is formed, consisting of C „ TT Nfl _L J_ 1-111 Mulder has attempted to explain by the fol lowing equation how the elements of protein may dispose themselves, in order to produce the compounds just described.
Equations of this kind, though sometimes of great service in simplifying complicated chemi cal changes, are always to be looked upon merely as representing possibilities, and should not be adopted without great caution ; much mischief has indeed already been done from the too ready credence in the truth of hypotheses which have thus been made to appear simple and striking, though really in the highest degree at variance with what further research has proved to be the truth.
The action of potash on protein and its com pounds derives additional interest from the cir cumstance that it may afford a clue to the man ner in which the gelatinous tissues of the body are formed from protein compounds, a problem at present very far from being satisfactorily solved. Both protid and erythroprotid are
somewhat similar in composition to chondrin and glu tin ; and leucin, which Mulder considers to be actually a constituent of protein, may be obtained also from gelatine, clearly showing some connection to exist between the protein and gelatine compounds moreover we find the gelatinous tissues forined in the herbivora, though not a trace of any analogous substance can be detected in their food. These circum stances tend to the conclusion that the chondrin and glutin of the herbivora at least, are in some way derived from the proteinaceous matters of the food, and Mulder has suggested that it may be owing to a change produced by the free alkali of the serum, not unlike that which I have described as the effect of the action of potash on the protein compounds. Glutin con sists, according to that chemist, of C, H 1 „ N2 05, and it is easy to represent by a chemical equation how such a compound may be formed from either protid or erythroprotid. When these latter substances are formed in the labo ratory by the decomposition of protein by potash, it is probable that two equivalents of ammonia are at the same time produced ; and we may conceive that in the living body the elements which, when not so circumstanced, unite to form ammonia, remain combined with those of protid and erythroprotid ; in that case we should have compounds containing protid plus ammonia, C, 3 H. NO„ NH. = C, 3 H, 2 N2 0, : and erythroprotid plus ammonia, C, 3 H. NO„ NH.= C, 3 11„ N2 05. If now we suppose that these two hypothetical substances, C, 3 H„ N2 0„ and C„ 11„ N2 05 become united, the one to three equiva lents, and the other with one equivalent of oxygen, a supply of which is always present in the arteries, we should have in the case of protid, C,3 H,2 N2 0, or C, H,,, N. 06 + 2 HO ; and in that of erythroprotid C, s H„ N. 06 or C„ 11,0 N2 05 + HO, so that m both cases glutin ?night be formed. This hypo thesis is highly ingenious and interesting, though the probability of its correctness is somewhat lessened by the circumstance that neither leu cin, protid, nor erythroprotid, have yet been detected in the animal organism ; and more over it is uncertain whether the alkaline re action of the blood is owing to the presence of free alkali, or of tribasic phosphate of soda.