PROTEIN. 



165 



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 ]<2 

 H 12 N0 4 + N0 5 + HO. 



2 Equivts. Erythroprotid C 2 . H. 6 N O, 



2 Protid ....C 26 H 18 N 2 8 



2 Leucin . . . . C 24 H 24 N 2 O 8 

 4 Ammonia .. H 12 N 4 



2 Carbonic acid C 2 O 4 } = 



1 Formic acid C H O_ 



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. 



C 2 Equivts. Protein . . C 80 H 62 

 I 9 Water . H 



O, 



C 80 H 71 N 10 : 



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 withoat 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 glutin ; 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 formed 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, ofC 13 H 10 N 

 O 5 , 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 13 H 9 NO 4 + NH 3 = C 13 

 H, , N- O 4 : and erythroprotid plus ammonia, 

 C 13 H 8 N0 5 + NH 3 = C 13 H 11 N 2 O 5 . If 

 now we suppose that these two hypothetical 



C 



N 10 33 



substances, C 13 H 12 N 2 O 4 and C l3 H tl 

 N 2 O 5 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 13 H 12 N 2 O, or C 13 H 10 N 2 O 5 

 + 2 HO ; and in that of erythroprotid C. , H, , 

 N 2 6 or C, 3 H 10 N 2 O 5 + HO, so that iA 

 both cases glutin might 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. 



We now come to the consideration of the 

 natural modifications of protein, which we find 

 composing the chief bulk of the bodies of 

 animals, viz.jfibrin, albumen, and casein. 



Fibrin. This is a substance of the highest 

 importance in the animal economy, since it is 

 the material of which the solid framework of 

 the muscles and some other tissues mainly 

 consist ; and it is also found dissolved in the 

 blood, from which it separates spontaneously 

 after removal from the body, forming the clot 

 or crassamentum. The following table shows 

 the average proportion of fibrin in several 

 animal products. 



100 parts Fibrine. 



Blood of the hog contain 0'46 



ox 0'37 



sheep 0'30 



Beef ( muscle of ) 20'0 



19-0 

 22-0 

 19-0 

 20'0 

 14-0 

 13'0 



Including a 

 little albu- 



men. 



Veal 

 Mutton 

 Pork 

 Chicken 

 Cod 



Haddock 

 Sole 



Calf's sweetbread (thy- 

 mus) .................. .'.. 8'0 



Fibrin may be obtained from lean animal 

 flesh by cutting it into thin slices and washing 

 with water till it is colourless ; it is, however, 

 impossible to obtain it pure in this way, as it 

 is always associated with fatty matters, nerves, 



M 3 



