14 



ORGANIC CONSTITUENTS. 



at about 340°. It contains no water of cry^stallization. It is 

 soluble in water and in alcohol^ but not in ether : its formula^ 

 is Cj^ Hj, N O^. According to Mulder it must be regarded 

 as an integral constituent of protein. It combines with nitric 

 acid and forms a crystalline acid to which the term nitro-leucic 

 acid has been given. 



We shall have occasion to revert to leucin in our observa- 

 tions on gelatin. 



Protid is the term applied to the extractive matter that re- 

 mains in solution after the removal of the erythropi^otid and 

 leucin. It is of a briglit yellow colour^ easily pulverizable, and 

 soluble in water and alcohol without coloming them. It is pre- 

 cipitable by the basic acetate of lead, but not by any other me- 

 tallic salts nor by tannin. The salts of lead serve to distinguish 

 it from erythroprotid. If a mixture of these two substances be 

 dissolved in water, the latter is precipitated by the neutral, the 

 former by the basic acetate of lead. 



Its formula2 is €,3 H^ NO^. 



The action of caustic potash on protein is evidently very com- 

 plicated. Mulder endeavours to show by the following formula 

 how these metamorphoses may occur. 



^80 ^62 N,^ Oj 



H„ 



Oa 



Ceo H„ N.„ 



33 



According to Liebig, protein is produced by vegetables alone, 

 and cannot be formed by animals, although the animal system 

 has the power of converting one modification of protein into 

 another ; it is never found as protein, in nature ; but occiu-s in 

 the shape of albumen, fibrin, or casein, both in vegetables and 

 animals. These modifications of protein are employed in the 

 formation of the different tissues, eacli of which bears a simple 

 relation to that substance, as will be seen by the follomng 

 table: — 



' See Appendix I, Note 5. 



* lb. Note 6. 



