THE U REIDS. 81 



C 5 H 5 N 5 O + 7H 2 O = 4NH 3 + 2CO 2 + CH 2 .NH 2 .COOH + H.COOH. 

 Guanin. 



C 5 H 4 N 4 O 2 -f 6H 2 O = 3NH 3 -f 2CO 2 + CH 2 .NH 2 .COOH + H.COOH. 

 Xanthin. 



C H 4 N 4 O 3 -f 5H 2 O == 3NH 3 + 3CO 2 + CH 2 .NH 2 .COOII. 

 Uric acid. 



According to Emil Fischer, the structural formula of uric acid can 

 be represented as follows : 



HN CO 



CO C NH\ 



I I >co 



HN C NH X 



Uric acid. 



and it is thus seen that, like the nucleinic bases, it contains the 

 purin radicle. 



All the nucleinic bases combine with acids and alkalies to form 

 salts, many of which are readily crystallizable. On boiling with 

 acetate of copper most of them are thrown down as insoluble com- 

 pounds. From their neutral solutions, or in the presence of a little 

 ammonia, they are precipitated by ammoniacal silver nitrate solu- 

 tion. This precipitate dissolves in nitric acid on the application of 

 heat, but reappears on cooling, Most of the nucleinic bases, more- 

 over, when evaporated to dryness in the presence of nitric acid, 

 leave a yellowish residue, which changes to orange and often to a 

 temporary purple on the addition of an alkali. In this respect also 

 these substances resemble uric acid, which gives a very similar reac- 

 tion (see Murexid test). When exposed to the action of putrefactive 

 organisms adenin is transformed into hypoxanthin, and guanin into 

 xanthin, so that these two only are found in decomposed material. 



For a description of the individual members of this group which 

 are found in the animal body, as well for the method of their isola- 

 tion and quantitative estimation, the reader is referred to subsequent 

 chapters (see especially pages 241 and 363). 



THE UREIDS. 



The ureids comprise a number of nitrogenous crystallizable bodies 

 characterized by the fact that on hydrolytie decomposition alone, or on 

 simultaneous oxidation, they yield urea. They are hence derivatives 

 of urea, and may contain one or more molecules of this substance, in 

 which one or more hydrogen atoms are replaced by radicles of mono- 

 basic or polybasic acids. On decomposition they yield either urea 

 and a non-nitrogenous acid directly, or they give rise to urea and a 

 less complex ureid, which is then further decomposed, as in the 

 first instance. They are accordingly divided into mono-ureids and 

 di-ureids. The former generally contain two atoms of nitrogen in 

 their molecule, while the latter possess four atoms of nitrogen. All 

 these bodies are closely related to each other and to the nucleinic 

 bases, from which they are, in part at least, derived. 



6 



