174 



ALLANTOIN. 



synthetised by prolonged heating to 100 of a mixture of glyoxylic acid 

 and urea 1 , or of the latter substance with mesoxalic acid 2 . 



As prepared artificially it crystallises readily in large prismatic 

 hexagonal crystals. 



FIG. 22. 



CRYSTALS OF ALLANTOIN PREPARED BY THE OXIDATION 

 OF URIC ACID. - (After Kiihne.) 



In addition to the crystalline form and precipitability with nitrates 

 of mercury and silver, allantoin is further characterised by yielding 

 Schiff's reaction with furfurol (see above, p. 159, sub urea), but less 

 readily and with less intense colouration than does urea. It also 

 reduces Fehling's fluid on prolonged boiling. 



THE XANTHIN GROUP 3 . 



This group comprises a number of substances closely related to uric 

 acid and to each other. Some of them occur in small amounts in 

 the tissues (muscles) and excretions (urine) of the body and are to be 

 regarded as being, like urea and uric acid, typical products of the 

 downward destructive metabolism of proteids. Some of them are 

 closely related to certain alkaloids which occur in plants (theobromin 

 and cafFei'n), and which probably play some not unimportant part in 

 the nutritional changes of the animal body, since they are constantly 

 consumed, in some form or other, by the larger part of the human race. 

 This relationship of the xanthin-bodies to certain vegetable alkaloids is 

 further interesting when it is remembered that the latter are regarded 

 by plant-physiologists as waste-products of the vegetable organism, and 

 are thus found chiefly in those parts of the plant which are on their 

 way to removal, viz. the bark, leaves and seeds. 



1 Grimaux, Compt. Rend. T. 83 (1876), p. 62. 



2 Michael, Amer. Ghem. Jl. Vol. v. (1883), p. 198. 



3 For a full statement of the general reactions of this group and the methods for 

 their separation and discrimination see Neubauer u. Vogel, Analyse des Harns, 

 1890. Sec. 200219. 



