56 ORGANIC CONSTITUENTS. 



peroxide of lead is gradually added to the boiling liquid, as long 

 as its colour is observed to cbange. The hot liquid is then 

 filtered and evaporated till crystals begin to form on its surface ; 

 on cooling they form in considerable quantity, and constitute 

 allantoin or allantoic acid, while urea remains in the mother 

 liquid, and oxalate of lead on the filter. 



The following symbolical representation may elucidate this 

 decomposition. 



It is on this reaction that Liebig founds his theory of uric 

 acid. He considers it to contain ready-formed urea and a hy- 

 pothetical substance, for which he proposes the term uril \]\, or 

 cyan-oxahc acid.^ 



For 1 At. Uric acid (C,,, H, N^ Og)— 1 At. Urea (C^ H, N^ 0^) = 2 (C, NO^) =2 UT. 



Hence the rational formula for uric acid appears to be — - 



1 At. Uric acid (C,o 11^ N, Og) = 2 uT-f 1 At. Urea (C, H, N^ 0^). 



In the production of allantoin from uric acid, the urea is 

 supposed to be set free, whilst the uril combines with oxygen 

 and water in order to form oxalic acid and allantoin. 

 The change may be illustrated in the following manner : 

 201 = C,N.,0^ = C^O^-fC^N,. 



By the addition of 2 at. oxygen to the former term, C^ O^, 

 we obtain 2C^ O^ (oxalic acid), and by the addition of 3 at. 

 water to the latter, C^ N,^, we obtain C,^ H3 N,^ O3 (allantoin.) 



This substance allantoin, or as it is frequently termed allantoic 

 acid, occurs ready formed in the allantoic fluid of the calf, from 

 which it crystallizes spontaneously on cooling, when the fluid 

 has been evaporated to one fourth of its volume. It then re- 

 quires to be purified by recrystallization. 



The crystals are colourless and transparent, tasteless and 



' 2 At. Hydrated oxalate of lead = 2 (Pb 0, C^ O3, 2 HO) 



= C, H^O.^Pb, 



* This name is suggested by its constitution ; for 1 at. Uril = C^ NO2 = C^ O2, Cy, 

 a formula that represents oxalic acid in which an equivalent of oxygen is replaced by 

 one of cyanogen. 



