1262 



ALLANTOIN. 



3. Oxaluric acid. C 3 H 4 N 2 O 4 . (Hydrated parabanic acid.) 



Occurs in minute traces in normal urine, from which it is 

 extracted by filtering a large quantity of urine very slowly 

 through a relatively small amount of animal charcoal. The 

 charcoal after being washed with distilled water is extracted 

 with boiling alcohol, to which it yields the oxaluric acid as an 

 ammonium salt. The free acid is a white crystalline powder, 

 not very soluble in water: its alkaline salts are readily soluble. 



4. Allan toin. C 4 H 6 N 4 O 3 . (Diureide of glyoxylic acid.) 



The characteristic constituent of the allantoic fluid, more 

 especially of the calf, as also of foetal urine and amniotic fluid; 

 it occurs also in the urine of many animals for a short period 

 after their birth. Traces of it are sometimes detected in this 

 excretion at a later date. It is obtained in urine after the 

 internal administration of uric acid. It has also been found in 



FIG. 215. CRYSTALS FROM CONCENTRATED URINE OF CALF. (After Kiihne.) 



The large central crystal composed of an aggregation of small prisms is 

 allantoin : those below it are crystals of creatine, creatinine, and oxalate of 

 lime. The large prisms in the upper part of the figure consist of magnesium 

 phosphate. 



vegetable tissues. It crystallizes in small, shining, colourless, 

 hexagonal prisms. They are soluble in 160 parts of cold water, 

 more soluble in hot, insoluble in cold alcohol and ether, soluble 

 in hot alcohol. Carbonates of the alkalis dissolve them, and 

 compounds may be formed of allantoin with metals but not 

 with acids. The salts with silver and mercury are important 

 as providing a means of separating allantoin from its solutions. 

 Allantoin gives no reactions which are sufficiently striking 

 to admit of its detection in urine or other fluids : it must there- 

 fore in all cases first be separated out and then examined. The 

 separation may be effected in several ways, of which those more 



