ELEMENTS OF COMPOSITION. 37 



grouped together in knots. Pyramidal or barrel-shaped pieces may 

 also be found among other figures (Schmidt, Lehmann). 



Uric acid, which has become deposited in urine (fig. 13, I), is dis- 

 coloured by the pigmentary matters of this fluid, its crystals appearing 

 yellow or brownish. The latter are met with as a rule either in the so- 

 called " whetstone form," i.e., as though they were transverse sections of 

 strongly biconvex lenses, or in rhombic tables with blunt or rounded 

 angles. Another very remarkable crystalline form 

 is that known as the " dumb-bell" (c). This may be 

 met with as a natural product, or may be produced 

 artificially by the decomposition of urate of potas- 

 sium (Funke). 



Uric acid, whose acid properties are very feeble, 

 requires about 14,000 parts of cold and 1800 

 parts of boiling water for its solution. With 

 bases it forms salts, seldom neutral, and as a rule 

 acid. Those of the first kind, w r hich contain two 

 atoms of base, are readily converted into the latter 

 kind by the action of carbonic acid. They are, 

 moreover, easier of solution than the acid salts, in gated ? 6 &, spheroidal 

 which only one atom of base exists. Among the 

 latter two of the most important alone need be mentioned, which are 

 very difficult of solution in cold water. 



ACID URATE OF SODIUM, C 5 H 3 NaN 4 3 . 



This salt forms short hexagonal prisms, or thick six-sided tables. It 

 is usually found under the microscope in the form of rounded bunches 

 of crystals (fig. 14). It also appears at times in strange spheroidal 

 masses with beaded processes (b, ft). 



ACID URATE OP AMMONIUM, C 5 H 3 (KH 4 )N 4 3 . 



A compound crystallizing in fine needles, usually combined to form 

 rounded tufts, in which the individual crystals are generally supposed to 

 be smaller than in the preceding salt (fig. 15). 



Both salts, as well as the acid itself, on being evaporated at a moderate 

 heat with nitric acid, leave behind a reddish residue, which assumes a 

 beauteous rose-red colour on the addition of ammonia, 

 and on the subsequent addition of caustic potash an 

 exquisite violet tint. This test, known as the mur- 

 exide test, accurate for the detection of uric acid. 



From its numerous decomposition products we are 

 as yet unable to gain any sure insight into the true 

 constitution of uric acid ; its being the source, how- R - is. Acid urate of 

 ever, of urea, allantoin, and oxalic acid, and, according 

 to Strecker, of glycin, renders it of great interest, and shows its import- 

 ance. 



Uric acid, as its name denotes, is a constant constituent of human urine. 

 It is present, in the latter, however, in far smaller quantity than urea, 

 amounting only to about one per thousand, and combined with soda more- 

 over. It is also met with, though in smaller proportion still, in the 

 urine of carnivorous mammals. Traces only of it are found in the urine 

 of phytophagous animals. Its amount in human urine appears to vary 



