588 THE CHEMISTRY OF THE URINE. 
likely that they can, under any circumstances, occur as physiological 
products. 1 
2. The acid urates or biurates, M'HU, are the most stable of the com- 
pounds of uric acid. They are prepared by dissolving the acid at boiling 
heat in weak solutions of the alkaline carbonates, from which they 
separate, after cooling, in stellar crystals. These and the foregoing salts 
were first studied by Bensch and Allan. 2 The acid urates are less 
soluble than the neutral salts. 
3. The quadriurates, H 2 U,M'HU. — These hyperacid salts, for the ex- 
istence and importance of which we have now satisfactory evidence, were 
first described by Schereiy and (independently) by Bence Jones, 4 but 
they have since been more carefully studied by Sir Wm. Roberts. 5 
They are best prepared by boiling uric acid with dilute solutions of 
acetate of potassium, and from solutions so obtained a quadriurate 
separates, as an amorphous precipitate, or in crystalline spheres. 6 They 
are very unstable, and when treated with water they split up into 
biurates and free uric acid. Owing to this instability it is impossible to 
determine directly their solubility in water; but they are probably less 
soluble than the preceding order of salts, as a strong solution of a 
biurate, when treated with acid-sodium phosphate, gives an abundant 
precipitate of a quadriurate. From analogy we might expect the three 
orders of salts, as described, to be in a descending series as regards 
solubility. 
Condition of uric acid in the urine: its spontaneous separation. — 
Coloured indicators which are sensitive to free uric acid give no 
indication of its presence, as such, in freshly-passed urine. Again, the 
quantity of uric acid present is generally greatly in excess of what 
would dissolve in a volume of water equal to that of the urine. The 
presence of neutral salts, and also, according to Riidel, 7 of urea, enhances 
this solubility, but not to a degree necessary for the retention of all the 
urinary uric acid in solution. We are led to expect, therefore, that it is 
present not as free acid but as a more soluble compound. Neverthe- 
less, most urines will, on cooling and prolonged standing, deposit a 
certain (and sometimes a large) proportion of their uric acid in a free 
condition. 
We have to explain, therefore, the nature of the original solution and 
the cause of the subsequent separation. The view generally held till 
recently, and still current with some authorities, is that the acid exists as 
biurates : and that these are slowly decomposed, with liberation of the 
free acid, by the action of the acid phosphates, according to the following 
simple reaction : — 
MHU + MH 2 P0 4 = H 2 U + M 2 HP0 4 
From acid urines, however, the uric acid is frequently deposited, in 
the first place, not as free acid, but in the form of urates, forming a 
precipitate which has long been known as the " lateritious deposit." 
A careful study of the chemistry of this deposit has led Sir William 
Roberts to conclude that the above equation does not rightly, or at least 
1 Roberts. 2 Ann. d. Chem., Leipzig, 1848, Bd. lxv. S. 181. 
3 Neubauer ii. Vogel, "Analyse des Haras," 9th edition, S. 192. 
4 Journ. Chem. Soc, London, 1862, vol. xv. p. 8. 
5 " Croonian Lectures," 1892. 6 Ibid, 
7 Arch./, exper. Path. u. PhannakoL, Leipzig, 1892, Bd. xxx. S. 469. 
