588 THE CHEMISTR Y OF THE URINE. 



likely that they can, under any circumstances, occur as physiological 

 products. 1 



2. The acid urates or biurates, M'HIJ, 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 Scherer, 3 and (independently) by Bence Jones, 4 but 

 they have since been more carefully studied by Sir Wm. Eoberts. 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 Eiidel, 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. Ixv. S. 181. 



3 Neubauer ii. Vogel, "Analyse des Harns," 9th edition, S. 192. 



4 Journ. Chem. Soc., London, 1862, vol. xv. p. 8. 



5 " Croonian Lectures," 1892. 6 Ibid. 

 7 Arch.f. exper. Path. u. PharmaJcoL, Leipzig, 1892, Bd. xxx. S. 469. 



