59 o THE CHEMISTRY OF THE URINE. 
In fact, quadriurates are thus re-formed, and become subject to the 
same influences as before. " These alternating reactions — breaking up 
of quadriurates by water into biurates and free uric acid, and recomposi- 
tion of quadriurates by double decomposition of biurates with mono- 
metallic phosphate — go on progressively, until all the uric acid may be 
set free." 
The quadriurates, therefore, are of great importance in the chemistry of 
urinary uric acid, and beyond all doubt form an intermediate step in the 
liberation of the free acid itself. Tbe evidence that they are tbe form in 
which the acid is actually excreted seems to be less conclusive. It is clear 
that the alternating reactions just discussed -would go on, whether the salts 
which leave the renal tubules are quadriurates or biurates. In the latter case, 
the interaction with the phosphates would be the first stage of the process, 
and the decomposition of the resulting quadriurates the second. Equations 
(1) and (2), above, would occur in reversed order, and the alternation would 
then continue as before. The fact that the solid excretion of birds and snakes 
consists of quadriurates, 1 may be held to support the view that these salts 
are the excretory form in man, as also the observation that certain urate con- 
cretions found in the kidneys of new-born children approximate in composition 
to the quadriurates.- But it may be fairly argued that when, as in the 
human adult, the mechanism of excretion has become more perfectly suited 
to the elimination of a liquid urine, the uric acid will tend to assume the 
more soluble form, and all the evidence points to the fact that this form is 
the biurate. I have frequently observed that when ammonium urate separates 
from a clear acid urine, as an effect of adding neutral ammonium chloride in 
excess (vide infra), it is wholly in the form of a biurate. "While it is not 
inconceivable that a migration of bases occurs under these circumstances, it 
is far more likely that the fact points to the pre-existence of biurates in the 
urine. 
Again, it will be found that many concentrated specimens of urine, when 
first passed and while perfectly clear, will, on slight acidification with acetic 
acid or with a mineral acid, give an immediate precipitate of quadriurates, 
while the same specimen may require hours before any urate deposit separates 
spontaneously. The explanation of this would seem to be that the urine 
originally contained the more soluble biurates, and that these are changed 
immediately upon artificial acidification, or more slowly by interaction with 
phosphates, into less soluble quadriurates. 
But whatever may be the primary form of the urates present, it is 
in any case important to recall the facts discussed on p. 578. The 
reaction between urates and phosphates is a reversible one ; with acid- 
phosphates, biurates yield quadriurates; with basic (monohydrogen) 
phosphates, quadriurates yield biurates. With a certain proportion- 
ate mixture of the two types of phosphate the uric acid salts will be 
therefore in equilibrium. 
In many urines this equilibrium between the phosphates and urates 
is established, and the determining reactions described above, therefore, 
cease before all the uric acid is liberated. In others, where the 
proportion of monohydrogen phosphate is at the outside large, the 
equilibrium occurs early, and little or no free uric acid separates. Only 
when the original excess of acid phosphate over basic phosphate reaches 
an adequate value is the whole of the uric acid set free. In other 
1 Roberts, loc. cit. 
- Flensbuvg, Jahresb. u. d. Fortsckr. d. Thier-Chem., Wiesbaden, 1894, Bd. xxiii. 
S. 581. 
