GENERAL CONSIDERATIONS. 571 
follows that- the urine is essentially a .solution of salts; its chemical and 
physical properties being those of a complex saline mixture. 
The chief bases are potassium, sodium, and ammonium ; calcium and 
magnesium ; urea, creatinin, and the xanthin bases. The chief acids are 
hydrochloric and sulphuric; phosphoric and carbonic; uric; oxalic; with 
hippuric and certain other aromatic acids. To the acid group belong also 
undoubtedly the pigments. 
The particular combinations formed in the urine by these various 
acids and bases depend primarily on their relative masses and avidities; 
the ultimate equilibrium of the fluid depending, secondarily, on the 
mutual influences, in solution, of the salts which potentially tend to form 
as a result of the two factors just mentioned. It should be understood 
that our present knowledge does not carry us far towards a calculation 
of this complex equilibrium in any particular case. When we have 
determined by analysis the proportions of the various bases and acids 
present, we may, for convenience, group them into various supposititious 
combinations one with another, and speak of the urine as containing so 
much sodium chloride, so much " earthy phosphates," and the like ; but 
such groupings can, with our present knowledge, be for the most part 
approximate only ; and, if insisted upon too closely, may be misleading. 
If the chemistry of urine had to be read merely as a final chapter in 
the history of metabolism, the actual condition of the acids and bases 
present would be of little importance to the physiologist or to the 
pathologist. The nature and amount of these constituents having been 
determined, each woidd be considered in connection with the organ or 
tissue the metabolism of which is responsible for its appearance in the 
urine, and the chemistry of the latter would be of no further import. 
But the case, as we have said, is otherwise. The two conditions of 
chemical equilibrium represented respectively by the expressions — 
(1) CaS0 4 + 2(NaH 2 P0 4 ) [three molecules] 
(2) Na 2 S0 4 + Ca(H 2 P0 4 ) 2 [two molecules] 
involve each of them the same amount of the bases and acids concerned ; 
but the presence of the first combination in the urine might involve a 
renal activity quantitatively as well as qualitatively different from that 
which would be indicated by the presence of the latter. Further, a 
knowledge merely of the percentage of uric acid in a given specimen 
of urine will by no means give us final information as to the power of 
the fluid to retain this constituent in solution. One individual may 
excrete a large percentage, and yet have no tendency to suffer from 
uric acid gravel ; another may not be free from this, though he habitually 
excrete a lower percentage. To explain this we must understand the 
influence of other urinary constituents on the solubility of uric acid ; in 
other words, we must study the properties of the urine as a whole. 
Enough has been said to show that we are not to remain content 
with analytical figures alone. The future study of the urine will con- 
cern itself also with the a] (plication of facts derived from that domain of 
chemistry which deals with the distribution of chemical forces in com- 
plex mixtures. At present we have but little available knowledge of 
this kind, and many urinary phenomena are consequently but imperfectly 
understood. We may instance, however, a generalisation made from the 
experimental and mathematical investigation of the mutual influence of 
salts in solution, which is capable of immediate application to our subject. 
