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 would 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 7 that we are not to remain content 

 with analytical figures alone. The future study of the urine will con- 

 cern itself also with the application 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. 



