CHEMICAL REACTION. 575 



urine confers upon them a definite acidic character. The acid oxides of 

 phosphorus and sulphur, which are the chief end-products of the meta- 

 bolism of these two elements, are eliminated almost entirely through 

 the kidneys. Eighty per cent, of the total sulphur ingested, and nearly 

 all the phosphorus, are eventually found in the urine as sulphuric and 

 phosphoric acids respectively. That these acids are eliminated as salts, 

 and not in the free state, depends in the main upon the fact that bases 

 are continually being ingested in the food in a form available for the 

 neutralisation of acids. The bases of the food are not all in the state 

 of stable neutral salts. Even animal food contains basic phosphates, 

 together with organic (proteid) combinations of the alkalies and 

 alkaline earths, and small quantities of alkaline carbonates; while 

 vegetable food contains, in addition, salts of the vegetable acids, which 

 in the body are converted into carbonates by oxidation. By the 

 ingestion of these unstable compounds of various bases, the organism 

 is saved from the necessity of eliminating free mineral acids. When 

 the supply of available bases is for any reason insufficient, a further 

 protective mechanism comes into action, metabolism being so modified 

 that a greater proportion of the nitrogen than usual is eliminated in 

 the strongly basic form of ammonia. All these factors are normally so 

 proportioned that, as we have seen, the urine, while containing no free 

 acid, is acid from acid salts. 



Phosphoric acid (H 3 P0 4 ) as a tribasic acid forms three orders of salts. 

 Those in which two out of the three hydrogen atoms of the acid molecule 

 are intact, are known as acid or superphosphates. They are soluble salts, and 

 react acid to litmus. The second type, in which two hydrogen atoms are 

 replaced by a base (monohydrogen phosphates), and the third, in which all 

 the hydrogen is replaced (normal phosphates), are alkaline to litmus. While 

 all varieties of the phosphates of sodium, potassium, and ammonium are freely 

 dissolved by water, of the alkaline earth metals only the superphosphates 

 are at all freely soluble. The monohydrogen and normal phosphates of 

 calcium, magnesium, and, we may add, of barium, are scarcely taken up by 

 water. 



If to a weak solution of, say, sodium - dihydrogen - phosphate 

 (NaH 2 P0 4 ) calcium chloride or barium chloride be added, no pre- 

 cipitation occurs ; the corresponding salts of these latter metals being 

 comparatively soluble. On the other hand, from a solution of di- 

 sodium-monohydrogen-phosphate (Na 2 HP0 4 ) nearly all of the phos- 

 phoric acid is precipitated on the addition of a calcium or barium 

 salt, in the form of the corresponding monohydrogen phosphate of the 

 alkaline earth. In any mixed solution of di- and mono-hydrogen 

 phosphates, the amount of phosphoric acid which is left unprecipitated 

 by, say, barium chloride, is a measure of the proportion of the di- 

 hydrogen phosphate originally present. Now, if we apply this test to 

 urine of average acidity, we find that about 60 per cent, of the total 

 phosphoric acid remains in solution after the addition of the barium 

 chloride. We are justified in concluding, therefore, that acid di- 

 hydrogen phosphates are present in about this proportion ; a fact in itself 

 sufficient to account for the acid reaction of the nuid towards litmus. The 

 composition of the barium precipitate from an acid urine proves that the 

 remaining phosphoric acid is mainly in the form of monohydrogen salts. 



If, now, we suppose the excretion to receive an increased quantity 



