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 cli- 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 fluid 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 
