726 THE REACTION OF THE URINE 



means that the blood contains some salt so weak that litmus can 

 take away its cation from it ; but phenol-phthalein is weaker 

 than litmus, and unable to take away the cation from any salt in 

 the blood. Normal urine is acid to litmus because litmus can 

 no longer successfully compete with any anion for its base, owing 

 to the great reduction in the relative number of cations to anions 

 present in urine. The stronger methyl- orange, however, is still 

 able to compete, and urine appears to be alkaline to it. It has 

 been found that in a large variety of conditions the acidity of 

 urine becomes so reduced that it may become alkaline to litmus. 

 Such conditions are diuresis, digestion, after the introduction of 

 a chloride or bromide into the alimentary canal or their injection 

 into the blood. But under no conditions does urine become 

 alkaline to phenol-phthalein, not even after the intravenous in- 

 jection of sodium carbonate. On the other hand, urine never 

 becomes acid to methyl-orange. Therefore the extreme varia- 

 tions in the reaction of urine correspond roughly to the interval 

 between Na 2 HP0 4 and NaH 2 P0 4 . Cushny has investigated 

 the mechanism by which this change in reaction is brought about 

 by the kidney. His method was to compare under different 

 conditions the urine from one ureter with that obtained from the 

 other, in which the flow of urine was reduced by the introduction 

 of a resistance equal to 15-30 mm. Hg. The object of the 

 resistance, as in his experiments previously referred to, was to 

 favour any absorption which might take place in the tubules. 

 He injected intravenously into dogs normal solutions of sodium 

 chloride or sulphate, and 10 per cent, solutions of dextrose or 

 sodium bromide, nitrate, malate, or tartrate. After the injection 

 of any of these solutions he found that the percentage acidity 

 always fell, so that the urine was alkaline to litmus and almost 

 neutral to phenol-phthalein, and was more or less equal on the two 

 sides. This result suggested that the fall in acidity was due not so 

 much to a too rapid flow through the tubules as to an absence of 

 acid-forming salts in the blood. He therefore injected Na 2 HP0 4 , 

 together with sodium chloride or dextrose, and found that the 

 urine in spite of the diuresis became much more acid to phenol- 

 phthalein, and might become acid even to litmus. In the urine 

 secreted under these conditions he found that the percentage of 

 phosphates and the percentage acidity to phenol-phthalein ran 

 more or less parallel to each other, and were greater on the ob- 

 structed than the free side ; this difference could be accounted for 



