VARIA TIONS IN A CIDITY. 5 79 
chemical energy of the system of mixed salts. The degree of acidity of 
the urine (or any analogous fluid) is in fact not an absolute quantity, 
but is wholly relative to the means which we employ to measure it. 
But by always employing the same means, he it, noted, we may obtain 
relative results which are strictly comparahle, and as an outcome of this 
somewhat difficult discussion, it may he suggested that we shall do well 
in the present state of our knowledge to continue to employ a simple 
titration method, by which we obtain comparable, if only relative, 
measurements. But we must employ an indicator which gives a more 
definite point of colour change than does litmus, and we must retain 
the same indicator for any one series of experiments; moreover, the 
nature of the indicator used must always be stated in stating the 
results. Phenolphthalein, and perhaps cochineal, will serve our pur- 
pose. If acid urine be gradually neutralised in the presence of the 
former of these, which is colourless when acid, a pink tinge is developed 
at a certain stage in the process, and we are justified in speaking of a 
specimen of urine which requires more alkali to produce this change 
as " more acid " than one which requires less. 
"What has been said in this section will have left a wrong impression if it 
be thought that such measurements are of no value. My endeavour has been 
to show that we have at present no means of expressing the acidity of the 
mine as an absolute quantity independent of the particular means adopted for 
measuring it. But, having chosen a method of estimation, and being careful 
always to use the same method, we may accurately follow the variations of 
urinary acidity, and obtain results with important bearings. 
Variations in acidity. — The degree of acidity as determined by 
titration is, as we have seen, in the main, a resultant of two opposing 
factors; on the one hand, acid production in metabolism: on the other, 
the ingestion of unsaturated or unstable basic compounds, supplemented 
by the production of ammonia within the body. To these, however, a 
third factor must be added — the elimination of acids or bases respect- 
ively by other than renal channels. 
The separation of the acid gastric juice and the consequent libera- 
tion of bases in the blood is associated with increased excretion of the 
latter in the urine. On the other hand, the flow of alkaline secretions 
— bile, pancreatic juice, etc. — diminishes the urinary bases. 
From these considerations, the reasons for the variations in acidity 
commonly met with become clear. The acidity increases with increased 
proteid metabolism, with exercise, and with the consumption of food, 
when this contains a small proportion of bases — in particular, with flesh 
food. It diminishes when the food taken contains abundant bases. The 
compounds of organic acids with the alkaline metals, which are so 
plentiful in vegetable food, become oxidised in the body to carbonates, 
and the excretion of bases thence derived tends to alkalise the urine. 
From this follows the familiar fact that the urine of herbivorous 
animals is alkaline, and that human urine may become alkaline (though 
seldom continuously so) when a vegetarian diet is maintained. 
The effect of the secretion of gastric juice is to produce what is called 
the alkaline tide. During the period of full gastric digestion the urine 
may become less acid, and may even (though this is rare) become alka- 
line to litmus. The occurrence of this phenomenon was first noted by 
Bence Jones. 
