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, be it noted, we may obtain 

 relative results which are strictly comparable, and as an outcome of this 

 somewhat difficult discussion, it may be 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 lias 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 

 urine 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. 



