VARIATIONS IN THE ACIDITY OF THE URINE. 291 
effect of the absorption of a meal into the blood, rather 
than of digestion? If it be true, as Liebig maintains, 
that the alkalescence of the blood —and in all animals 
that possess blood its reaction is alkaline — depends 
simply on the chemical composition of the alimentary sub- 
stances, is there not here a solution for our question ? 
In his twenty-eighth Letter,* Liebig points out that 
phosphoric acid and the alkalies are present in such pro- 
portion in bread, meat, and our ordinary food, that if we 
suppose them dissolved the alkalies invariably prepon- 
derate. Hence arises, he says, the alkalinity of the blood. 
If this be so, every meal that is dissolved and absorbed 
into the blood must increase the alkaline reaction of that 
fluid and raise it for a time above the natural level. 
But it is well known that when salts of the fixed alka- 
lies which have an alkaline reaction — such as carbonates, 
basic phosphates and borates, or vegetable salts, which 
become carbonates in the system — are artificially exhi- 
bited, they change the reaction of the urine from acid to 
alkaline; evidently from inducing an excessive alkales- 
cence of the blood, which it is the function of the kid- 
neys to diminish by allowing the excess to escape in the 
urine. Conformably to this hypothesis, the earthy and 
alkaline phosphates were found greatly increased in the 
urine after meals. 
A meal, therefore, viewed in this light, is a dose of 
alkali, which, when digested and absorbed, necessarily 
adds to the alkalinity of the blood; and, as a more remote 
but equally inevitable consequence, lowers the acidity of 
the urine or, if in sufficient quantity, renders it actually 
alkaline. It has been already pointed out that the set- 
ting-in of the alkaline tide coincides, in point of time, 
with the passage of the digested food into the blood, as 
indicated by the increased amount of solid urine secreted 
* Familiar Letters, Letter 28. 
VOL. XV. QQ 
