OF FHEA AKD TJEINAEY WATEE. 
825 
it in an increasing ratio ; and in like manner the action of barometric elevation was 
inverse ; but in reference to urea with food and exertion both relations are direct, and 
the urea increases as they increase. The effect upon the elimination of carbonic acid 
was immediate, but upon the excretion of urea it was not usually evident until the 
following day. 
6. Both excretions are influenced by excess of food, as has been shown by the “basis 
quantities but the influence upon urea was much greater than that upon carbonic acid. 
7. Exertion influences the excretion of carbonic acid much more than urea. 
8. Hence the relation of urea and carbonic acid is one of a general character only. 
11. The Eelation of Urea to Nutrition. 
This is associated with both the ingestion and the egestion of nitrogen. 
The Ingestion of Nitrogen . — The ruling theory in reference to the ingestion of nitro- 
gen is, that the elements of food are absolutely divisible into two classes, and that the 
nitrogenous are expended in repairing nitrogenous tissues, and the hydrocarbons in 
supplying heat. But I proved in my former paper on carbonic acid on the one hand, 
that starch and fat, apart from nitrogen, do not increase the emission of carbonic acid 
(the accepted evidence of the activity of the heat-forming function), and, on the other, 
that nitrogenous foods do increase the emission of that substance, and are thereby 
respiratory and heat-forming excitants. It had also been shown by several eminent 
chemists, that the fattening properties of fodder are in proportion, not to the carbon, 
but to the nitrogen which it contains ; and thence I infer, in accordance with my expe^ 
riments, that, in tlie experiments on animals, the fat was due in some way to the nitro- 
gen. It is therefore, I think, impossible to defend this theoretical division of foods, 
and nitrogen should probably be regarded both as an element of tissue and an excitant 
of vital actions. 
The Excretion of Nitrogen . — The Hews in reference to the excretion of nitrogen are 
three ; — 
1. That of Liebig and his pupils, as Bischoff and Voit — that the excretion of urea is 
in proportion to the metamorphosis of tissue, and is as the bulk of the tissue, the nitro- 
genous fluids contained in the tissue, and the oxygen supplied, or, in other words, as 
the wants of the tissues, the supply of nitrogenous food, and the activity of the vital 
processes. They affirm that in every condition in which the elimination of urea is 
increased there is increased \ital action ; and the following Table, derived from my own 
experiments, shows that even the ingestion of fluid on a day of entire abstinence from 
food causes temporary increase of pulsation. 
* Philosophical Transactions, 1859. 
