308 DR ALISON’S OBSERVATIONS ON 
in the metamorphoses to which starch is liable in a living body ; and as we know 
the importance of oxygen in maintaining (in one way or other) all vital action, the 
latter supposition is more probable. 
If, e. g., we suppose 4 atoms of starch to yield 2 of fat, we must subtract from 
48 C 40 H 40 O 
24 C 20 H 20 
leaving 24 C 20 H 38 O=20 HO 9 CO?2+15 C;* 
so that on this supposition 15 atoms of carbon are set free, and as these do not 
appear, they must unite with the oxygen of the air, and take the form of carbonic 
acid ; and then the fat which appears, together with the water and carbonic acid 
thrown off, will account for all the elements concerned in the action. In this pro- 
cess, therefore, supposing the quantities of starch taken in, and of fat formed to 
be as above, 30 equivalents of oxygen must be absorbed ; so that we perceive the 
use of oxygen in the change, and the necessity of its presence, although the fat 
formed contains so much less oxygen than the starch. 
That this should be the real nature of the change is just what we ought to ex- 
pect, if, agreeably to the supposition formerly made, the starch taken into the 
blood of a living animal, is acted on at certain parts of the body by two powers, 
and divides itself between them, viz., a vital affinity, in which carbon is the chief 
agent, which leads to the formation of fat, and the simply chemical affinities, ex- 
erted chiefly by oxygen (continually taken into the blood), by which, if removed 
from the living body, we know that it would gradually be resolved into carbonic 
acid and water. And that this is the real state of the case we are fully assured 
by a simple but very important observation, viz., attending to the effect of exercise 
on the formation or deposition of fat in the living animal body. As we see by the 
numbers given above, that a certain amount of oxygen must be absorbed, and a 
certain quantity of carbonic acid and water, formed by its help, must be excreted, 
to enable starch to yield oil or fat by the process there represented, we can un- 
derstand that moderate exercise should favour the change; but when exercise 
is carried beyond a very moderate extent, we know that the circulation and res- 
piration being much accelerated, and the quantity of oxygen taken into the 
living blood being much increased, the effect is, to increase the exhalation of car- 
bonic acid and water, and proportionally to diminish the deposition of fat ; 2. e., to 
give a preponderance to the simply chemical affinities exerted by the oxygen, 
over the vital affinity, which would tend to the formation of fat. 
From this simple fact we may infer, 1. That the vital affinity by which oil is 
* It need hardly be said, that all these numbers are given, not as indicating the exact changes 
which take place when the organic compounds are formed, but only as illustrating their general nature. 

