268 ON THE EXTERNAL CAUSES OF DISEASE. 
of wliicli the mean was 17 86 cenlilitres, or about 10*90 cubic indies ; and 
that the absorption, likewise nearly the same in all, was on an average 9*30 
centil., or5'67 cubic inches, and that it was at tlie expense of the oxygen. 
We shall now see what light this first experiment throws on the nature of 
the gas absorbed. Is it pure oxygen, or carbonic acid, which mixes with 
the air during respiration; or, lastly, a mixture of both? We shall, for the 
present, suppose, that the two first hypotheses only are possible; we shall 
afterwards examine the last. 
In the early stages of the experiment, when the respiration begins, there 
is searcely any carbonic acid in the vessel; but the quantity increases as the 
experiment proceeds. If, therefore, it is this gas which is absorbed, the 
absorption will be scarcely sensible at first, will go on increasing, and be at 
its maximum at the end of the experiment. On the other hand, if it be 
oxygen, the reverse will take place, since its proportion is largest at the 
commencement, and dimini.shes progressively. Now^the following are the 
phenomena presented in the experiment. As soon as the animals are intro¬ 
duced, scarcely any of the expansion of the air which must necessarily take 
place from the rise of temperature, is indicated by the mercury in the tube; 
w hence it results that absorption takes plaee from the beginning, and, con¬ 
sequently, that it is the oxygen w hich is absorbed. In confirmation of this 
we may observe, that when the absorption is evident by the ascension of the 
mercury in the tube, it is more rapid at the beginning than towards the eon- 
clusion of the experiment. Three puppies, like the preecding, were placed 
in the same conditions as in the first series of experiments; but instead of 
remaining five hours they remained only two. The average quantity of 
carbonie acid produced was 14*86 centil., or 9 cubic inches, and 7 eentil.,or 
4*27 cubic inches of gas absorbed. In this instance the gas absorbed was 
rather less than half the carbonie acid produced, and in the former rather 
more than half. We see from this how* little influence the lengthened con¬ 
finement of these animals in air containing a pretty large proportion of car¬ 
bonic acid, had on the proportion betw een ti)e two quantities, whence it is 
evident that the gas absorbed is principally oxygen. T'hrce very young 
cabias,or guinea-pigs, were subjected to the same kind of experiment for one 
hour and forty-tw o minutes. They produced each, on an average, 21'69 centil., 
or 13*23 cubic inches of carbonic acid, and absorbed 5*44 centil., or 3*32 
cubie inches of oxygen. 'I’he ehange of species occasioned a striking 
change in the proportion of the oxygen lost to the carbonic acid produced. 
In the case of the cabias it is as one to four, iii the puppies of the second 
series a little less than one to two. Besides the dilfercncc depending on 
their species, the one being carnivorous and the other herbivorous, there is 
another very remarkable, one to w hich we have frequently alluded, depend¬ 
ing on their development. The new-born cabias come into the world in a 
more advanced state, w hich gives them the pow er of producing more heat. 
Hitherto w e have seen that the proportion of gas absorbed to the carbonic 
acid produced, varies principally according to the species and the age; for 
in casting a glance at the tables, it is evident that the mean quantities differ 
but little from the particular results. Butw e shall now observe the results 
of experiments upon other species, in which the individuals differed much 
in this respect. These examples are taken from birds, both in laborious 
inspiration, or in that w Inch may be regarded as nearly free and natural. 
The conditions of the experiment may be so modified as to approach inde¬ 
finitely to natural respiration, by increasing the quality of air in proportion 
to the bulk of the animal, and by shortening the ])eriod of confinement. 
I chose ten adult yellow-hanuners, w hose bulk w as equal to a little more 
than 3 centilitres, or 18 cubic inches. Each of these were placed in a 
