280 MESSRS. ALLEN AND PEPYS ON THE RESPIRATION OF BIRDS. 
The production of carbonic acid in this case was somewhat greater than in 
atmospheric air ; but the remarkable feature of this experiment is, that, except 
in the formation of carbonic acid, the oxygen remains nearly unchanged, while 
the whole loss falls upon the hydrogen ; so that the blood appears to have 
absorbed a quantity of hydrogen, and given out a proportionate quantity of 
azote, the total volume of gas before and after the experiment remaining nearly 
the same. 
The present experiments tend to strengthen and confirm our former con- 
clusions, and prove that when atmospheric air alone is respired in a natural 
way, the proportion of azote is not altered, and that there is only a change of 
a certain portion of oxygen for an equal portion of carbonic acid gas ; that 
when a larger proportion of oxygen than is contained in atmospheric air is re- 
spired, a quantity of oxygen is absorbed by the blood, and an equal quantity 
of azote gas evolved. 
That when a mixture of hydrogen, oxygen, and azote are respired, the oxygen 
being in the same proportion as in atmospheric air, there is no loss of oxygen, 
but a quantity of hydrogen disappears and is replaced by the same quantity 
of azote. 
The circulation of the blood is quicker in birds than in other animals ; and 
if we may judge from the effects produced upon the pigeon, they are more 
sensible to the stimulus of oxygen. 
