299 
mode in which the air contributes to change the 
colour of the blood, it cannot be by imparting to it 
any portion of its ponderable matter. Consequently, 
although these facts prove that oxygen gas possesses 
the power of changing the colour of the blood, as well 
through dead, as through living, animal membranes, 
yet they afford no evidence of the combination of 
oxygen with that fluid, but shew only the conversion 
of that gas into carbonic acid, precisely in the same 
manner as this acid gas is formed when the blood is 
reddened in the ordinary process of respiration. 
595. Even when the air and blood are brought into 
contact, they only exert a reciprocal action on each 
other, by which carbonic acid is formed, but no oxy- 
gen appears to combine with the blood. We have 
already given various proofs (97. ), that, when the 
blood is changed in colour by the agency of the air, 
the oxygen gas of the air disappears, and carbonic 
acid is produced. These facts are confirmed by the 
experiments of M. Berthollet, who confined recent 
blood in a vessel of common air, and, at the end of 
twenty-four hours, the air, on analysis, afforded nearly 
rra of carbonic acid. In two other experiments, SN 
milar results were afforded ; and in all these experi- 
ments, the acid gas produced was exactly equal to the 
volume of oxygen that disappeared *. Unless, there- 
fore, it be maintained, that the same oxygen can, at 
the same time, exist in two combinations, we must 
suppose, that, in these experiments, no oxygen coim 
* Mem. d'Arcueil, t. ii. p. 46?. 
