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gen gas ; and may not the base of this gas produce 
this effect by combining with the blood ? Mercury, 
lead, and iron, says M. Lavoisier, become red when 
they combine with oxygen, and since the blood is 
rendered red by pure air, may not its redness pro- 
ceed from a similar combination * ? By very deci- 
sive experiments, M. Lavoisier shewed, that the 
base of oxygen gas combined with metals, du- 
ring their conversion to the state of oxides; but 
we have suggested doubts (427. ), whether the red 
colour, even of metallic oxides, depends on any co- 
louring property in oxygen ; and were this, for the 
present, conceded, it would not serve the case before 
us. since we possess no direct evidence that oxygen 
combines with the blood. It is, therefore, incumbent 
upon those who believe the blood to be rendered red 
in consequence of its oxygenation, to afford some 
palpable evidence that the base of oxygen gas really 
combines with that fluid, in a manner similar to that 
in which it unites with metals. 
579. It does, indeed, appear to us somewhat re- 
markable, that M. Lavoisier, whose accurate and 
comprehensive views respecting the combinations 
of oxygen, enabled him to introduce such great im- 
provements into the general doctrines of chemistry, 
and who first clearly remarked (118.) the difference 
between the effects produced in the air by combus- 
tion and by respiration, should still, apparently from 
the mere circumstance of similarity of colour, have 
concluded that respiration was nothing but a simple 
* Mem. de TAcad. des Sciences, an. 1777, p. 192. 
