464 The Atmosphere Considered [Odtober, 
earth than has been restored to the atmosphere by decom- 
position, and that therefore the quantity of carbonic acid in 
the air has been gradually lessening from remote periods up 
to the present time. This appears anything but improvable, 
remembering the arguments already noticed in favour of the 
supposed highly-carbonated atmosphere of the carboniferous 
period ; and although the calculations leading to such a 
conclusion are necessarily based on very imperfedl data, it 
may be safely affirmed, at least, that such a state of affairs 
is not only possible, but probable. 
In these calculations we are not only to consider the 
carbon of the vegetable kingdom, for it will be obvious that 
any animal carbon which may remain in rocks is also more 
or less diredlly derived from the carbonic acid of the atmo- 
sphere. Taking the extreme case of the Carnivora , it is 
clear that they must ultimately depend on the air for their 
supplies of flesh-forming material. Say a tiger dines off a 
cow ; the carbon and nitrogen of her flesh have been ob- 
tained from vegetation, which in turn extracted them from 
the air; so that we have a kind of physiological “ House 
that Jack built.” “ This is the Tiger that ate the Cow that 
devoured the Grass that absorbed the Carbon,” &c. Viewed 
in this way it seems that “ living on air ” is a more substan- 
tial kind of existence than has usually been supposed. 
Now this which is true of the higher animals applies 
equally with regard to lower forms. There will be a vege- 
tarian somewhere to fall a prey to a carnivorous marauder, 
who in his turn may be the vidtim of a stronger individual ; 
and the successive appropriations may go through any 
number of steps. Thus the carbon and nitrogen of forms 
of animal life now fossil have been also derived from the 
atmosphere. We do not find much, if indeed any, of this 
carbon in its original form now, or diredlly traceable to ani- 
mal agency, because highly nitrogenous organic substances 
decay very rapidly, but it is not unlikely that their results 
are to be seen in carbonaceous and bituminous shales, and 
oleiferous rocks such as those in the neighbourhood of 
petroleum springs; for, as Dr. Sterry Hunt remarks, since 
animal tissues contain the elements of cellulose, plus water 
and ammonia, they may give rise to similar hydrocarbon- 
aceous bodies to those derived from vegetable substances.* 
In many cases, also, the decomposition of these animal 
tissues would result in the formation of carbonates, so that 
on the whole there must be through this source a vast 
* Chem. and Geol. Essays. “ On Bitumens and Pyroschists,” p. 179. 
