466 
The Atmosphere Considered 
[October, 
In course of time, when vegetable and animal life had 
made their debut, the withdrawal of the carbonic acid from 
the air must have proceeded much more rapidly, and the 
atmosphere gradually cleared to such a. condition as to per- 
mit of the existence of air-breathing animals. It may be 
here remarked that the very gradual introduction, in more 
recent periods, of warm-blooded beings, would also coincide 
with the hypothesis of the originally highly mephitic state 
of the atmosphere. 
Carbonic Acid now Increasing or Decreasing ? 
An important question now arises — Is the amount of car- 
bonic acid increasing or decreasing, and what may the result 
be in either case ? To begin with the last part of the 
question : — Any considerable difference one way or the other 
must result in a diminution of animal life ; in its higher 
forms in the former event, in all divisions in the latter. 
Beyond a certain proportion very little above the ordinary 
standard — at most io times, equal to about 5 vols. in 1000,* 
or o‘5 per cent ! — carbonic acid in air becomes a deadly 
poison to all warm-blooded animals. On the other hand, a 
diminution in the percentage of carbonic acid would tell 
even more severely. Vegetable life would languish, gramin- 
ivorous animals would eventually have nothing to eat, and, 
finally, the Carnivora, being obliged to prey upon each other, 
would of course become extinft. And this would be appli- 
cable to all divisions of the animal kingdom. The result 
would be a completely barren and desolate planet, perhaps 
in some degree resembling the moon. Doubtless that planet 
has passed through phases of existence alike to those which 
have obtained upon the earth ; and Mr. ProCtorf is of 
opinion that the moon certainly had originally an atmo- 
sphere, which is now either altogether absent or is attenuated 
to an extreme degree. It can well be imagined that this 
result, and its consequent azoic addition, has been brought 
about by some such absorption of the constituents of the 
moon’s atmosphere as that which I have endeavoured to 
sketch out above as regards the earth. 
Probable Withdrawal of Oxygen . — It may seem a little 
paradoxical that such dire effects would more immediately 
follow the withdrawal of a poisonous gas, and that the latter 
is on the whole more important to the continuance of life 
than oxygen gas, which is almost inseparable from our ideas 
* Watts, Chem. Didt., 1862, vol. i., p. 438. 
f Quart. Journ. Science, July, 1874. “ On the Past History of our Moon.” 
