63 
Agricultural Chemistry, would lead us to imagine; for when 
I consider the immensity of those beds which are deposited 
in various parts of the earth's surface, one of which in our 
own little island extends to a depth of some hundreds of 
feet, and occupies an area of several hundreds of square 
miles, we are bound to believe that some appreciable differ- 
ence must have been produced; and particularly so when we 
further consider that every cubic foot of this limestone con- 
tains about 1,000 cubic feet of gas, every particle of which 
must have been extracted from the air. It is true that the 
present quantity of carbonic acid in our atmosphere amounts 
to about 3,867 cubic miles, or, estimated at the mean pres- 
sure on the earth's surface, to about 276 cubic miles; but 
this quantity, large as it may appear, is not greater than 
we find in a little more than one-fourth of a cubic mile of 
limestone. Hence every person conversant with the rudi- 
ments of geology must at once perceive that, in our own 
bed of mountain limestone, we have an amount of carbonic 
acid which, if set free, would increase the quantity in the 
atmosphere very considerably, and, together with that con- 
tained in our chalk, oolites, and coal, would probably increase 
it to five or six times its present quantity. But what is this 
compared with the whole surface of the globe ? and what 
the mere basset edge of limestone, which is exposed to our 
mining operations, compared with that which we have every 
reason to believe extends far beyond our reach ? A mere 
fraction ! Then, I think, we may safely conclude that the 
quantity of carbonic acid which existed in the atmosphere 
prior to these depositions, was at least one hundred times 
greater than at the present day. Admitting this, we may 
then understand, independent of other considerations, why 
the globe, during the long period of its cooling, prior to 
the deposition of the carboniferous system of rocks, was not 
suited for the existence of animal life; and we may also 
