1 877.] 
465 
in its Geological Relations. 
quantity of carbon — originally drawn from the air — locked 
up in the crust of the earth. And to all must be added the 
immense amount of carbon combined as carbonate of lime 
due to the direct solvent adlion of atmospheric water on 
calcareous rocks and minerals. If we add all this to the 
vegetable carbon already considered, there can hardly be a 
question but that the amount of carbon abstracted from the 
atmosphere and hidden away in our globe very, very far, ex- 
ceeds the proportion present in the air of this age. If this be 
granted — and I cannot see any possible evasion of it — we 
must admit that the more ancient atmospheres contained 
far more carbonic acid than that which now envelopes us, 
and must renounce the dodtrine of Uniformity in this con- 
nection at any rate. 
Origin of Carbonic Acid. 
H aving got so far, we are naturally led to inquire as to 
the origin of the carbonic acid in the first instance. Carbon 
is so thoroughly associated in our minds with organic matter, 
or in fa Cl with life, that it is difficult to conceive the possi- 
bility of its existence in an azoic world, and the difficulty is 
aggravated by the recollection that the earth must have been 
at the beginning in a state of incandescence, not to go fur- 
ther and say a gaseous condition. However, under the 
influence of extreme heat, many elements are isolated which 
at lower degrees of temperature — but still very great — com- 
bine and form chemical compounds. For example, hydrogen 
and oxygen at a high temperature unite to form water, but 
at a still higher are again dissociated ; and we know that 
hydrogen exists in a state of incandescence, not combustion, 
in the sun’s photosphere.* Similarly free carbon might have 
been one of the gaseous constituents of the earth in its 
nebulous phase, t and as the temperature lowered might 
have been consumed, or united with oxygen, and gone to 
form part of the primeval atmosphere. In this way all the 
carbon now in the crust of the earth would necessarily have 
been at first confined to the atmosphere. Then when rains 
began to fall, the carbonic acid, being carried down upon the 
earth, would soon decompose the silicates which must have 
resulted from the cooling down of the original heated mass ; 
carbonates would be formed and carried down into the pri- 
mitive oceans, and clayey residues would be left behind. 
* Prof. Henry Draper has just announced the discovery of oxygen in the 
sun. Nature, August 30, 1877. 
f According to Mr. J. Lawrence Smith, carbon in the gaseous form is 
spectroscopically manifest in the attenuated matter of comets. Am. Journ. 
Sci. 9 June, 1876. 
VOL. VII. (N.S.) 2 K 
