1 877.] in its Geological Relations . 461 
proportion of carbonic acid is returned to the atmosphere* 
And that there must he, and always has been, this constant 
circulation of carbon between the earth and the atmosphere 
is self-evident. What time it originated must he beyond 
our ken, hut so far back as we have any knowledge of there 
are evidences in the rocks of vegetable or animal life. And 
the decomposition of such carbonaceous matters, whether 
at the surface, immediately after death, or whilst buried 
under a depth of strata, — as in the case of coal-seams, — has 
always yielded carbonic acid to the atmosphere. At the 
same time the carbon returned in this way falls far short of 
what has been abstracted. But, as Bischof points out, the 
carbon aCts as a carrier of oxygen between the mineral king- 
dom and the air. 
Formerly Greater Abundance of Atmospheric Carbonic Acid . 
It has long been considered probable that in remote ages 
the proportion of carbonic acid was greater than it now is, 
more especially during the Carboniferous period. The re- 
markable luxuriance of vegetation of a tropical facies during 
that era, in every part of the globe, — even the polar regions, 
— indicates a very warm climate universally, and it is also 
thought to imply a much larger supply of carbonic acid 
than is now noticeable in the atmosphere. The rarity of 
warm-blooded animals has been pointed to as corroboration 
of this view ; but strictly this is only negative evidence, the 
absence of fossil forms affording no proof as to the non- 
existence in bygone time of animals of any particular type. 
However, a very curious faCt bearing on the question has 
resulted from Prof. Tyndall’s researches on radiant heat. 
It appears that a very small addition of carbonic acid to air 
renders it absorptive and retentive of radiant heat, and a 
slight increase in the percentage of carbonic acid in the 
atmosphere would have a very distinct result. The visible 
rays of the sun could pass through the atmosphere to the 
earth ; but the radiant heat from the earth, instead of being 
dissipated into space, would be imprisoned by the atmo- 
sphere, which would thus form a warm envelope around the 
earth, converting it in faCt into an immense greenhouse. 
The glass roof of a conservatory aCts in precisely the same 
way: it permits the solar rays to penetrate freely, but absorbs 
and cuts off the escape of the radiant heat, and the interior tem- 
perature is thereby rendered tropical. Granting, then, the 
former abundance of carbonic acid, the extreme richness of 
the carboniferous vegetation, its tropical character and wide 
