Letts & BurakE—The Carbonic Anhydride of the Atmosphere. 159 
alternately or together, and a condition of balance or equilibrium 
ig maintained. 
Regarding the first of these it seems to us that subterranean 
and volcanic action is alone of importance. Cotopaxi, according 
to Boussingault, evolves more carbonic anhydride annually than a 
whole city like Paris, while Lecog has calculated that of the 
mineral springs, those of Auvergne alone give off in the same time 
7,000,000,000 cubic metres of the gas, an amount rather less than 
jo the volume produced by the annual combustion of the coal 
employed throughout the whole of Kurope.! 
Therefore it may be assumed that even at the present time 
considerable quantities of carbonic anhydride are being poured 
into the air from the interior of the earth, while in the past, when 
voleanic action was so much more active, it is only reasonable to 
- suppose that the amount was vastly larger. 
The remarkable theory of Sterry Hunt? supposes another source 
of augmentation in the amount of atmospheric carbonic anhydride. 
According to his calculations, ‘the carbonic anhydride absorbed 
in the process of rock decay during the long geological ages, and 
now represented in the form of carbonates in the earth’s crust, 
must have probably equalled two hundred times the entire volume 
of the present atmosphere. This amount could not, of course, 
exist at any one time in the air: it would at ordinary tempera- 
tures be liquified at the earth’s surface. Whence came this vast 
quantity ?” he asks; and, having dismissed de Beaumont’s hypo- 
thesis, which supposed a reservoir of carbonic andydride in the 
interior of the earth, and also the idea that the gas was of volcanic 
origin, he came to the conclusion that it has reached us from 
celestial sources—a theory which is not new, he says, ‘but was 
put forth by Sir William Grove in 18438, and developed by 
Matthieu Williams in ‘The Fuel of the Sun,’ and has lately 
been noticed by Dr. P. M. Duncan in its geological bearings.” 
Could it be definitely proved that the subterranean sources of 
carbonic anhydride are to be traced to carbonates produced in 
previous ages, as Sterry Hunt believed, then, undoubtedly, colour 
would be given to his theory; but, in the absence of any over- 
1 See Varigny, ‘‘ Air and Life.”’? Smithsonian Miscellaneous Collections. 
2 British Assoc. Reports [1878], p. 544. 
