in the Air upon the Temperature of the Ground. 27 i 



removed from the atmosphere in all times, namely the 

 chemical weathering of siliceous minerals, is of the same 

 order of magnitude as a process of contrary effect, which is 

 caused by the industrial development of our time, and which 

 must be couceived of as being of a temporary nature. 



" In comparison with the quantity of carbonic acid which 

 is fixed in limestone (and other carbonates), the carbonic acid 

 of the air vanishes. With regard to the thickness of sedi- 

 mentary formations and the great part of them that is formed 

 by limestone and other carbonates, it seems not improbable 

 that the total quantity of carbonates would cover the whole 

 earth's surface to a height of hundreds of metres. If we 

 assume 100 metres, — a number that may be inexact in a high 

 degree, but probably is underestimated, — we find that about 

 25,000 times as much carbonic acid is fixed to lime in the 

 sedimentary formations as exists free in the air. Every 

 molecule of carbonic acid in this mass of limestone has, 

 however, existed in and passed through the atmosphere in 

 the course of time. Although we neglect all other factors 

 which may have influenced the quantity of carbonic acid in 

 the air, this number lends but very slight probability to the 

 hypothesis, that this quantity should in former geological 

 epochs have changed within limits which do not differ much 

 from the present amount. As the process of weathering 

 has consumed quantities of carbonic acid many thousand 

 times greater than the amount now disposable in the air, 

 and as this process from different geographical, climato- 

 logical and other causes has in all likelihood proceeded with 

 very different intensity at different epochs, the probability 

 of important variations in the quantity of carbonic acid 

 seems to be very great, even if we take into account the com- 

 pensating processes which, as we shall see in what follows, 

 are called forth as soon as, for one reason or another, 

 the production or consumption of carbonic acid tends to 

 displace the equilibrium to any considerable degree. One 

 often hears the opinion expressed, that the quantity of 

 carbonic acid in the air ought to have been very much 

 greater formerly than now, and that the diminution should 

 arise from the circumstance that carbonic acid has been taken 

 from the air and stored in the earth's crust in the form of 

 coal and carbonates. In many cases this hypothetical dimi- 

 nution is ascribed only to the formation of coal, whilst the 

 much more important formation of carbonates is wholly over- 

 looked. This whole method of reasoning on a continuous 

 diminution of the carbonic acid in the air loses all foundation 

 in fact, notwithstanding that enormous quantities of carbonic 



U2 



