1 877.] 
in its Geological Relations. 
457 
form, dissolves the lime as bicarbonate, carries it down to 
the ocean, and finally gives it up to become part of the 
skeleton of a coral or mollusc, which in its turn may form 
a portion of an immense mass of limestone rock. 
The atmosphere mainly consists of a mechanical mixture 
of oxygen and nitrogen ; these, however, bear to each other 
an almost constant proportion, any variations being ex- 
tremely minute. The composition by volume is found to be 
as follows : — 
Oxygen 20*80 
Nitrogen ...... JQ'20 
Carbonic acid,* 3 vols to 10 vols. in 10,000 vols. 
Ammonia, a trace; o*i to 135 vols. in 1,000,000. 
Nitric and sulphuric acids, traces occasionally. 
The respective amounts of oxygen and nitrogen do not 
vary to the extent of as much as 1 per cent, even in excep- 
tional cases. Regnault’s analyses of samples of air col- 
lected in various parts of the globe gave very close results, 
the percentage of oxygen being to all intents and purposes 
identical, viz., 20*9 per cent. Air collected by Sir James 
Ross in the ArCtic Regions did not differ in this respeCt from 
that collected at Paris, or at Ecuador in South America ; 
the very slight differences that have been observed not ex- 
ceeding those noticed in air collected at the same place at 
different times : and the same results have been obtained 
from air collected at the summit of Mont Blanc, and even 
from that taken at a height of 21,000 feet by Gay-Lussac 
during a balloon ascent. There is therefore a marked uni- 
formity in the aerial mixture under all circumstances. t 
It has not yet been explained how it is that a mere me- 
chanical mixture should have this constant composition, but 
it is certain that the gases are not chemically combined — 
1. Because the proportion of the constituents bear no 
simple relation to the atomic or combining weights 
of those elements. 
2. When they are mixed in the proper quantities there is 
no contraction, nor is there any evolution of heat, and 
the mixture aCts in every way as air. 
* Stridtly carbonic anhydride ; but I shall use the less scientific but more 
familiar term in this paper to designate it, in accordance with geological 
custom as regards this gas. Indeed in its geological relations it may be 
regarded as a true acid when dissolved in water. 
f From some recent observations by Boussingault, and Miller, it would 
appear the amount of oxygen slightly differs at various heights. Mendeleeff 
thinks Gay-Lussac’s results are probably incorred ( Bull . Sac. Chim . [2], xxv., 
394). However, we have hardly decisive information yet on this point. 
