474 
The Atmosphere Considered 
[Odtober, 
that the nitrogen has been originally drawn from that source. 
We may fitly conclude this part of the subjedt with the 
mention of the native sulphate of ammonium, Mascagnine, 
of which it may be said that every constituent could have 
been obtained from the atmosphere. 
Nitrogen . 
It is obvious that much of what has been said regarding 
ammonia will apply to nitrogen, but on the whole the latter 
in its free state appears to have but little influence as a 
geological agent. 
Sulphuric and Sulphurous Acids . 
The exceedingly minute traces of these acids make but a 
slight effedb on rocks when compared with the gases already 
touched upon. That they are not altogether inert may be 
taken for granted, but both their absorption and re-evolution 
are of a local nature, being chiefly apparent in the neigh- 
bourhood of large towns and about volcanic regions. They 
may he “ withdrawn from circulation ” as sulphates and 
sulphides, and be returned in their original shape, or decom- 
posed into sulphur or sulphuretted hydrogen. 
Variations of Atmospheric Pressure. 
These cannot but have an appreciable effedt on certain 
classes of geological phenomena. The emanations of gases 
from the interior of the earth are influenced in some degree. 
It is well known that explosions in coal-mines sometimes 
follow a sudden fall of the barometer, which can be well 
understood on comparing the pressure corresponding to 
different barometric heights. 
Barometer at 28 inches. Atmospheric pressure 1370 lbs. 
9 9 29 99 99 99 14 I 9 99 
99 3° 99 99 99 14 99 
99 3 i 99 99 99 15*17 9, 
It is usual to refer to the atmospheric pressure as about 
15 pounds on the square inch, but the above table shows 
that a considerable variation makes itself felt within the 
barometrical range. This must not only control evolution 
of gases from coal-seams, but also exhalations from open 
grottoes and caves, mineral springs both thermal and other- 
wise, and probably from intermittent adtive volcanoes, such 
as Stromboli, where the periodical explosion of gases is an 
