THE ATMOSPHERE AS RELATED TO VEGETATION. 101 
stantly, bubbles begin to escape rapidly from the bottom 
of the tube through the water of the wine-glass, thus 
See that hydrogen passes into the cup faster 
than air can escape outwards 
through its pores. If the bell be 
removed, the cup is at once bathed 
again externally in common air, the 
light hydrogen floating instantly 
upwards, and now the water begins 
to rise in the tube in consequence of 
the return to the outer atmosphere 
of the hydrogen which before had 
diffused into the cup. 
It is the perpetual action of this 
diffusive tendency which maintains 
the atmosphere in a state of such 
uniform mixture that accurate ana- 
lyses of it give for oxygen and 
nitrogen almost identical figures, at 
all t' mes of the day, at all seasons, 
all altitudes, and all situations, ex- 
cept near the central surface of 
large bodies of still water. Here, 
the fact that oxygen is more largely 
absorbed by water than nitrogen,. 
diminishes by a minute amount the 
Fig. 7. usual proportion of the former gas, 
If in a limited volume of a mixture of several gases a 
solid or liquid body be placed, which is capable of chemic-, 
ally uniting with, or otherwise destroying the aeriform 
condition of one of the gases, it will at once absorb those 
particles of this gas which lie in its immediate vicinity, 
and thus disturb the uniformity of the remaining mixture, 
Uniformity at once tends to be restored by diffusion of a 
portion of the unabsorbed gas into the space that has been 
deprived of it, and thus the absorption and the diffusion 
