12 HANDY BOOK OF 
entangled with one another, serve to protect the rocks on 
which they grow from the effects of frost from changes 
likewise consequent on the disengagement of carbonic acid 
gas from fissures eyen in granite rocks, as in the neigh- 
bourhood of Clermont, in Auvergne. These changes, 
called by Dolomieu "la maladie du granite," resemble the 
dry-rot in wood, for the hardest blocks become soft, and 
readily crumble in the hand. Where, however, mosses 
assert their empire, the effect is neutralized by their ab- 
sorption of the otherwise injurious carbonic acid, according 
to the well-known fact, that when two gasses of different 
specific gravities are brought into contact, even though the 
heaviest be the lowermost, they soon become uniformly 
diffused, by mutual absorption, through the whole space. 
By virtue of this law the heavy carbonic acid finds its way 
upwards in the lighter air of the atmosphere, and conveys 
nourishment to the tiniest moss or lichen which grows on 
the mountain-top. 
In regions, on the contrary, where devastating torrents of 
rain fall suddenly, their transporting power is counteracted 
by a greater luxuriance of vegetation. A geologist, who 
carefully explored many parts of the tropical regions, 
observes, with reference to the fitness of all plants for the 
places they occupy, that tho softer rocks would speedily be 
washed away in such portions of the globe, if the roots of 
parasitic and creeping plants were not so entwined as to 
present considerable resistance to the direct action of heavy 
rain. Unlike their forest brethren whose giant arms are 
vainly spread forth as if to repel the coming storm, and 
which are often hurled by its fury from stations in which 
they have stood for ages, the plants of which we speak serve 
to shoot off the fast-falling stream, and again lift up 
their heads when returning sunbeams light the dripping 
landscape. Mosses, in like manner, are not affected by 
running water, even if mountain springs assume in winter 
the character of copious floods, and rush impetuously over 
them. 
