192 
water from the influence of the atmosphere, the 
temperature of which in northern winters is usually 
very much below the freezing point ; and this 
water becomes the first nourishment of the plant 
in early spring. The expansion of water during 
its congelation, at which time its volume increases 
-j3l, and its contraction of bulk during a thaw, tend 
to pulverise the soil ; to separate its parts from each 
other, and to make it more permeable to the influ- 
ence of the air. 
If a solution of lime in water be exposed to the 
air, a pellicle will speedily form upon it, and a solid 
matter will gradually fall to the bottom of the water, 
and in a certain time the water will become taste- 
less ; this is owing to the combination of the lime, 
which was dissolved in the water, with carbonic 
acid gas which existed in the atmosphere, as may 
be proved by collecting the film and the solid mat- 
ter, and igniting them strongly in a little tube of 
platina or iron ; they will give off carbonic acid 
gas, and will become quicklime, which, added to 
the same water, will again bring it to the state of 
lime-water. 
The quantity of carbonic acid gas in the atmo- 
sphere is very small. It is not easy to determine it 
with precision, and it must differ in different situa- 
tions ; but where there is a free circulation of air, 
it is probably never more than nor less than 
^ of the volume of air. Carbonic acid gas is 
nearly * heavier than the other elastic parts of 
the atmosphere in their mixed state : hence, at first 
view, it might be supposed that it would be most 
abundant in the lower regions of the atmosphere ; 
