444 
Waste . 
[August, 
the same property of preventing the escape of the low- 
tension heat-rays as does carbonic acid, frost would be un- 
known outside the polar regions, and the Ardtic ice-cap 
would not be able to send us cold currents of air and water 
to the ruin of our crops. The attributes of our atmosphere 
form, then, an additional source of the waste of heat. 
There is another process by which the soil is locally 
chilled, even if the total sum of heat in our globe is not 
reduced. If drops of water, such as rain, fall upon a porous 
soil, they sink in and draw with them mechanically a por- 
tion of air which becomes condensed in the soil, and by this 
means, as well as by the chemical adtion of the oxygen 
present, raises the temperature of such soil. But if imme- 
diately after the rainfall a strong wind arises, as is very fre- 
quently the case, this process is very gravely interfered with. 
The stronger and drier the wind, the more of the water 
which has fallen, instead of sinking into the soil, evaporates 
from the surface. For its conversion into vapour a quantity 
of heat, taken from the soil, is rendered latent, and thus 
vegetation is chilled and more or less injured. Every expe- 
• rienced gardener, when working with plants under glass, 
and consequently completely or nearly so under his control, 
takes good care immediately after watering to shut the frame 
or the house up close, so as to minimise evaporation and 
give the water time to disappear from the surface of the soil 
by absorption. 
To carry out this method of working in Nature would, 
however, involve a serious difficulty. For rain is due to 
watery vapour becoming condensed into the liquid state and 
falling to the earth. By condensation it necessarily occupies 
less room and a partial vacuum, or at least, a reduction of 
pressure being thus created, air rushes in from other districts 
to restore the equilibrium. Hence the prevalence of wind 
after rain, ruinous to vegetation as experience no less than 
theory proves it to be, seems, according to our present know- 
ledge, a physical necessity. 
We come now to some points connected with the behaviour 
of water when in the solid state, as ice and snow, which in- 
volve no inconsiderable waste of heat. When these sub- 
stances melt an enormous amount of heat is rendered latent, 
being withdrawn from surrounding obje(5ts, from the lower 
stratum of the air, &c. We might at first sight think that 
a pound of ice or of snow at o° C. would be easily, and at 
little cost, transformed into a pound of water at the same 
temperature. Not so : as much heat is required as would 
raise a pound of water from o° to 8o° C. It may be argued, 
