on the Temperature of Soils. 
121 
heat and moisture adapted, so far as their agency is concerned, 
for bringing a given soil, in a given latitude or situation, to its 
maximum state of fertility. The researches of different philo- 
sophers have elucidated the laws which pertain to water, in 
its several states, as a fluid, a solid, and a vapour or steam. 
There is, probably, no natural substance which has been inves- 
tigated with greater success, and there is perhaps no other sub- 
stance which performs more numerous or more important parts 
in its action on soil, and in the economy of vegetable life, than 
water. In its chemical relations to the solid, saline, and gaseous 
constituents of soil there may still be something to discover ; 
but its physical properties as regards heat, its operation as a 
solvent, and its mechanical laws, are sufficiently ascertained to 
enable us to understand, and explain satisfactorily, the various 
benefits that are afforded to wet soils by drainage. 
If a soil be saturated with water, the nobler classes of plants 
cannot flourish ; they vegetate more or less imperfectly, until the 
quantity of water be so diminished as to suit their habits. The 
reduction of the excess of water to the due proportion can only 
be effected, naturally, by its gradual evaporation, i. e. by its 
conversion into vapour ; and its transition from the fluid to the 
aeriform state is accompanied by the absorption of so large a 
quantity of heat from the soil in contact with it, that it may be 
convenient to consider its action in this respect first, and to en- 
deavour to appreciate its amount. 
When water is set over a fire in an open vessel, its temperature, 
as indicated by the thermometer, cannot be made by any force of 
fire to exceed 212°, under the mean atmospheric pressure of about 
30 inches of mercury. The temperature of the water then be- 
comes stationary, and the heat of the fire is afterwards expended 
in converting the water into steam or vapour. The temperature 
of the steam continues to be precisely that of the water, and it 
has been found that it requires about six times as much heat to 
boil off any given volume of water as would raise the temperature 
of that volume from .'lO*^ to 212°. Hence it is concluded that the 
difference, or 162 x 6 = 972 degrees of heat, have passed through 
the water, and entered into the composition of every atom of steam. 
Steam, therefore, has a much greater capacity for heat than water. 
These continual accessions of heat are absorbed by the steam in 
the act of its formation, and become what is termed latent, i. e. 
insensible to the thermometer, which, plunged in the steam, 
marks only the same temperature as that of the water from which 
it was generated, viz., 212°. This latter is termed the sensible or 
thermnmetric heat of the steam. That the whole of the heat thus 
expended in changing water from its fluid into its gaseous state, 
has entered into the steam, is proved, conversely, by condensing a 
