198 Causes of Fertiliti/ or Barrenness of Soils. 
the means so frequently in his own hands, he ridicules the idea 
of making use of them. 
IV. — Warmth or Coldness of Soils. 
The temperature of a soil is a most important condition, since 
vegetation is increased, checked, or actually retarded, according 
to the warmth or coldness of the soil : to be convinced of this 
we have only to compare crops growing on wet undrained clay, 
which we find starved in appearance and yielding a wretchedly 
small produce, with the luxuriant dark-coloured shoots made in 
a dry, well-cultivated soil. Warmth is dependent upon dryness, 
porosity, and colour. Clay soils are cool, even when well drained 
and cultivated ; in a natural state they are very cold. This is 
due to their cohesive property keeping them moist, preventing 
the sun's rays from penetrating, and the evaporation constantly 
going on from their surface. It has been proved that water 
evaporated from the soil extracts for the same volume an equal 
amount of heat as when converted over a fire into steam. When 
we remember that it requires six times as much heat to convert a 
pound of water into steam as to raise the same from 50° to the 
boiling point, we can form some idea of the coldness of a soil 
constantly full of moisture. Water possesses little or no con- 
ducting power, and tlierefore keeps a soil cold by preventing the 
heat descending : it also radiates heat more rapidly than the soil 
itself, and, owing to its peculiar laws of density, may farther 
reduce temperature by changing places with the warmer water 
below, until the whole mass is reduced to 42°. By draining 
we entirely alter all this, converting the water into a carrier of 
heat. The rain-water, often warmer than the soil, especially in 
winter, instead of evaporating from the surface, passes slowly 
downwards, supplying the wants of vegetation and increasing 
the temperature by giving up a portion of its own heat to the 
surrounding soil. If the rain is cooler than the soil, which is 
generally the case in summer, it abstracts heat from the surface, 
giving it up again to the subsoil, and thus equalising tlie tem- 
perature of the whole mass. The experiments of Mr. Parkes, 
already detailed in a foimer number of the Journal, are very 
valuable, as showing the important part free circulation of water 
plays in regulating the temperature of the soil. For the benefit 
of those who may not have the opportunity of studying his 
valuable paper (vol. v., p. 119), I may be pardoned for briefly 
alluding to them. The site chosen was a peat-bog in Lancashire, 
his object to determine the temperature of both surface and sub- 
stratum in the natural semifluid state, and after drainage and 
cultivation. His observations were conducted by means of ther- 
