AGRICULTURAL CHEMISTRY. 163 



the snow. In our latitude every one has had opportunity to observe 

 that snow thaws most rapidly when covered by or lying on black 

 earth. The reason is that snow absorbs heat of low intensity with 

 greatest facility. The heat of the sun is converted from a high to a 

 low intensity by being absorbed and then radiated by the black mate- 

 rial. But it is not color that detejjmines this difference of absorptive 

 p*ower, for indigo and Prussian blue, though of nearly the same color, 

 have very different absorptive powers. So far, however, as our 

 observations extend, it appears that usually, dark colored soils absorb 

 heat most rapidly, and that the sun' s rays have least effect on light 

 colored soils. 



2. The degree of moisture present is of great influence on the tem- 

 perature of the soil. All soils when thoroughly wet seem to be nearly 

 alike in their power of absorbing and retaining warmth. The vast 

 quantity of heat needful to gratify the demand of the vapor that is 

 constantly forming, explains this. From this cause the difference in 

 temperature between dry and wet soil may often amount from 10° to 18°. 

 According to the observation of Dickinson, made at Abbot's Hill, 

 Hertfordshire, England, and continued through eight years, 90 pev 

 cent, of the water falling between April 1st .and October 1st, evapo- 

 rates from the surface of the soil, only 10 per cent, finding its way into 

 drains laid three and four feet deep. The total quantity of water that 

 fell during this time, amounted to about 2,900,000 lbs. per acre; of 

 this more than 2,600,000 evaporated from the surface. It has been 

 calculated that to evaporate. artificially this enormous mass of water, 

 more than seventy-five tons of coal must be consumed. 



Thorough draining, by loosening the soil and causing a rapid re- 

 moval from below of the surplus water, has a most decided influence, 

 especially in spring time, in warming the soil and bringing it into a 

 suitable condition for the support of vegetation. 



It is plain then that even if we knew with accuracy what are the 

 physical characters of a surface soil, and if we were able to estimate 

 correctly the influence of these characters on its fertility, still we must 

 investigate those circumstances which affect its wetness or dryness, 

 whether they be an impervious sub -soil, or springs coming to the sur- 

 face, or the amount and frequency of rain-falls, taken in connexion 

 with other meteorological causes. We cannot decide that a clay is 

 too wet or a sand too dry, until we know its situation and the climate 

 it is subjected to. 



The great deserts of the globe do not owe their barrenness to neces- 

 sary poverty of soil, but to meteorological influences — to the continued 

 prevalence of parching winds, and the absence of mountains to con- 

 dense the atmospheric water and establish a system of rivers and 

 streams. This is not the place to enter into a discussion of the causes 

 that may determine or modify climate, but to illustrate the effect that 

 may be produced by means within human control, it may be stated that 

 previous to the year 1821, the French district Provence was a fertile 

 and well-watered region. In 1822, the olive trees which were largely 

 cultivated there were injured by frost, and the inhabitants began to 

 cut them up root and branch. This amounted to clearing off a forest, 



