Action of the AtmospJierc upon neiolij-dcepened Soil. 419 



in the country, and is in fact an aggregation of minute crystals 

 and earthy matter ; in each pebble and fragment of undecom- 

 posed stone these crystals are interlaced in all directions. Bodies 

 of this nature exposed to the air and moisture absorb water, which 

 enters into the interstices and crevices between the faces of these 

 crystals, or into the pores of the raw clots of earth ; should frost 

 then take place, this contained water changes into ice, and in so 

 doing, by a well-known law, ex})ands with irresistible force, 

 shivering the substance in which it is lodged into a number of 

 particles, which number is measured by the quantity of crevices 

 or })ores into which the moisture and frost have penetrated : so 

 long as the frost lasts these particles are all bound together by 

 the enveloping ice, but immediately when tliaw comes they 

 separate and fall down ; then the lump which before showed but 

 one surface to the air has now a multitude of surfaces, and the 

 atmosphere acts upon it in an infinitely multiplied degree. It 

 will be shown that the property of condensing some* important 

 gases within a porous body is in proportion to the extent of 

 superficies that the gas can meet with : the comminution or 

 crumbling down of the particles of the soil is therefore, even in 

 this respect alone, an important feature ; the earthy salts of the 

 soil are also thereby more extensively exposed to the action of 

 the atmospheric influences in bringing about beneficial changes 

 in their constitution. The practical importance of this is well 

 known to farmers, who, in preparing their stubbles for the next 

 season's crop of turnips, or such-like roots, always endeavour to 

 get as great a breadth of land well turned over before the numth 

 of December as they possibly can, in order that it may get the 

 full benefit of the winter's frost. 



In clear koen weather during the winter months, when the 

 thermometer in the shade may be osclUating between i\0^ and 

 40^, the skin of the soil, where it is not much covered with snow, 

 will undergo the alternation of frost and thaw almost every four- 

 and-twenty hours ; for althougli the register thermometer may 

 show only a minimum of say 35°, or 3" aljove freezing, yet from 

 the effects of radiation and evaporation the soil will jirobably fall 

 during the night to at least SO"^, or 2'' below it ; and although the 

 maximum air-temperature in the shade may not rise above the 

 freezing point all day, yet the surface of the ground, exposed 

 freely to the effect of any sunshine there may be, will rarely be 

 unaffected bv thaw to a greater or less deptli. The effect of a 

 thin cf)verini; of snow will be, as has been shown, to cause in 

 clear still nights a very intense degree of frost, by reason of radi- 

 ation, even when the thermometer in the air may be hovering 

 about 32'^ V . : the more intense the frost the deeper it will pene- 

 trate, and the greater will be the disintegration. 



