CONDUCTIVITY OF SOIL CONSTITUENTS 171 



of a powder of similar composition is well shown by the ex- 

 periments made by Forbes and Thomson on the conductivity 

 of the sand of the Experimental Garden at Edinburgh, and of 

 the sandstone of the Craigleith Quarry ; the latter was found 

 to conduct heat about four times better than the former. 



Illustrations of what has just been said will be found in 

 the statement of Pott's results already given. It will be seen 

 that the compression of a dry powder increased its power 

 of conducting heat. The coarse sand conducted better than 

 the fine sand. A mixture of equal bulks of stones and clay 

 conducted heat much better than clay alone. 



The circumstance having the greatest effect on the con- 

 ductivity of sand, clay, and chalk was, however, the presence 

 of water. Water is not itself a good conductor of heat ; it 

 is indeed in this respect inferior to the solid constituents of 

 soil, but it is a much better conductor of heat than air; the 

 displacement of air by water thus serves greatly to facilitate 

 the transmission of heat. The dry quartz powder had its 

 conductivity doubled when thoroughly wetted. The presence 

 of a little water is sufficient to produce a large effect in this 

 direction. The moist sand contained only 9-9 per cent, of 

 its volume of water; its conductivity was, however, 74 per 

 cent, greater than that of the dry sand. The presence of 

 much water is probably not favourable to the propagation 

 of temperature, owing to the high specific heat which water 

 possesses. It was perhaps for this reason that Pott found 

 that wet peat was apparently a much worse conductor of heat 

 than wet chalk, clay, or sand. 



It appears from the facts now mentioned, that a fine, dry, 

 loose soil is the one which conducts heat worst ; such a soil will 

 have its surface much heated by the sun's rays, but the heat 



