390 VETEEINAEY HYGIENE 



foundation such as concrete. A building owing to its higher 

 temperature acts as a suction pump to the soil on which it 

 stands, and impurities both from below and laterally may 

 be drawn in. 



This is particularly the case in the matter of ' made 

 soils ' such as are frequently met with in towns and cities. 

 A made soil is produced by filling in excavations, or 

 levelling uneven ground, with the rubbish of the neigh- 

 bourhood. Practically anything and everything is employed 

 for this purpose, the land being afterwards used for building 

 purposes. Observations have shown that it takes at least 

 three years for most animal and vegetable debris to dis- 

 appear in the soil. 



Soil Temperature. — There are some soils like sand which 

 are hot, others like clay which are cold. Soil temperature 

 depends upon the capability of the soil for absorbing heat. 

 The power of the soil to absorb heat depends largely on its 

 colour, being least for light-coloured soils, and greatest for 

 dark soils. In the deep layers of the soil the heat pene- 

 trates slowly, so much so that it may be many weeks after 

 the maximum temperature of the air has been reached, 

 before that of the deep layers of the soil is arrived at. 

 There is a daily and an annual variation of soil tempera- 

 ture, the former in the United Kingdom cannot be deter- 

 mined at a greater depth than 4 feet, while the annual 

 variation ceases to be felt at 40 feet from the surface. 

 Below 40 feet the temperature of the soil increases 1° 

 Fahrenheit for every additional 50 feet of depth. 



The radiation of heat from soils depends to an extent on 

 their capacity tor absorbing it, but also on their colour and 

 the degree of vegetation covering it. The more vegetation 

 the more rapid the cooling ; the darker the soil the longer 

 it holds its heat. Soils, however, always cool quicker than 

 they heat. The colour of dark soils is due to humus, 

 while the red, yellow, and brown soils are coloured by 

 ferric oxide. 



The Micro-organisms foimd in the Soil have been the sub- 

 ject of considerable research. Some of them of service to 



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