in.] WATER LIFTED BY SURFACE TENSION 81 



it ; a certain amount of consolidation has taken place 

 through the addition of the dry earth and the subsequent 

 striking off when wet The numbers in the columns 

 headed Minimum in the table on p. 6g were obtained 

 in this way: they show that though the maximum 

 capacity for water, or pore space, may not vary very 

 greatly for different soils, there is a much wider and 

 more important divergence between the amounts of 

 water they will retain by surface tension alone, this 

 latter being the important factor in judging of the power 

 of the soil to retain a reserve of moisture for crops. 



Variations in Surface Tension. 



The surface tension of water is very high, but it is 

 easily raised or lowered by the presence of small 

 amounts of material in solution. The effect of altering 

 the surface tension of a film at any point is to cause 

 motion, as is seen in the well-known experiment of 

 covering a plate with a thin film of coloured water and 

 dropping a little alcohol into the middle of the film. 

 The alcohol immediately weakens the surface tension of 

 the film in the middle to such an extent that all the 

 liquid runs to the outer edge and leaves the plate bare 

 in the middle. Most of the salts which are used as 

 artificial manures and are soluble in water increase the 

 surface tension of the soil water, hence an application of 

 salt or nitrate of soda may, by increasing the tension of 

 the surface film, lift more water from the subsoil and 

 maintain the top layer of soil in a moister condition. 

 Per contra^ solutions of organic matter, particularly of the 

 many organic substances used as manure which have a 

 little oil in them, extracts of dung, etc., have a surface 

 tension below that of water. To this fact may be 

 attributed the "burning" of soils which is sometimes 



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