H4 SCIENCE PROGRESS 



or even in any simple proportion. This complex is decomposable 

 by change in temperature, concentration, etc. , but the process 

 is continuous and does not show the per saltern character of 

 chemical change ; the complex can be acted upon by various 

 solutions (as for example added fertilisers), absorbing certain 

 substances as a whole, e.g. organic substances, or simply giving 

 up to the solution an amount of base equivalent to what it has 

 absorbed. These changes, he has shown, follow the ordinary 

 laws of absorption of colloids [2, 3]. On this view the con- 

 centration of the soil solution directly depends on the com- 

 position of the colloidal complex and may be altered within 

 limits by addition of soluble salts. 



Thus we are up against a question of fact — whether the soil 

 solution is or is not constant in composition in the case of 

 different soils. Unfortunately the difficulty of extracting the 

 soil solution has not been satisfactorily overcome and recourse 

 must be had to devices which give its composition only approxi- 

 mately. If we suppose that the water of a well represents the 

 soil solution then there is clearly no sort of constancy of 

 composition in passing from soil to soil, as can be seen by 

 looking through the numerous recorded analyses. If we 

 suppose that the drainage water from the top layer of soil, 

 which is rather different in composition from well v/ater, more 

 nearly represents the soil solution we again find no evidence of 

 constancy, although in the case of a particular field the drainage 

 water may show but little variation throughout the whole 

 season. 1 Unfortunately sufficient data are not yet collected. 

 But perhaps it is fairest in this discussion to accept Whitney's 

 method and to analyse the figures obtained by Taylor and 

 Mooney on which he based his statements. Considerable 

 variations are found ; the highest and lowest values are : 



Highest. Lowest. 



Phosphoric acid 40*60 0*59 



Nitric acid 62*00 trace 



Calcium 102*85 trace 



Potassium 62*20 493 



1 Von Seelhorst {Journal fiir Landwirtschaft 1901, 49, 251) made complete 

 analyses, at first each week and afterwards each fortnight, of drainage water 

 running from a field during the year in August 1899 to August 1900. The results 

 in parts per million are : 



CaO MgO SO s K 2 NaO, 



Highest figures . 184 46*4 59*2 37 8*2 



Lowest figures . 157 31*3 435 17 i*o 



