412 SCIENCE PROGRESS 



development of the classical work of Way and that of Lemberg 

 on the transformation of minerals by contact with salt solutions 

 has much to support it. Armsby as long ago as 1877 pointed 

 out that CaClg and an artificial Na-aluminosilicate may interact 

 to form two slightly soluble substances' — Na- and Ca-alumino- 

 silicates. In such a case the reaction does not proceed to the 

 formation of one of these two substances exclusively, but an 

 equilibrium is reached in which both slightly soluble compounds 

 are present ; a definite relationship then prevails between the 

 concentrations of the reacting substances in the solution : the 

 amounts of different bases absorbed will vary with the solubilities 

 of their respective alumino-silicates, while change in temperature, 

 by altering the relative solubilities, may lead to readjustments 

 in the equilibrium. Such an absorption is in general an 

 exponential function of the concentration, as also is the solubility 

 of alkaline earth sulphates in acids of varying concentration 

 (Ostwald) and the action of KgCOg on BaS04 (Meyerhoffer). 

 This form of curve is not therefore necessarily proof of adsorption, 

 as is frequently supposed by agricultural workers. Such inter- 

 changes were found to occur by Sullivan with many other 

 silicates and alumino-silicates and some soils. Al is replaced 

 by the base of a neutral salt : and this accords with Veitch's 

 conclusion that an interchange of bases occurs between the 

 neutral salt and the hydrated neutral silicates or alumino- 

 silicates of the soil by which Al is brought into solution, the 

 acidity developed being due to the hydrolysis of the Al salt. 

 Daikuhara, Rice, and more recently Knight came to somewhat 

 similar conclusions, and this aspect has been further developed 

 by the work of Spurway on the " hydro lytic ratio " of soils and 

 that of Hartwell and Pember on the part played by the Al ion 

 in the toxicity of acid soils. This, however, is probably not 

 the whole story, and the very complexity of the soil militates 

 against any single factor being the sole cause of soil acidity. 



(D) Mmeral Acid Theory 



A fourth theory proposed by O. Loew, and later strongly 

 supported by Truog, attributes the acidity of certain Porto 

 Rican clay soils deficient in humus to an alumino-silicic acid in 

 the clay which he calls " argillic acid," and to which he gives 

 the formula (->. 



/^\ 



/ \ 



HO— Si— O— Al— OH 



(a) >0 (6) 



HO— Si— O— Al— OH 



\ / 



\o/ 



