N. M. Comber 383 



which are slowly hydrolysed on washing. The full discussion of the re- 

 tention of lime by soils is therefore impossible until more is known about 

 these borderline phenomena on the one hand and about the actual con- 

 stitution of the soil colloids on the other. Meanwhile it seems clear that 



(i) The absorption of lime is mainly confined to the colloidal surface. 



(ii) The absorptive power must be largely satisfied before lime causes 

 the abnormal flocculation of clay. 



The partial ignition of soils. 



Ignition and solubility. Quite independent evidence that the particles 

 making up the soil aggregates are bound by the cementing action of their 

 gelatinous surfaces, is found in the experiments on the ignition of soils. 

 The experiments show in the first place that partial ignition normally 

 increases the amounts of iron, aluminium, etc. which are extracted by 

 dilute acid. This increase can only be due to one of two general causes. 

 Either the elements concerned are transferred by ignition to some other 

 chemical combination or to some other physical state of higher solubility 

 in acid, or else the amount of effectively exposed surface is increased. 

 The formation of more soluble compounds is most improbable. It is 

 scarcely likely that ferrous iron, ferric iron, aluminium and phosphorus 

 would all be converted to more soluble compounds, and in any case the 

 formation of more soluble forms of iron and aluminium compounds by 

 ignition is the very opposite of usual experience. 



Ferric hydroxide powder obtained in a variety of ways failed to show 

 any increased solubihty on ignition. WTien, however, the particles of 

 ferric hydroxide were suspended in a suitable solution and precipitated 

 so that they became aggregated by some gelatinous substance such as 

 silica or alumina, the first effect of ignition was to increase the amount of 

 iron dissolving in acid. 



The conclusion is that the increased dissolution is due to the drying 

 up of the colloidal matter, the separation of the particles and the conse- 

 quent exposure of a larger surface to the acid subsequently added. 



This increase in the amount dissolved is not a true increase in 

 solubility. Ignition depresses the solubility of iron, etc. In the early 

 stages of ignition, however, the effect of increased surface more than 

 counteracts that depression of solubiUty. 



Ignition and base absorbing power. The base absorbing power of 

 various soils and subsoils, before and after 5 minutes' ignition, was 

 examined by the Hutchinson-McLennan method. The results recorded in 

 Table IV present three cases for consideration. 



