376 The Flocculation of Solh. Ill 



abandoned. After extraction with iV/3 acid the felspar showed a " lime 

 requirement" of 0-01 per cent. CaCOj and the granite showed no "hrae 

 requirement ' measurable by the Hutchinson- McLennan method. 



E. The effect of heat on soil aggregates. 



It has been contended in these papers that the aggregation of soil 

 particles is brought about and maintained by the binding or cementing 

 action of gel material. If this is true, one of the first effects of heating a 

 soil will be tlie dehydration, slirinkage and cracking of the gels and 

 presumably a loosening of the aggregate and a consequent increase in 

 the exposed surface. 



It is well known that heating a soil under certain conditions increases 

 the soluble matter^. The experiments recorded in this section have been 

 made in order to find evidence for or against the view that this increase 

 is in part due to the increased surface exposed by the destruction of the 

 cementing gels. 



1. («) It has been claimed by Fraps^ that after the ignition of a soil 

 for a few minutes over a bunsen flame the amounts of iron, aluminium 

 and phosphorus soluble in dilute IIC'l arc increased. Repetitions of these 

 experiments with a number of English soils abundantly confirm Fraps' 

 results. For example, the following figures show the weights of FegOj and 

 AUOg extracted from 100 gm. London Clay subsoil by 250 c.c. Njiy HCl 

 before and after 5 minutes' ignition. 



(i) In order to test qualitatively the elTect of the partial ignition of 

 soils and other systems, on the solubility of iron in dilute acid, equal 

 amounts of the material under examination were weighed out, and one 

 portion ignited over a bunsen flame for a few minutes. Each portion was 

 then extracted with the same volume of iV/5 HCl for 15 minutes and 

 filtered. 1 c.c. of a standard solution of NH4CNS was added to equal 

 volumes of the filtrates and the colours compared. In this way an 

 increase in the solubility of iron in dilute acid was shown to be brought 

 about by the partial ignition of a large number of soils, sub.soils and clays, 

 and of some synthetic systems to be presently described. 



(c) Experiments similar to the foregoing were made using KgFeCgNg 

 as the reagent, and it was found that a considerable amount of ferrous 

 iron in the acid solution resulted from partial ignition. 



• For n review of the literature see Gu.stapon, Soil Science, 1922, 13. 

 ' Journ. Iml. Hiujin. Chem. 1911, 3; J.C.S. Abstracts, ii. 1912. 



