Decembeb 30, 1921] 



SCIENCE 



653 



to enter upon what is perhaps the most 

 promising nest step in the development of 

 the sciences: namely cooperative undertak- 

 ings on a large scale involving chemistry- 

 physics, chemistry-engineering, chemistry- 

 geology, chemistry-biology, and the like. 

 Many of the pressing problems of the im- 

 mediate future are too large for any indi- 

 vidual or for any single department. In this 

 way, on its scientific side, the university may 

 best serve the community. Thus it may 

 better perform the prime function of every 

 true university — the advancement of Jcnowl- 



EdWAED L. ISTlCHOLS 



Cornell IlNrvERSiTT 



THE ORIGIN OF SOIL COLLOIDS AND 



THE REASON FOR THE EXISTENCE 



OF THIS STATE OF MATTERS 



In the mechanical analysis of hundreds of 

 samples of soil by the beaker method, the mi- 

 croscopical control of the subsidence of the 

 clay group indicated that the smallest diam- 

 eter of a clay particle is about 0.0001 mm. 

 while the water from which the sediment 

 subsided was clear and transparent. 



At first thought it would appear that in a 

 soil which has weathered under many agen- 

 cies, such as the grinding of glacial ice, the 

 abrasion of flood waters, the pounding of 

 ocean waves, and other agencies of attrition 

 due to soil movements operating through un- 

 told ages, material of every degree of fineness 

 would accumulate, passing down below the 

 limit of microscopic vision. Practically how- 

 ever this does not appear to be the case as 

 the finest material of the soil, called the clay 

 group, excluding the colloidal material, to be 

 discussed later, ranges in diameter from .005 

 to .0001 mm. The question naturally arises 

 as to what has become of the material of 

 smaller size. 



■ My present view is that particles of matter 

 derived from silicate rocks and other soil- 

 forming minerals when they approach a 

 diameter of .0001 mm. contain relatively so 

 few molecules that the bombardment of the 

 water molecules in which the particle is im- 



mersed shatters the particle beyond the ability 

 of the molecules in the solid to hold together 

 as a solid mass. The atoms of calcium, mag- 

 nesium, ix)tassium and sodium in the mole- 

 cule of the silicate would go for the most 

 part into true solution, while the atoms of 

 silicon, aluminum, and iron would go chiefly 

 into colloidal solution forming the basis of 

 the colloidal matter or the ultra clay of the 

 soil. It should be possible for the mathemat- 

 ical physical chemist, from physical constants 

 now known, to determine empirically the rel- 

 ative size of the particle of matter which 

 could withstand such bombardment without 

 complete disintegration. This is a problem 

 which has not yet been worked out.-"- 



There appears to be a certain equilibrium 

 established between the colloidal state and 

 the truly soluble state as there is always 

 a small proportion of silicon, aluminum, and 

 iron which seem to be in real solution, as 

 they pass through a Pasteur-Chamberland 

 filter and separate out on evaporating the 

 solution not as a colloid but as an amorphous 

 mass of hard scale-like material, like a boiler 

 scale, without absorptive properties. 



It is that portion of the silicon, aluminum, 

 and iron which collects on the outside of the 

 Pasteur-Ohamberland filter in a truly colloid- 

 al condition which is recognized as the 

 ultra clay. 



This colloidal matter is very absorptive and 

 takes into itself a considerable quantity of 

 salts of calcium, magnesium, potassium, and 



1 AnjOther way of looking at this is from the 

 point of view of the internal energy of the system. 

 The molecular attraction between the molecules of 

 the solid and the molecular attraction between 

 water molecules themselves and between the mole- 

 cules of the solid and of the water must come to 

 equilibrium. If the solid particle becomes rela- 

 tively small in diameter there will be relatively few 

 molecules in the solid to hold together against the 

 attraction of the increasing number of water mole- 

 cules surrounding them as the size of the solid par- 

 ticle diminishes. The attraction of water mole- 

 cules for solids which it wets, as, for instance, glass, 

 is seen in the relatively high temperatures and 

 therefore high energies required to remove the last 

 traces of water from the solid. 



