34 SOILS AND MANURES 



The analysis of the smallest particles given above is the 

 mean of several samples of soil, all of which gave very 

 similar results. This material prohahly consists of about 

 75 per cent, of kaolin mixed with about 25 per cent, of 

 other minerals, chiefly quartz and oxide of iron. The 

 particles larger than O004 m.m. diameter evidently con- 

 sist mainly of undecomposed minerals, in this case quartz. 

 It appears, therefore, that in chemical composition sand 

 and clay may differ widely or they may be much alike. 

 The characteristic of the one is grittiness, and of the other 

 impalpability, or, when wet, plasticity. These properties 

 have reference to the sense of touch, and it is impossible 

 to fix upon any precise limit of demarcation. As the 

 particles become finer the material loses the character 

 of sand and takes on that of clay. Whatever the minimum 

 limit of size of the particles may be for sand, that will 

 be the maximum limit for clay. It has been suggested 

 that this limit will generally be recognised somewhere 

 about 0*025 m.m. diameter. The finest particles of clay 

 are indistinguishable under ordinary powers of the micro- 

 scope, and it is not known what the ultimate limit of 

 division may be. It has recently been discovered, however, 

 that some, hitherto indistinguishable, can be rendered 

 visible by the use of stains or dyes, and, by using high 

 power objectives, with oil immersion, particles of O'OOOlm.m. 

 diameter have been successfully measured. The average 

 diameter of the particles in the finest group that can be 

 separated by processes of elutriation is about 0'002 m.m. 



The coherence, plasticity and other characteristic pro- 

 perties of clay depend mainly upon the proportion of the 

 more minute particles, and, to some extent, upon the pre- 

 sence of gelatinous hydrates of iron, alumina and silica 

 and colloidal organic compounds. 



In the sense of mass, clay is a light substance ; its true 

 density is only 2*5, whereas that of quartz is 2*62, and 



