COMTOSITIOX OF PORCELAIN AND AG HIC ULT TR/L CLAYS. C3l 



district. The strongest clay soils which are anywhere in culiivation 

 rarely contain more than 35 per cent, of alumina.' 



Soils in general consist in great part of the three substances abpve 

 named in a state of^ mechanical mixture. This is always the case with 

 the siliceous sand and with the carbonate of lime — but in the clays the 

 silica and the alumina are, for the most part, in a state of clieinical com- 

 bination. Thus, if a portion of a stiff clay soil be kneaded or boiled 

 with repeated portions of water till its coherence is entirely destroyed, 

 and if the water, with the finer parts which float in it, be then poured 

 into a second vessel, the whole of the soil will be separated into two por- 

 tions — a fine impalpable powder consisting chiefly of clay, poured off 

 with the water, and a quantity of siliceous or other sand in particles of 

 various sizes, which will remain in the first vessel. This sand was 

 only mechanically mixed with the soil. The fine clay retains still some 

 mechanical admixtures, but consists chiefly of silica and alumina chem- 

 ically combined. 



Of the porcelain clays above alludwd to, there are several varieties, 

 three of which, containing the largest proportion of alumina, co*»Jst res- 

 pectively of — 



100-00 100-00 lOO-Of 

 But, as already stated, these clays rarely form a soil — the stiffest 

 clays treated by the agriculturist containing a further portion of silica, 

 some of which is mechanically mixed, and can be partially separated by 

 mechanical means. 



The strongest agricultural clays {pipe-clays) of which trustworthy 

 analyses have yet been published, consist, in the dry state, of 56 to 62 

 of silica, from 36 to 40 of alumina, 3 or 4 of oxide of iron, and a trace of 

 lime. Clays of this composition are distinguished by the foreign agri- 

 cultural writers as pure clays. They are all probably made up of some 

 of the varieties of porcelain clay, more or less intimately mixed with 

 siliceous and ochrey particles — in so minute a state of division that they 

 cannot be separated by the method of decantation above described. 



These clays are adopted by the German and French writers as a 

 standard to which they can liken clay soils in general, and by compari- 

 son with which they are enabled distinctly to classify and name "them. 

 As the use of the term clay in this sense has been introduced into Eng- 



• In an interesting paper on subsoil ploughing by Mr. H. S. Thompson, in the report of 

 the Yorkshire Agricultural Society for 1837, p. 47, it is stated that the lias clays, which form 

 the subsoil in certain parts of Yorkshire, contain sometimes, in the dry state, as much cw 51 

 per cent, of alumina (?) 



t When heated to redness the whole of U» - water is driven off from these clays, and they 

 then consist respectively of— 



Silica 54-5 574 534 



Alumina 45-5 42 6 46 6 



1000 1000 ¥300 



Which numbers are in accordance with those given at the foot of the preceding page. 



