On the Causes of the Efficacy of Burnt Clay. 505 



position of agricultural clays, and pointed out those substances in 

 clays on which their fertilizing properties chiefly depend. 



Clays generally result from the disintegration and degradation 

 of gratutic and felspathic rocks. Felspar, a mineral composed of 

 silicate of potash or soda and silicate of alumina, exposed for a 

 long time to the united action of the atmosphere and water, 

 suffers a gradual decomposition, and falls altogether to powder. 

 Sdicate of potash, a soluble salt, is worked out by the rain 

 falling on the decomposed rock, and converted in its turn, by the 

 carbonic acid of the atmosphere, into carbonate of potash and 

 silica, which remains behind in a gelatinous or soluble state with 

 insoluble silicate of alumina, the chief constituent of clays. The 

 analyses of different samples of agricultural clays exhibit many 

 differences in their composition, and show that agricultural clays 

 are never pure silicate of alumina, but mixtures of pure clay 

 (silicate of alumina) with more or less sand, undecomposed frag- 

 ments of felspar, mica, granite, and other minerals, carbonate 

 of lime, magnesia, free or uncombined alumina, oxide of iron, 

 soluble silicate of potash, traces of phosphoric and sulphuric acid 

 and chlorine. In some clays lime occurs in so large quantities, 

 that on this account they cannot be used for the making of tiles. 

 Of such a kind is Bradford clay, from the neighbourhood of 

 Cirencester, in which I have found no less than 19*92 per cent, 

 of carbonate of lime. In others little lime, but considerable 

 quantities of free alumina and oxide of iron are found. To this 

 kind of clays belongs an agricultural clay, the analysis of which 

 I subjoin, in order to show its complex composition : — 



Analysis of Clay from Huntstile Farm^ near Bridgewater. 



Water of combination, and a little organic matter . . 3*38 



Insoluble siliceous matter (fine clay) . . . . 54'89 



Finely-divided silica (soluble in dilute caustic potash) . 17*94 



Oxides of iron ........ 8*82 



Alumina 6*67 



Lime . . ] '44 



Magnesia 0*92 



Phosphoric acid . . . . . . . 0'.51 



Potash 1-48 



Soda , 1-08 



Traces of sulphuric acid and chlorine, carbonic acid, 



and loss 2*87 



100*00 



On these foreign admixtures, which all agricultural clays con- 

 tain, and the state of division of the sand and other constituents, the 

 plasticity, tenacity, porosity, and other physical characters depend. 

 In an examination of an agricultural clay, therefore, not merely its 

 ultimate chemical composition should be ascertained, but likewise 



