12 



or consider the action to be entirely dependant on the altered 

 physical state which clay aft^er burning presents. Of recent 

 writers on the subject, Professor Johnston, however, entertains 

 much more rational and wider views than any of his predecessors. 

 Giving all due importance to the mechanical effects of burning 

 upon clay, Professor Johnston, in an excellent paper in his 

 Experimental Agriculture, p. 254, shows that the mechanical 

 effects of burning upon a clay are insufficient to explain the bene- 

 ficial action of burnt clay, and demonstrates experimentally that 

 the chemical changes produced in burning are of even a greater 

 importance than the mechanical. " These chemical changes, 1 ' the 

 learned Professor says, " are of such a kind as to render the 

 constituents of the clay more soluble that is, soluble to a greater 

 extent than before the burning both in water and in acids." He 

 further found by analysis, that by over-burning, new changes 

 among the constituents of the clay take place, by which they are 

 again rendered less soluble than when slightly burnt. The soluble 

 matter consisted of potash, soda, lime, magnesia, chlorine, sulphuric 

 acid, silica. The relative proportion of these substances, however, 

 is not stated in the above-mentioned work, from which I infer 

 that the nature of the soluble matters has been examined by 

 Professor Johnston only qualitatively. 



By my own experiments. 1 am enabled to confirm Kersten's and 

 Johnston's observations of the greater solubility of burnt clay, 

 and the observation of Johnston, that clay again becomes less 

 soluble on over-burning. In addition to this, the quantitative 

 analyses of a clay, burnt in three different modes, has given me 

 results which will throw considerable light on the causes of the 

 action of burnt clay. The mere fact that clay becomes more 

 soluble in water and acids, appears to me insufficient to explain the 

 beneficial effects of burnt clay ; for it is quite possible that alumina, 

 oxide of iron, or any other unimportant element of clay, of which 

 most soils contain already sufficient quantities, is rendered soluble. 

 It is evident that the greater solubility of any of such substances 

 would increase the quantity of soluble matter, without adding 

 anything to the aggregate fertilising effects of burnt clay. Pure 

 pipe-clay, slightly burnt, is indeed more soluble in acids than 

 the unburntclay. Sulphuric acid decomposes moderately burnt clay, 

 and dissolves, on boiling, all alumina, leaving the silica, with which 

 the latter was combined, behind, whereas concentrated sulphuric 

 acid has but little action on clay in the unburnt state. The greater 

 solubility of burnt clay in itself, then, is insufficient to account for 

 the effects of burnt clay. But this objection stands no longer in 

 our way, since we are in a position to show that, in burning, one of 

 the most important fertilising substances which is found in clay, if 

 not the most important of all, is rendered more soluble.* 



* Want of space obliges us to postpone the remainder of this interesting paper to 



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