24 



grs. of clay gave 0-33 grs. of bichloride of platin. and ammonium, 

 or 



100 grs. gave 0.018 per cent of ammonium. 



These quantities of ammonia are nearly identical. Ammonia, 

 accordingly, is not formed, as Sprengel supposes, by the decom- 

 position of water under the influence of protoxide of iron and the 

 atmosphere, in a larger quantity, in which ammonia is absorbed 

 by dry clay from the atmosphere. 



Thus, under no circumstance do we find ammonia in burnt 

 clay in larger quantities than in unburnt clay. The effects of 

 burnt clay, therefore, cannot be explained by the absorption of 

 ammonia only. 



11. What is the reason that burnt clay improves especially 

 root and other green crops as Mr Woodward states ? 



Mr Woodward's observation that root-crops are particularly 

 benefited by burnt clay also receives an easy explanation from 

 the mode of its action, which we have explained ; for we must 

 recollect that turnips, swedes, maugold-wurzel, potatoes, &c., 

 require much potash to mature their growth. 



It may be mentioned, in conclusion, that I have determined the 

 whole amount of alkalies which the clay from Huntstile farm is 

 capable of furnishing, when fused with carbonate of baryta. The 

 quantity of potash and soda present in the clay, for the greater 

 part in a state of insoluble silicates, I find to be : Potash=4*726 

 per cent, and Soda -88 per cent. As one of the characteristics 

 of a clay fit for burning, we have pointed out the undecomposed 

 alkaline silicates which good clays should contain. Thus, finding 

 the proportion of potash and soda so considerable, as in this clay, 

 we are justified in suggesting that this clay is most likely to prove 

 very efficacious after burning. With this theoretical speculation 

 agrees well the fact mentioned by Mr Danger, the tenant of 

 Huntstile farm, that by burning this clay the land is very much 

 improved. Mr Danger says : " Of course I can only speak to 

 the fact. A soil which I have found quite sterile, on which this 

 process has been used, became totally changed" 



Having thus considered each of our propositions separately, 

 and deduced from them what appeared to us the most prominent 

 and legitimate conclusions, we shall conclude by merely recapitu- 

 lating the principal and most practical facts which depend on 

 them, leaving the discriminating reader to form his own opinion 

 of the whole subject. 



Summary. 



1. The mechanical changes produced on clay upon burning, 

 which by no means are unimportant, nevertheless do not suffi- 

 ciently explain the fertilising effects of burnt clay. 



