Oil the Causes of the Efficacy of Burnt Clay. 



499 



such soils. The mechanical effects of burnt clay^ by which the 

 physical condition of soils is altered so materially, are therefore 

 very important, and by no means to be undervalued in an inquiry 

 into the causes of the efficacy of burnt clay. The beneficial 

 effects of burnt clay are ascribed by many chemists and agricul- 

 tural writers entirely to its greater porosity, or rather to ammonia, 

 which, according to their views, is absorbed from the atmosphere 

 by burnt clay more extensively than by clay in its natural state. 

 In order to put this theory to the test, I made the following ex- 

 periments with — 



1. Clay, from Huntstile, near Bridgewater, in its natural state. 



2. Clay, from the same locality, moderately burnt. 



Both portions were moistened with water, and exposed in 

 glass beakers to the atmosphere for a period of two months and 

 twelve days, without, however, renewing the evaporated water. 

 After that period the quantity of ammonia in each sample was 

 determined by combustion with soda-lime in the usual manner. 

 The following are the results : — 



1. Clay, from Huntstile, in its natural state, 239 "15 grains, on com- 

 bustion furnished 4-94 grains of bi-chloride of platinum and ammonium ; or 



100 parts of air-dry clay contained 0*240 per cent, of ammonia 

 (NH„0). 



2. Clay from Huntstile, moderately burnt, 210*15 grains, on combustion 

 gave 0*36 of bi-chloride of platinum and ammonium ; or 



100 parts of air-dry clay contained 0-019 of ammonia (NH^, O). 



The clay when unburnt, it will be observed, furnished a much 

 larger quantity of ammonia than the same clay after moderate 

 calcination. We must, however, not conclude that unburnt clay 

 possesses a greater power of absorbing ammonia from the atmos- 

 phere, for the ammonia obtained in the analysis is partly the 

 result of the decomposition of nitrogenised organic matters 

 which were present in the clay, and which were destroyed on 

 burning. 



At all events, the above analyses show that unburnt clay con- 

 tains ammonia, or the elements from which ammonia is formed, 

 in larger quantities than burnt clay. The quantity of ammonia 

 absorbed by burnt clay besides is so inconsiderable that it cannot 

 be justly regarded as likely to influence in a sensible degree the 

 fertdity of the sod to which burnt clay is applied. For that 

 reason I cannot attach much value to the ammonia theory. 



Sprengel ascribes the beneficial action of burnt clay to am- 

 monia, which he says is formed in burnt clay under the influence 

 of protoxide of iron by the decomposition of water and the 

 nitrogen of the atmosphere. Burnt clay, he thinks, will lose its 

 effects as soon as the protoxide of iron is changed into peroxide, 

 because then the further decomposition of the water and the 



2 k2 



