9 



porous after burning, they suppose absorbs more ammonia, and 

 acts, consequently, more beneficial on vegetation than unburnt. 



At first Sprengel attributed the effects of burnt clay to the cir- 

 cumstance that, in burning, the protoxide of iron, existing in many 

 natural clays, is changed into peroxide, which he considers to be 

 more beneficial to vegetation than the protoxide. SprengeFs 

 second explanation entirely contradicts this statement, inasmuch 

 as, according to it, during the burning process the peroxide of iron 

 in clays is changed into protoxide to which now he ascribes the 

 greatest importance, as being the chief agent in the formation of 

 ammonia in burnt clay ; for, in slightly burnt clay, protoxide of 

 iron is always present ; and, as it has been ascertained by Haus- 

 mann and others, ammonia is formed when protoxide of iron, 

 moistened with water, is kept in contact with nitrogen. Sprengel 

 explains the beneficial action of burnt clay by the formation of 

 ammonia, which is generated in it in the following manner : The 

 protoxide of iron, of which burnt clay usually contains more than 

 unburnt clay, when exposed to the atmosphere in a moist state, is 

 converted into peroxide by the oxygen of the water ; the hydrogen 

 of the decomposed water, in the moment of its liberation, unites 

 with the free nitrogen of the atmosphere to ammonia, which is 

 retained by the humic acids present in all cultivated plants. 

 According to SprengeFs views, the more protoxide of iron clay 

 after burning contains, the more certain it will appear in its effects, 

 because more ammonia will be formed : burnt clay will cease to 

 exhibit the same fertilising effects when all the protoxide of iron 

 has become changed by oxidation into peroxide, because then no 

 more ammonia can be formed. This is the case in overburnt clay, 

 which contains peroxide of iron only, and no protoxide : overburnt 

 clay thus exercises no beneficial effects on vegetation, because no 

 ammonia is formed in it on exposure to the air. 



So much for SprengeFs theory. The ammonia, then, which in 

 burnt clay is formed during the oxidation of the protoxide of iron, 

 Sprengel considers as the chief cause of action of burnt clay; at 

 the same time he ascribes to the necessary constituents of clay, as 

 potash, soda, lime, magnesia, &c., some influence in promoting the 

 growth of plants ; and agrees likewise with Lampadius^s opinion, 

 that humate of alumina, which he considers an important sub- 

 stance in the vegetable processes, is more readily produced in burnt 

 clay than in unburnt. 



The recent analyses of the ashes of most cultivated plants have 

 shown the entire absence of alumina in plants ; Lampadius and 

 SprengeFs theory concerning the action of humate of alumina, 

 therefore, falls to the ground. That ammonia exists in burnt clay 

 Sprengel has demonstrated, by heating clay, free from organic 

 matters; exposing the same, in a moist state, for three days to the 

 atmosphere; and after that time heating the clay in a retort, to 

 which a receiver was attached, containing water acidulated with 



