10 



hydrochloric acid. On evaporation of the liquid in the receiver, 

 distinct crystals of sal-ammoniac were left behind in the basin in 

 which the liquid was evaporated. The repetition of this experi- 

 ment gave me the same result. However, Sprengel erred in think- 

 ing that ammonia is formed only in those clays which contain 

 protoxide of iron, for I have found that in clay which contains not 

 a trace of protoxide of iron, ammonia is found after exposure to 

 the atmosphere for some time. It cannot be denied that protoxide 

 of iron, in contact with moisture and atmospheric air, gives rise to 

 the formation of ammonia; but the proof that in burnt clay a 



freater quantity of ammonia is found, because it contains, as 

 prengel supposes, more protoxide of iron, he has not furnished ; 

 and his theory loses much in probability, by the consideration 

 that ammonia is found in clay containing only peroxide of iron ; 

 and further, that certain blue clays, in their natural state, contain 

 protoxide of iron in preference, with but little peroxide. If it was 

 true that the ammonia is the chief cause of the effects of burnt 

 clay, and that it is formed in clay under the influence of pro- 

 toxide of iron, from water and atmospheric air, these blue clays 

 ought to possess the same effects as burnt clay, or exhibit even 

 greater fertilising effects than most burnt clays. The contrary, 

 however, is the case. Sprengel appears to have felt the difficulty 

 which presents itself in explaining why certain blue clays, which 

 contain a large quantity of protoxide of iron, do not act so beneficially 

 as burnt clays, containing much less protoxide ; and he endeavours 

 to meet it, by a reference to the changes which protoxide of iron 

 undergoes during heating. He says, in unburnt clay, protoxide of 

 iron occurs in a state of hydrate ; on burning, it is changed into 

 anhydrous protoxide, in which state it possesses the greatest gal- 

 vanic energy, in consequence of which a larger decomposition of 

 water, and also more considerable formation of ammonia, results. 

 This explanation, probable as it may appear at first sight, never- 

 theless wants the only proof on which any explanation can be 

 founded namely, direct experiment and amounts, therefore, to 

 nothing more than explaining one difficulty by assuming another. 

 For my own part, I cannot see why anhydrous protoxide of iron 

 should be in a state of greater galvanic energy, and consequently 

 produce more ammonia than the hydrated protoxide; and as 

 Sprengel has not shown, by direct experiment, that this is really 

 the case, the difficulty which presents itself to his ammonia-theory 

 in certain blue clays cannot be said to be removed. We shall see 

 hereafter that ammonia, which is found in burnt clay, indeed con- 

 tributes to the general effect of the same; but we shall likewise see 

 that it is not the chief cause. 'The part ammonia plays in pro- 

 ducing these effects will be discussed afterwards. 



It is now time to allude to the opinions which Professor Zierl 

 entertained respecting the causes of the influence of burnt clay. 

 Unlike Lampadius, Kersten, and Sprengel, he considers the acci- 



