FORMATION OF NITRE. 219 



adapted for its formation, without the co-operation of anima. 

 matter." 



This result of very numerous and correct experiments contra- 

 dicts completely the view that nitre may be generated in mixtures 

 of earth destitute of animal matter, and therefore at the expense 

 of the constituents of the air. The ad vocal es of this view cite in 

 defence of it the following experiments : — When earth forming 

 nitre is freed from all its soluble salts by lixiviation, and is then 

 exposed for several years to the action of the air, it yields a se- 

 cond crop of nitre, and these crops may be obtained three or four 

 times in succession, although in different proportions. The ad- 

 vocates of this theory, considering that all the substances con- 

 taining nitrogen are removed, argue that the nitrogen of the nitre 

 formed afterwards, must have been derived from the air. But 

 this conclusion is opposed to all rules of inductive science. When 

 a known cause produces the same action in all cases submitted 

 to examination, we must revert to the same cause in considering 

 the same action in cases not examined ; for we have no right to 

 assign to it a new cause, in order to save us the trouble of a 

 closer investigation. 



The advocates of the opinion that the nitrogen of the air is con- 

 verted into nitric acid in the nitre-beds, have never estimated the 

 amount of substances containing nitrogen existing in those beds ; 

 and they have never compared with this amount the quantity of 

 nitric acid actually generated. Those who, like Gay Lussac, 

 have taken this trouble, found that the quantity of nitric acid 

 formed corresponded to the quantity of animal matters present in 

 the mixture ; less nitre being formed, when the amount of the 

 latter was decreased, and by its increase, a greater quantity of 

 nitre was produced. 



Another reason for the opinion was, that nitrates were formed 

 in certain limestone caverns in Ceylon, where, according to Dr. 

 Davy, nitrates of potash and lime occur in a limestone containing 

 felspar, but quite destitute of animal matter. But the latter as- 

 sertion is very questionable, as there is scarcely a limestone in 

 existence that does not yield ammoniacal liquid on being subjected 

 to distillation. An experiment with materials expressly prepared 

 for this purpose (carbonate of lime, felspar, and water free from 



