the natural production of Ealipetre. 525 
air on the interior surface of the wall having been prevented 
by the intervention of a glass, the formation of nitre took place 
only to a very slight extent, is in a great measure at variance 
with such a supposition. 
Again, though it is evident that the natural production of 
the saltpetre is closely connected with changes in the state of 
the atmosphere, those changes relating not only to its tem- 
perature but also to the degree of its moisture and barometrical 
pressure; yet a much longer series of observations, and these 
not partial like the present, but carried on in various parts of 
the world, is requisite, before even this part of the problem 
can be accurately solved. There still would remain a part of 
much more difficult solution, namely, the source of the metallic 
base of the alkali of the nitre. 
With respect to this difficult question, if we compare the 
elements present in the composition of the saltpetre formed, 
with the elements of the substances present during its forma- 
tion, it seems a reasonable conclusion that the potassium, or 
the metallic base of the potash of the saltpetre, is either a 
simple principle of some of the elements present, or that it 
results from the union of two or more of those principles, or 
of two or more of the elements themselves.* 
The gross compounds present during the formation of salt- 
petre, in the instances mentioned in this paper, are atmosphe- 
rical air and the limestone on which the saltpetre effloresces; 
• The terms ‘‘ element” and principle” are here used in the same relation to 
each other as the early physical philosophers used the terms i.toix^Tx and They 
supposed that the former, though incapable of decomposition by common means, weie 
not necessarily to be considered as absolutely simple substances : those absolutely 
simple substances they expressed by the term 
MDCCCXIV. 3 Y 
