524 Dr. Kidd’s Observations respecting 
where the saltpetre had been detaclied, provided it had been 
formed during the winter. I could not extend the experiment 
on the saltpetre formed during the summer, having only 
reserved a portion from one spot. 
Judging from such experiments as I have made, it appears 
that the saltpetre formed in the situations described in this 
paper, consists of full gg parts in 100 of nitrate of potash; 
with a very minute proportion of some calcareous salt, which 
is either not at all present in the saltpetre formed during 
summer, or is present in smaller quantity than in that formed 
during winter — though even in the latter instance it scarcely 
amounts to one part in 200. 
The proportion of sulphuric and of muriatic acid, and the 
bases with which these acids are combined, I have not 
attempted to ascertain, on account of the minuteness of the 
proportion in which they evidently exist. 
In considering the relative situations of the different parts 
of the laboratory, and the other buildings also, in which salt- 
petre is naturally formed, it is evident that the efflorescence 
takes place only where the exterior of the wall on which it is 
formed, is either exposed to the direct influence of the weather, 
or is in contact with the adjacent ground ; not taking place at 
all in those instances in which the wall is neither exposed to 
the weather on either side, nor is in contact with the adjacent 
grcund : from which it might be argued, that the eflect de- 
pends upon the action of the external air, or of the moisture 
of the ground transmitted through the substance of the wall : 
but the result of that experiment in which, the action of the 
