• the natural production of Saltpetre. 519 
glass, the edges of the frame of which were covered with a 
cement, so as to exclude any access of air between the glass 
and the wall. As the depth of the frame was not more than 
the third of an inch, the inclosed space contained but an 
inconsiderable quantity of atmospherical air. On the following 
day, October 29, 1 observed that a formation of nitre had taken 
■place on the part of the wall within the glass, but that it was 
not so extensively diffused nor so abundantly deposited, as had 
been usual before the application of the glass ; and the crystals, 
which were distinctly, prismatic, were much more accurately 
defined, and larger than I had ever before observed them ; in 
consequence, probably, of the tranquil state of the medium in 
which they were formed. On the day following, October 30, 
the quantity was increased ; but it remained stationary from 
that time to November 12, when it began to diminish; and 
on November 16, there was no longer any appearance of it: 
nor did it reappear in the interval between November 16 and 
29. In the mean time the efflorescence had not only taken 
place on the adjacent parts of the wall in the usual manner, 
but had been more than once brushed off and again deposited, 
while no increase had taken place in that originally formed 
within the glass. 
On Nov. 29, the exterior surface of the glass was, for a 
purpose to be mentioned presently, covered over with white- 
wash ; which was not removed till January 8, 1814. No ob- 
servation could of course be made on the appearance of the 
surface within the glass during the interval that the glass it- 
self was covered with whitewash ; but no nitre was visible on 
the removal of the whitewash on January 8. It had, however, 
reappeared before the morning of January 10, partly in separate 
3X2 
