422 Observations respecting 



would by congelation have been rendered incapable of dissolving 

 the nitre. 



It is deserving of notice, that such a spontaneous disappear- 

 ance of the nitre, as has been just mentioned, took place ante- 

 cedentlv to and during the late heavy fall of snow on January 

 18 and "19. 



Wisliinr; to ascertain vvhetlier tlie free presence of atmosphe- 

 rical air be necessary in the natural process under consideration, 

 I selected a part of the wall on which the iormation of saltpetre 

 usually takes ])lace to a considerable extent ; and insulated about 

 a square foot of its surface which had been previously Inushed 

 quite clean. It was insulated by means of a plate of 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 dej)th of tlie frame was not more than the tliird of an inch, 

 the inclosed space contained but an inconsiderable quantity of 

 atmosplicrical air. On the following day, October 29, I ob- 

 served that a formation of nitre had takoi place on the part of 

 the wall within the glass, but that it was not so extensively dif- 

 fused, nor so abundantly deposited, as had been usual before the 

 application of the glass ; and the crystals, which were distinctly 

 prisniatic, were mucli more accurately defined, and larger, than 

 J had ever before observed them ; in consequence, probably, of 

 the tranquil state of the medium in which they were formed. 

 On the day foliowing, October oO, the quantity was increased ; 

 but it remained stationary from that time to November 12, when 

 it began to dinsinish ; and on November IG there was no longer 

 any appearance of it : nor did it reappear in the interval between 

 November IG 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 bceii more than once brushed off and 

 again deposited, while no increase had taken place in that ori- 

 ginally formed within the glass. 



On November 29, the exterior surface of the glass was, for a 

 jjurpose to be mentioned presently, covered over with whitewash; 

 which was not removed till Janiiary 8, 1814. No observation 

 could of course be made on the appearance of the.smface within 

 the glass during the interval that the glass itself 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 capillary prismatic 

 crystals which were umch larger than on the former occasion, 

 and reached from the surface of tlie Avail to the inner surface of 

 the glass ; and partly in small flocculent tufts; the whole quan- 

 tity of the nitre formed being much greater than, and the man- 

 J}er of its formation being very different from, that formation 



which 



