5^8 Dr. Kidd's Ohservafmis respecting 
besides which, had it fallen from tlie wall, I should most pro- 
bably have found it on the pavement beneath, which never 
happened to me. It afterwards appeared probable, although 
I have never met with any condensed moisture on the surfaces 
submitted to the foregoing observations, that the aqueous 
vapour precipitated from the atmosphere in the state of it 
above alluded to, might dissolve minute particles of the nitre, 
and be absorbed with them into the substance of the wall; 
but on this supposition nitre ought to be found in lixiviating a 
portion of the stone taken near the surface. I have however 
made the experiment without detecting any nitre in the stone 
so taken. But, in opposition to the idea of the absorption of 
the nitre into the substance of the stone, I found the efflo- 
rescence disappear in more places than one during the severe 
frost of the present year (1814), at a time, when from the 
temperature of the stone, if not of the air also, the absorption 
here supposed could not have taken place ; since, that tem- 
perature being below the freezing point, the aqueous particles 
would by congelation have been rendered incapable of dis- 
solving the nitre. 
It is deserving of notice, that such a spontaneous disap- 
pearance of the nitre, as has been just mentioned, took place 
antecedently to, and during the late heavy fall of snow on 
January 18 and ig. 
Wishing to ascertain whether the free presence of atmosphe- 
rical air be necessary in the natural process under consideration, 
I selected a part of the wall on which the formation of saltpetre 
usually takes place to a considerable extent ; and insulated 
about a square foot of its surface which had been previously 
brushed quite clean. It was insulated by means of a plate of 
