5^4 Kidd's Observations respectmg 
appearance took place in frosty weather, and that its reap- 
pearance about the i6th of March had been preceded by a 
frost of a few days ; whereas during the intervals, in which it 
disappeared and was not a^ain produced, the weather had 
been mild. The wall was again brushed quite clean on the 
i6th of March ; between which day and the 4th of April, a 
considerable quantity had again collected on the same part; 
the process having been more rapid during the last four or 
five days, which were cold. 
On April the 12th the front of this part of the wall was 
renewed by scraping, for the purpose of observing what would 
be the effect of a fresh surface, and before the 25th of April, 
nitre had formed on many parts of this new surface; but 
having first increased and then continued stationary for some 
days, it began to diminish about the beginning of May, the 
state of the air having been very moist during the two or 
three preceding days. 
About the middle of May, 1813 , 1 selected several distinct 
parts on which the spontaneous formation of nitre usually 
takes place, some within and some without the building of the 
Museum, and began to make daily observations on the phe- 
nomena of that process, which I noted down at the time : but 
as a register of this kind would be unnecessarily tedious in its 
recital, I shall beg leave to give the following general results 
of those observations ; requesting it may be kept in mind that 
I do not presume to lay a greater stress on them than is pro- 
portional to the short period of time, and the circumscribed 
space in which they were made. 
It appears then from the observations I have hitherto been 
able to make, that the spontaneous formation of saltpetre is. 
