5^2 Kidd^s Observations respecting 
brick wall : but in that instance the substance of those bricks 
on which the nitre appeared had crumbled away to some 
depth; and if this destruction of their texture be owing to the 
presence of an unusual proportion of lime in the clay of which 
they are made (a supposition not improbable, since many parts 
of the stratum of clay from which bricks in this neighbour- 
hood are made do contain an unusual proportion of lime) the 
reason of the exception in the case of this brick wall will cor- 
respond with the truth of the general observation above stated. 
The following circumstance is particularly deserving of 
notice. A part of the north wall of the laboratory, on which 
saltpetre usually effloresced, having been covered with wain- 
scot some months since, and the wainscot having been painted 
with common white paint, I was surprised after a time in ob- 
serving an efflorescence on particular parts of the paint, similar 
to what might have been expected on the wall itself. Where 
this efflorescence had taken place the paint was loosened from 
the wainscot, and might be readily peeled off in small flakes. 
The saline particles of which this efflorescence consisted 1 at 
first supposed to be nitrate of lead; but upon examination in 
various ways no trace of lead could be found in them, and 
they exhibited the principal characters of common nitre : they 
deflagrated, for instance, with charcoal, leaving a deliquescent 
alkaline residuum. Many weeks have elapsed since that saline 
efflorescence was brushed off”, but I have not yet observed any 
renewal of it. 
Though the production of saltpetre had been pointed out to 
me in the laboratory of the Ashmole Museum as long since 
as the year 1802, I was prevented by many circumstances 
from observing with any degree of regularity or precision 
