the natural production of Saltpetre. 515 
generally speaking, much more extensive and rapid in winter 
than in summer, whether it take place on the interior or 
exterior of a building : thus during the period of the cold 
weather in January, 1814, it became visible not only on parts 
of the walls where I had never before observed it, but even 
on the pavement of the laboratory. The part of the pavement 
on which it appeared is adjacent to the north wall; but as the 
greater part of the pavement is covered by a flooring of wood, 
it is impossible to say whether or not it took place on other 
parts also. The stone forming the pavement, is the same kind 
of limestone as that of which the walls are built. 
Again, whereas in summer its reproduction is most rapid and 
extensive in proportion to the degree of light present, the 
reverse of this (though not universally) takes place in winter. 
Wherever the saline efflorescence in question occurs, the sur- 
face of the stone becomes permanently discoloured, as if from 
the effect of damp; but this discoloration is merely superfi- 
cial. If these discoloured parts be whitewashed, the process 
still goes on ; and the whitewash is gradually detached in 
flakes : but it is difficult to ascertain whether the nitre is 
formed on the whitewash, or on the wall which it covers ; 
though probably the latter. 
When the spontaneous formation of nitre takes place slowly 
and in a sheltered situation, it is at first visible in the form of 
minute prismatic crystals, which usually project from the 
surface of the wall nearly at right angles ; but sometimes 
they are scattered in different directions, lying upon its sur- 
face so lightly as scarcely to appear in contact wdth it. In 
general, however, the saline efflorescence makes its appear- 
ance in extremely minute capillary crystals, either accumulated 
