420 Observations respecting 



face of ttie stone becomes permanently discoloured, as if from 

 the effect of damp ; but this discoloration is merely superficial. 

 If these discoloured parts be wliitewashed, 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 scat- 

 tered in diflfercnt directions, lying upon its surface so lightly as 

 scarcely to appear in contact with it. In general, however, the 

 saline efHorescence makes its appearance in extremely minute 

 capillary crystals, either accumulated in groups, which resemble 

 recently fallen flakes of snow, or investing the wall like a fine 

 down. 



During the severely cold weather of January 1814, it ap- 

 peared in some places in the form of minute dense grains closely 

 aggregated ; %vhile in others it still continued to wear the ap- 

 pearance of down or wool : and the local circumstances most 

 obviously connected with this difl^erence iri the manner of crystal- 

 lization, were the presence of a greater degree of light, where the 

 granular deposition took place, and a less degree of shelter from 

 the influence of cold air. 



In some instances the production of the saltpetre is accom- 

 panied with a disintegration of the substance of the stone on 

 which it is formed : but this circumstance is only observable on 

 stones of a loose texture. 



The shortest interval I have observed between the time of its 

 having been brushed away and its reappearance, is four hours : 

 but it was then in full efflorescence, and would probably have 

 been visible much earlier. The observation was made on No- 

 vember the 1 7th, about midnight : there had been snow in the 

 niidlde of the day, and the night was frosty. 



The spontaneous formation of nitre takes place indifferently 

 on the surface of the stones composing a wall, and of the mortar 

 by which those stones are cemented ; and near the close of the 

 late frost, I observed it for the first time on the surface of a par- 

 tition consisting entirely of laths plastered over with the mortar 

 or stucco commonly used for that purpose. 



It accumulates in greater quantity on some parts of a given 

 surface than on others ; and this difference in the degree of its 

 accumulation, is probably connected with some slight difference 

 in the texture or composition of the stone on which it is formed: 

 for I have repeatedly observed that after a careful removal of it, 



its 



