the vaturol Production of Saltpetre. 421 



its reproduction takes place to the same proportional extent in 

 the same parts : and this diifercnce in the degree of the efflores- 

 cence often takes place not by a gradual transition, but as 

 abruptly as if the line of separation had been marked by a graver ; 

 so that the part of the surface on one side of the line shall be 

 almost totally devoid of any efflorescence, while on tlie other 

 side it shall resemble the accumulation of hoar-frost on the small 

 branches of a tree. 



The saltpetre formed in summer scarcely appears to contain 

 a particle of any calcareous salt : that i'ornied in winter contains 

 most evident traces of such a sak, tJiough probably even in win- 

 ter the amount of this is not much above one part in two hun- 

 dred of the whole mass. 



A frosty, clear, and dry state of the atmosjjhere is particularly 

 favourable to the natural production of nitre : but there seems 

 to be a limit to its formation, on the same spot, even under the 

 most favourable circumstances ; the quantity of the nitre not 

 continuing to increase after it has proceeded to a certain ex- 

 tent. 



In a moist state of the atmosphere the formation either does 

 not take place at all, or goes on slowly : and if that state of the 

 atmosphere which is unfavourable to the production of nitre 

 continue a sufficient length of time, the nitre already formed 

 gradually disappears. At the commencement of these observa- 

 tions I attributed the occasional disappearance of the nitre to its 

 mechanical removal from the wall, and supposed that it must 

 have been by accident brushed off: but repeated observations 

 convinced me this was not the case, its disappearance proceeding 

 as gradually as its previous appearance ; besides which, had it 

 fallen from the wall, I should most probably have found it on 

 the pavement beneath, which never happened to me. It after- 

 wards appeared probable, although I have never met with any 

 condensed moisture on the surfaces submitted to the foregoing 

 observations, that the a([ueous vapour precipitated from the at- 

 mosphere in the state of it above alluded to, might dissolve mi- 

 nute 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. 

 1 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 efflorescence disappear in more places than one during tlie 

 severe frost of the present year (1814), at a time when, from 

 the temperature of the stone, if not of the air also, the aI)sorp- 

 tif)n here supposed could not Iiave taken place ; since, that tem- 

 perature being below the freezing point, the atjueous particles 



D (i 3 would 



