Dr. Kidd's Observations respecting 
capillary prismatic crystals which were much larger than on the 
former occasion, and reached from the surface of the wall to 
the inner surface of the glass ; and partly in small flocculent 
tufts : the whole quantity of the nitre formed being much 
greater than, and the manner of its formation being very dif- 
ferent from, that formation which took place within the glass 
on October eq and 30. Still the whole quantity was not nearly 
equal to that which was usually formed on the same surface 
when exposed to the free action of the atmosphere. The in- 
creased quantity however formed in January, compared with 
that formed in October, corresponds with the observation that 
the natural production of nitre is much influenced by temper- 
ature : but it is W’orth mentioning that w'hile the nitre formed 
within the glass in January occurred either in distinctly pris- 
matic crystals or in flocculent tufts, that which w^as formed on 
the adjacent parts of the wall was of the granular character 
above described, (pag. 516, lin. 3.) 
Having by the foregoing experiment satisfied myself wdth 
respect to the necessity of the. free presence of atmospherical 
air for the full effect in the spontaneous production of nitre, I 
wished to ascertain the nature of the connexion between the 
nitre formed and the stone on which it is formed ; and parti- 
cularly wdiether carbonate of lime simply would be sufficient 
for its production. I therefore made a nearly liquid paste with 
distilled water and prepared chalk,* with which I covered the 
exterior surface of the glass that had been attached to the wall 
for the purpose already mentioned : but though nitre was 
* As chalk sometimes contains magnesia, and almost always a small proportion of 
silex and alumine, I intend on a future occasion to repeat the experiment, using very 
finely pulverised crystallised calcareous spar, instead of chalk. 
