12 
president’s address. 
Nitre is also found in tlie plains of India, being most abundant 
in the parts distant from mountain ranges. The soil is composed 
of a very uniform alluvium. Nor a depth of more than two hundred 
feet the uniformity of this is only broken by occasional thin layers 
of clay indicating former river-courses, and still more rarely by 
rough nodules made up of granules of carbonate of lime, so 
agglomerated that they enclose much of the surrounding sand, the 
proportion of carbonate of lime varying from fifteen to seventy per 
cent. These beds lie in a horizontal plane at depths varying from 
one to twenty feet. They are from six inches to three or four feet 
thick, from one to three yards wide, and from one to several miles 
in length, with occasional interruptions. The yield of nitre is 
abundant only where these calcareous beds exist in the soil, and 
■where, at the same time, the natural water-level is from twenty to 
forty feet below the surface. In parts where the surface of the 
well-water is only a few feet from the level of the plain, no nitre is 
gathered : it is probably carried away and diffused as soon as formed. 
AVhere, on the other hand, the water-level is more than twenty feet 
from the surface, all the watery parts of fluids falling on the surface 
are drawn ujiwards and evaporated by the powerful sun, leaving the 
solid particles to be gathered or washed away when the new rain 
falls. 
Thus much for the occurrence of natural nitre. But during the 
long European wars of the last and present century, saltpetre often 
became scarce and dear, and, moreover, being contraband of Avar, 
Avas difficult to procure. Necessity became the mother of invention, 
and hence chemists set their Avits to Avork to obtain the material in 
other Avays. It Avas found that Avhcn nitrogenous organic matters 
of various kinds Avere heaped together Avith calcareous substances, 
moistened and exposed to free air, that a continuous production of 
nitre occurred, Avhich could from time to time be Avashed out, and 
the material being again left for a time, a fresh cro]) of nitre could 
be obtained, and so on time after time almost indefinitely. The 
occurrence of this so-called artificial nitre became an interesting 
subject of investigation to a number of leading scientific men, and 
