136 NITRE. 
surgeons under the name of potential cautery, to open 
abscesses, and to destroy useless or hurtful excrescences. 
Potash, after it has been made red hot, is rendered 
whiter and more pure. In this state it has the name of 
pearl ash. 
206. NITRE, or SALTPETRE, is a salt which consists 
of potash in combination with nitric acid (30). 
Its colour is whitish or limpid ; and it does not liquefy by the 
action of the air. It is usually observed in the form of fine 
capillary crystals, though it is sometimes found in a massive 
state. When pure, it crystallizes into six-sided prisms (Fig* 
15) which have a rectangular base. It is denominated by che- 
mists nitrat of potash. 
Nitre is found incrusted on the surface of the 
earth, in some parts of India, Africa, and Spain, and, 
in such abundance, as to admit of being swept off 
at certain seasons of the year, twice or three times a 
week. In our own country it not unfrequently occurs 
in a state of white efflorescence, on old plaster walls 
that are sheltered from rain. Nitre is also produced in 
stables and cart-houses, from the mixture of animal and 
vegetable substances in a state of putrescence. 
Many kinds of plants, which grow in soils favour- 
able to the production of it, contain nitre : this is parti- 
cularly the case with pellitory, borage, and the large 
sunflower. 
Immense quantities of nitre are annually required for 
the purposes of war. From its constituting one of the 
most important substances in the composition of gun- 
powder,, it has been found necessary to adopt artificial 
modes of procuring it. In several districts of the East 
Indies there are places called saltpetre grounds. From 
these large quantities of the earth are dug, and put into 
cavities through which water is passed. This brings 
away with it the salt which the earth contains, and which 
is afterwards separated from the water by boiling. The 
East India Company, for more than a century past, has 
been under engagements to import into this country, and 
supply the board of ordnance, for his Majesty's service, 
