Ix Historical Sketch of the Physical Sciences, 1818. 
of Philosophy, xiii. 50); as he acknowledges that it is an im~- 
perfect process. 
VIII. SALTS. 
The salts constitute, by far, the richest department of che- 
mistry ; many of them still remain to be examined. It is there- 
fore m the power of every industrious chemist to add new facts 
to the science, almost at pleasure, by examining this easy, though 
somewhat neglected, department of chemistry. The additions 
made to this province of the science during the last year have 
not been very numerous ; though some of them are of consider- 
able importance. 
1. Saltpetre-—The process followed in France for purifying 
saltpetre, at least in the manufactories carried on by govern- 
ment, is the following: The salt, such as it is procured from 
the saltpetre-makers, is dissolved in the fifth part of its weight 
of water. The common salt, which exists in too great quantity 
in it to be dissolved, is removed from the bottom of the vessel, 
and the scum which collects on the surface is skimmed off. 
The liquid is claritied by a solution of glue, and then poured, 
while boiling hot, mto a large copper bason, where it is con- 
tinually agitated till it becomes cold. By this means it is sepa- 
rated in very small crystals. These crystals are put into wooden 
boxes, and sprinkled with water till that liquid passes off pure. 
The object in view, in agitating the liquid during the crystalli- 
zation, 1s to make the size of the crystals as small as possible, in 
order to enable the subsequent process of washing to carry off 
the whole of the mother liquor attached to the crystals, in which 
alone the foreign salts, constituting the impurities, exist. Salt- 
petre, purified by this process, contains about =), part of its 
weight of common salt.—(See Longchamp, Ann. de Chim. et 
Phys. ix. 200.) 
2. Carbonate of Potash.—Chemists are aware that one of the 
easiest methods of obtaining this salt in a state of purity, is to 
burn a mixture of nitre and cream of tartar. M. Guibourt has 
shown that the best proportions are two parts of bitartrate of 
potash and one part of saltpetre ; and that the mixture should 
be thrown into a crucible, heated rather below redness. If the 
crucible be at a strong red heat, there is always formed a con-~ 
siderable portion of cyadide of potassium, and it is difficult to 
get rid of the hydrocyanic acid, which is formed when the sub- 
stance is dissolved in water. To avoid the formation of this 
substance, it is better to expose the mixture of salts only to a 
heat below redness.— (Jour. de Pharmacie, 1819, p. 59.) 
3. Ferro-chyaxate of Potash.—This is the salt usually called 
triple prussiate of potash. It has a fine yellow colour and crys~ 
tallizes in square tables, with bevelled edges ; it is transparent, 
and when seen by transmitted light is green, by reflected light 
topaz yellow ; its specific gravity is 1:833. Its taste is saline 
‘ 
