The _nitrate of potass_ [nitre, saltpetre, sal-prunelle], is a dangerous poison. It has been often mistaken for the saline laxatives, especially the sulphate of soda, and has thus been the source of fatal accidents. SECTION I.—_Of the Chemical Tests for Nitrate of Potass._ It exists in commerce and the arts in two forms, fused and crystallized. The fused nitre [sal-prunelle] is sold in little button-shaped masses, spheres of the size of musket-balls, or larger circular cakes, of a snow-white tint. The crystallized salt [sal-petre] is sold in whitish, sulcated crystals, which are often regular and large. They are six-sided prisms, more or lest flattened, and terminated by two converging planes. In both forms nitre has a peculiar, cool, but sharp taste. Its chemical properties are characteristic. In the solid form, it animates the combustion of burning fuel, and yields nitrous fumes when heated with strong sulphuric acid. In solution it is precipitated yellow by the chloride of platinum, and yields, when not greatly diluted, a crystalline precipitate with perchloric acid. The crude salt of commerce contains chloride of sodium; and hence the odour disengaged by sulphuric acid may be mixed with that of chlorine or hydrochloric acid gas. When mixed with any vegetable or animal infusion by which it is coloured, crystals may sometimes be easily procured in a state of sufficient purity by filtration and evaporation. But if not, then the same process must be resorted to with that formerly recommended for nitric acid (p. 143), the first step of neutralization with potass being of course dispensed with.—A process nearly the same with this has been suggested by M. Kramer of Milan. He proposes to free the liquid in part of animal matter by adding acetate of lead, transmitting sulphuretted-hydrogen through the filtered fluid to remove any excess of lead, boiling the fluid after another filtration, and then proceeding with acetate of silver to remove chlorides, as in the process I have adopted. In this way he found nitre even in the blood.[447] SECTION II.—_Of the Action of Nitrate of Potass and its Symptoms in Man._ This substance forms an exception to the general law formerly laid down with regard to the effect of chemical neutralization on the local irritants. Both its acid and its alkali are simple irritants; yet the compound salt, though certainly much inferior in power, is still energetic. Nay, the experiment of Orfila and the particulars of some recently published cases tend even to prove, that the action of its alkali and acid is materially altered in kind by their combination with one another; for, besides inflaming the part to which it is applied, nitre has at times produced symptoms of a secondary disorder of the brain and nerves.