294

ELEMENTS OF MATERIA MEDICA.

In Prussia nitre-walls are employed instead of nitre-beds. Thesehave two advantages,—they economize land, and they expose a large sur-face to the air, (Dumas, op. cit.)

Properties. —Nitrate of potash usually crystallizes in six-sidedprisms with dieclral summits. Its primitive form is the right rhombicprism, and, therefore, it has tw o axes of double refraction along which aray of light is not split into two.

Double system of rings seen by looking through a slice of nitre (cut perpendicularly tothe axis of the crystal) placed between two plates of tourmaline (cut parallelto the axis of the crystal).

Flg. 52. Fig. 53.

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Fig. 52 is seen when the plane of the axis of nitre is parallel or perpendicular to theplane of polarization.—F ig. 53 is seen when the nitre is turned 45°.

When pure the crystals are transparent and colourless, have a sharp cool-ing taste, and undergo no change by exposure to the air. When heated,nitrate of potash fuses, and when cast in moulds forms the nitrum tabula-tum, or salprunelle of the shops. At a strong red heat it is,decomposed,with the evolution of oxygen and the formation of hyponitrite of potash,which when rubbed to powder and mixed with sulphuric acid emits redfumes (composed of nitrous acid and binoxide of nitrogen.) One hundredparts of water at 32° dissolve I3‘32 parts of this salt, but at 77° theydissolve 38 parts. During the solution cold is generated. In pure alcoholnitre is insoluble.

Composition.— Nitrate of potash has the following composition :—

Eq. Eq.Wt. PerCent. Wollaston.!

Nitric Acid . . 1 . . . 54 . . . 52*9 .... 53*332

Potasli . . . . 1 . . . 48 . . . 47*1 .... 46*668

IsitraieotPotaslil . . . 102 . . . 100*0 .... 100*0001

Eq. Eq.Wt.Per Cent.(Nitrogen . 1 . . 14 . . 13*75

or < Oxygen. . 6 . . 48 .. 47*10

(Potassium 1 . . 40 . . 39*15

.1 . . 102 . 100*00

The crystals contain interstitial water, but no water chemically combined.

Characteristics. —This salt is known to be a nitrate by the charac-ters already detailed (p. 160) for this class of salts. That its base ispotash is shewn by the tests before mentioned (p. 274) for this substance.

Physiological Effects. (a.) On vegetables. —Nitrate of potash dis-solved in 300 times its weight of water promotes vegetation : but a solu-tion containing part of nitre is injurious to the growth of plants (Davy,Agricult. Chemistry).

(b.) On animals generally. —Orfila (Toxicol. Gen.) found that introducedinto the stomach of dogs it acts as an irritant poison, and is capable ofproducing death, when it is not vomited, in doses of two or three drachms.Its operation is that of a narcotico-acrid poison. When applied to the■cellular tissue it produces, according to this experimentalist, local effectsonly, and does not become absorbed. But Devergie (Med. Leg.) states

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