NITRE. 465 



(b.) More commonly, however, it is in crystalline, striated or 

 channeled masses, which, when of considerable length, are called 

 stick nitre. 



Sc.) Primitive form, a right rhombic prism incidence of the lat- 

 planes, 109.50; ratio between one side of the base and the 

 height, nearly : 1 : 0.48. 



Cleavage, " parallel to all the faces of the primitive, and also to a 

 plane passing through the two short diagonals of the bases.* 



Nitre sometimes crystallizes in tables, or laminae, and in the prism 

 of six sides, the two opposite ones are commonly broad ; the prism 

 is sometimes terminated by 18 faces at each extremity, arranged in 

 three rows, each having six faces, " as if three truncated pyramids 

 were piled on each other." Sp. gr. 1 .9603. 



(d.) Taste, bitterish and cool. 



(e.) Brittle, and easily pulverized. 



CHIEF CHEMICAL PROPERTIES. 



1. ACTION OF HEAT. This salt is anhydrous, and the small 

 portion of water that is lodged mechanically between the plates of 

 the crystals is easily dissipated by low ignition. 



(a.) It melts quietly into an oil-like liquid, and if cooled, congeals 

 into a smooth white mass.f 



(b.) If the heat be increased to redness, we obtain oxygen gas, to the 

 amount of about J of the weight of the nitre employed. The first 

 portions are pure, but after about } part has been withdrawn, it is 

 obtained more or less mixed with nitric oxide gas, and with nitrogen, 

 which prevail most towards the end. 



(c.) If the heat be continued, the decomposition is entire, and po- 

 tassa remains behind. If the salt be removed from thejire, when only 

 a part of the oxygen gas has made its escape, it is found reduced to 

 the state of nitrite. 



This is an easy process for oxygen gas, and answers very well, 

 where we do not want it very pure. It is usually saved, when it 

 is so good as to re-light a candle just blown out, but having a red wick. 



In a gun barrel or iron bottle, the salt should be melted at the up- 

 per part first, and then the remainder by degrees ; otherwise there 

 is danger of an explosion. One pound of nitre yields about 1200 

 cubic inches of oxygen gas. 



2. ACTION OF WATER. 



(a.) Soluble in 7 parts of water at 60 Fahr. and in nearly its own 

 weight of boiling water ;{ crystallizes on cooling. When mixed with 



* Levy, Quart. Jour, Vol. XV, 284, and Henry. 



t When melted, it is sometimes poured into moulds, and sold in round lumps like 

 bullets, under the name Sal prunella. In this state it is preferred by jewellers, for 

 heightening the color of their wares. J. G. 



\ According to Dr. Hope, it is soluble in 4 or 5 times its weight of water at 60. 



59 



