POTASSIUM. BUBIDIUM, CESIUM, AttD LITHIUM 555 



repeated crystallisation. The greater part of the nitre used for 

 making gunpowder is now obtained from the sodium salt Chili salt- 

 petre or cubic nitre, which occurs in nature, as already mentioned. 

 The conversion of this salt into common nitre is also carried on by 

 means of a double decomposition. This is done either by adding 

 potassium carbonate (when, on mixing the strong and hot solutions, 

 sodium carbonate is directly obtained as a precipitate), or, as is now 

 most frequent, potassium chloride. When a mixture of strong solu- 

 tions of potassium chloride and sodium nitrate is evaporated, sodium 

 chloride first separates, because this salt, Vhich is formed by the 

 double decomposition KC1 + NaN0 3 = KNO 3 -f NaCl, is almost 

 equally soluble in hot and cold water ; on cooling, therefore, a large 

 amount of potassium nitrate separates from the saturated solution, 

 while the sodium chloride remains dissolved. The nitre is ultimately 

 purified by recrystallisation and by washing with a saturated solu- 

 tion of nitre, which cannot dissolve a further quantity of nitre but only 

 the impurities. 



Nitre is a colourless salt having a peculiar cool taste. Tt crystal- 

 lises easily in long striated six-sided rhombic prisms terminating in 

 rhombic pyramids. Its crystals (sp. gr. 1*93) do not contain water, but 

 their cavities generally contain a certain quantity of the solution from 

 which they have crystallised. For this reason in refining nitre, the 

 production of large crystals is prevented, saltpetre flour being prepared. 

 At a low red heat (339) nitre melts to a colourless liquid. 14bi3 

 Potassium nitrate at the ordinary temperature and in a solid form is 

 inactive and stable, but at a high temperature it acts as a powerful 

 oxidising agent, giving up a considerable amount of oxygen to substances 



M bit Before fusing,, the crystals of potassium nitrate change their form, and take the* 

 same form as sodium nitrate that is, they change into rhombohedra. Nitre crystal- 

 lises from hot solutions, and in general under the influence of a rise of temperature, in a 

 different form from that given at the ordinary or lower temperatures. Fused nitre solidi- 

 fies to a radiated crystalline mass; but it does not exhibit this structure if metallic 

 chlorides be present, so that this method may.be taken advantage of to determine the 

 degree of purity of nitre. 



Carnelley and Thomson (1888) determined the fusing point of mixtures of potassium 

 and sodium nitrates. The first salt fuses at 889 and the second at 816, and if p be 

 the percentage amount of potassium nitrate, then the results obtained were 



p = 10 20 80 40 60 60 70 80 90 



298 283 268 242 231 281 242 284 806 



which confirms Shaffgotsch' s observation (1857) that the lowest fusing- point (about 281) 

 is given by mixing molecular quantities ( p = 54'8) of the salts that is, in the formation 

 of the alloy, KNO 3 ,NaN0 3 . 



A somewhat similar result was discovered by the same observers for the solubility o! 

 mixtures of these salts at 20 in 100 parts of water. Thus, if p be the weight of potas- 

 sium nitrate mixed with- 100 -p parts' by weight of sodium nitrate taken for solution, 



