546 PRINCIPLES OF CHEMISTRY 



is to add a solution of potassium carbonate, or to simply treat it with 

 ashes containing this substance. The potassium carbonate enters into- 

 double decomposition with the calcium and magnesium salts, forming 

 insoluble carbonates of these bases and leaving the nitre in solution. 

 Thus, for instance, K 2 CO 3 + Ca(NO 3 ) 2 :=2KNO3 + CaCO 3 . Both cal- 

 cium and magnesium carbonates are insoluble, and therefore after 

 treatment with potassium carbonate the solution no longer contains 

 salts of these metals but only the salts of sodium and potassium together 

 with organic matter. The latter partially separates on heating in an 

 insoluble form, and is entirely destroyed by heating the nitre to a 

 low red heat. The nitre thus obtained is easily purified by repeated 

 crystallisation. The greater part of the nitre used for making gun- 

 powder is now obtained from the sodium salt Chili saltpetre or cubic 

 nitre, which occurs, as has been already mentioned, in nature. 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 car- 

 bonate is directly obtained as a precipitate), or as is now most fre- 

 quent, potassium chloride. When a mixture of strong solutions of 

 potassium chloride and sodium nitrate is evaporated, sodium chloride 

 first separates, because this salt, which is formed by the double decom- 

 position KCl + NaNO 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 solution of nitre, which cannot dis- 

 solve a further quantity of nitre but only the impurities. 



Nitre is a colourless salt having a peculiar cool taste. It easily 

 crystallises in long striated six-sided rhombic prisms terminating in 

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

 their cavities generally contain a certain quantity of the solution from j 

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

 crystals are not obtained but saltpetre flour is prepared. At a low 

 red heat (339) nitre melts to a colourless liquid. 14 Potassium nitrate j 

 at the ordinary temperature and in a solid form is inactive and stable, 



14 Before fusing, the crystals of potassium nitrate change their form, and take the j 

 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- into 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. With a small trace of sodium chloride, the mass of nitre 

 solidified after fusion does not present any signs of crystallisation in the middle. 



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



