122 Dr. Ure on Gunpowders 



adopt the general language of chemists, and say that three 

 parts of nitre are soluble in one of boiling water, since the 

 liquid has a much higher heat and greater solvent power than 

 this expression implies. 



Water at 60 dissolves only one-fourth of its weight of nitre ; 

 or, more exactly, this saturated solution contains 21 per cent, 

 of salt. Its specific gravity is 1.1415 ; 100 parts in volume 

 of the two constituents occupy now 97.91 parts. From 

 these data we may perceive that little advantage could 

 be gained in refining crude nitre, by making a boiling-hot 

 saturated solution of it ; since, on cooling, the whole would 

 concrete into a moist saline mass, consisting by weight of 2 

 parts of salt, mixed with 1 part of water holding J of salt in 

 solution, and in bulk of 1 -J of salt with about 1 of liquid : for 

 the specific gravity of nitre is 2.005, or very nearly the 

 double of water. It is better, therefore, to use equal weights 

 of saltpetre and water in making the boiling-hot solution. 

 When the filtered liquid is allowed to cool slowly, somewhat 

 less than three-fourths of the nitre will separate in regular 

 crystals ; while the foreign salts that were present will remain 

 with fully one-fourth of nitre in the mother liquor. On redis- 

 solving these crystals with heat, in about two-thirds of their 

 weight of water, a solution will result, from which crystalline 

 nitre, fit for every purpose, will concrete on cooling. 



As the principal saline impurity of saltpetre is muriate of 

 soda, (a substance scarcely more soluble in hot than in cold 

 water,) a ready mode thence arises of separating that salt from 

 the nitre in mother waters that contain them in nearly equal 

 proportion. Place an iron ladle or basin, perforated with 

 small holes, on the bottom of the boiler in which the solution 

 is concentrating. The muriate, as it separates by the evapo- 

 ration of the water, will fall down and fill the basin, and may 

 be removed from time to time. When small nitrous needles 

 begin to appear, the solution must be run off into the crystal- 

 lizing cooler, in which moderately pure nitre will be obtained, 

 to be refined by another similar operation. 



At the Waltham Abbey gunpowder-works the nitre is ren- 

 dered so pure by successive solutions and crystallizations, that 

 it causes no opalescence in a solution of nitrate of silver. Such 



