100 JUNIOR GRADE SCIENCE 



produced by heating a crystal very gently for a short time, and the 

 cause is the same in each case, viz. the loss of a portion of its water 

 of crystallisation. This property of losing water of crystallisation 



FlO. 76. Fn-sh fry>t;ils of soda. FlO. 77. Crystals of soda after 



(From a photograph by Mr. H. E. exposure to tin- uir. ( From a photo- 



Hadlcy.) graph by Mr. H. E. Hadley.) 



at ordinary temperatures is called efflorescence. Soda melts at a low 

 temperature, and gives off its water of crystallisation, of which it 

 contains nearly 63 per cent. The anhydrous soda melts on very 

 strong heating and is more difficult to dissolve than when crystalline. 



43. EXAMINATION OF NITRE. 



i. Examine as described in Chapter XVIII. 



ii. Saturated solution of nitre. Procure a supply of nitre and powder 

 it. Put some of the powdered solid in a test-tube and half fill with water. 

 Shake them up together for some time, and if the powder dissolves, add more 

 and shake again. Continue this addition of the powder and the shaking 

 until some of the powder remains undissolved however much it is shaken. 

 The solution is now saturated, that is, it contains as much of the solid as 

 it will hold. 



Now warm the cold saturated solution. The powder which before 

 remained on the bottom of the test-tube dissolves. Continue to add more 

 nitre, and notice that a great deal must be added before a hot saturated 

 solution is obtained. 



Place the test-tube on one side to cool. As cooling proceeds some of 

 the nitre separates out in clear, well-formed crystals, because as the solution 

 cools it cannot dissolve as much nitre as before. 



Nitre. Nitre is a transparent crystalline solid. If supplied in the 

 form of powder it is found on close examination to consist of small 

 crystals. When heated it melts to a clear liquid ; it melts, but does 

 not give off water. 



Nitre is used in the preparation of explosives and fireworks. It 

 is also used as a preservative in the pickling of meat. 



