66 COLOUR, FORM, AND MOVEMENT. 



4. Common Salt crystallises in cubes. Here the number 

 and arrangement of the axes is the same as in aluin. 

 This should be demonstrated by means of a diagram. 



5. Copper Sulphate. In this type of crystal all the 

 axes are unequal and are placed obliquely to each other. 



Growth of Crystals. 



The experiment of growing crystals should be performed. 

 The teacher should make a saturated solution in water of 

 any of the three foregoing substances or of any other he 

 may find more serviceable to his purpose, by dissolving as 

 much as the hot water will take up. Keep the water near 

 boiling point, continuing to add more of the solid as it dis- 

 solves. In the case of common salt a very large quantity 

 of substance will dissolve in this way, but as it is readily 

 available in quantity it should be taken for this experi- 

 ment. 



When the solution is saturated, which will be indicated 

 by the liquid ceasing to dissolve any more solid, it should 

 be poured into another clean vessel. A quite shallow 

 vessel with a wide opening is best, and if all the liquid is 

 not used the excess may be put aside for future use. In 

 the shallow vessel as cooling and evaporation proceed the 

 dissolved substance will reappear in crystalline form. 



If the crystals are not too crowded, perfect forms may 

 be picked out. They may be separated from each other 

 with the point of a needle. In this way small but perfect 

 cubes of common salt will be got. In the case of the alum, 

 if the small crystals can be kept apart from each other and 

 turned from time to time so that the faces may grow equally 

 quite large crystals may be obtained. So also with copper 

 sulphate, but this substance being poisonous is not so con- 

 venient for school use. 



If these experiments are successfully performed, and with 

 care and patience they may be, it will be well in conclusion 

 to compare the growth of a crystal with that of a plant or 

 animal. The teacher should make clear the fact that 



