EXAMINATION OF COMMON SUBSTANCES 



103 



How crystals can be made. Warm water when saturated with 

 any soluble substance often contains more of the solid dissolved than 

 an equal quantity of a cold saturated solution. The consequence of 

 this is, that if a warm saturated 

 solution is allowed to get cold, the 

 water can no longer keep all the 

 substance in solution, and it separ- 

 ates out in the solid state, which, 

 in these circumstances, always takes 

 a crystalline character. The crystals 

 of alum, formed in this way, gener- 

 ally have eight sides, or the shape of 

 the crystal is the same as the solid 

 called the octahedron, shown in 

 Fig. 80. But in some circumstances 

 the crystals only have six sides, or 

 are cubes. 



It is found that when a solution 

 is cooled rapidly, that the crystals 

 resulting are very small, but that when allowed to cool slowly larger 

 crystals are formed. The best crystals are formed when a solution 

 just saturated at the temperature of the air, or very slightly above it, 

 is allowed to evaporate, in a place free from dust. By selecting a small 

 perfect crystal of alum, and suspending it by means of a hair in the 

 solution from which it was obtained (all the other crystals being removed) 

 a crystal similar to that shown in Fig. 80 may be grown. It is im- 

 portant that wide differences of temperature should be avoided during 

 the growth of the crystal. 



FIG. 80. An eight-sided crystal of 

 alum. (From a photograph by Mr. 

 H. E. Hadley.) 



46. EXAMINATION OF WOOD. 



i. Examine as described in Chapter XVII. 



ii. Dry distillation of wood. As a result of heating wood in a test-tube, 

 it is noticed that both a liquid and a gas are given off. Arrangements must 

 be made to collect and examine all the products given off. For this purpose 

 the apparatus shown in Fig. 81 may be used. 



If necessary, in order to secure sufficient of the products for examination, 

 the test-tube may be recharged with wood, without interfering with the 

 other apparatus. 



Each of the final products must be examined separately. 



Solid. Examine and identify if possible, using any test you think suitable. 



Liquid. Note that there are two liquids, one like tar, and the other 

 lighter and clear in colour. Record the odour, taste and action on litmus. 

 Does the liquid at all resemble any liquid with which you are familiar ? 



