CHEMISTRY AND THE MICROSCOPE 



known as the crystal layer, where crystals of calcium 

 oxalate occur as tiny balls called crystal sand. In 

 the leaf stalks of begonias very beautiful and 

 occasionally very large crystals of this substance may 

 be found, whilst in shapes they are strikingly varied. 

 In orris root there are enormous crystals of calcium 

 oxalate ; in fact it is common in many plants. 



If we have a photographic friend who will supply 

 us with quite a small quantity of gold chloride, we 

 shall be in a position to try three most interesting 

 experiments and to obtain some curious crystals. 

 We require a very weak solution of gold chloride in 

 water, not more than 3.5 per cent, for our first 

 experiment. Mix one drop of this solution with the 

 same quantity of hydrochloric acid on a slide and 

 heat over a flame till dry. The microscope will now 

 show us probably the most curious crystals we have 

 ever examined ; some are long, some short, and some 

 a zigzag in form; mixed with these there will be 

 a few flat plate-like crystals with rectangular pro- 

 jections. All these curious crystals are yellow. 



If instead of hydrochloric acid we use a solution 

 of common salt in water and repeat the experiment 

 as before, we shall obtain pale yellow prisms and 

 some crystals of common salt. Gold is costly, so it 

 is perhaps lucky that one of the tests for this rare 

 metal is one of the most delicate known to chemists ; 

 it is possible, in fact, to detect very minute quanti- 

 ties of gold. For this experiment we may take an 

 exceedingly weak solution of gold chloride and place 

 a drop on our slide ; we also require a solution of the 



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