100 CHWI.CAL DISCOVERY AND INVENTION 



and characterisation of minerals, and the importance of crystalline 

 form in its relation to chemical constitution has been recognised 

 only within recent years. It is also doubtless in part attributable 

 to insufficient acquaintance with the necessary mathematics, 

 but as the mathematical treatment of chemical problems of all 

 kinds is now common, the study of crystals, their external form 

 and optical properties, will doubtless receive in future a larger 

 share of attention from chemical students. 



In conclusion mention must be made of the application of 

 fused silica or quartz glass to the construction of chemical 

 apparatus. This substance melts at about the same temperature 

 as platinum, and is manufactured in two qualities, opaque and 

 transparent. The former is made in an electric furnace from 

 the less pure varieties of massive quartz ; the amount of im- 

 purity is, however, very small, not usually exceeding J per cent, 

 the opacity being due to air bubbles. Muffles, trays, and large 

 evaporating dishes are made of this material, as it resists the 

 action of all acids except hydrofluoric acid, and, at high tempera- 

 tures to a small extent, phosphoric acid. The transparent silica 

 ware is made from rock crystal, and for some years was made 

 exclusively by fusion in an oxyhydrogen flame. Small flasks, 

 beakers, tubes, crucibles, dishes, and other vessels are now in 

 use in all laboratories with great advantage, owing to the remark- 

 able properties of this substance in relation to heat and chemical 

 agents. Its coefficient of expansion being very small, only 

 about T V that of common glass, a silica vessel bears Sudden 

 changes of temperature without damage. It is an interesting 

 and surprising experiment to see a silica flask heated to redness 

 in a flame plunged at once into cold water without a crack. 



At the same time it is not advisable to subject large or thick 

 masses of the material to so severe a test. And of course it 

 must be remembered that although silica resists the action of 

 ordinary acids it is not proof against alkalis. Contact with hot 

 alkaline solutions and especially fused potash or soda and lime 

 or baryta must be avoided. 



