THAT GLASS IS VERY HARD 



E are led to believe that this error is very preva- 

 lent because the expression "as hard as glass" 

 is used as a comparison by some manufacturing 

 firms in their advertisements of goods in which 

 hardness is a specially desirable quality. And we are 

 confirmed in this view by the fact that the editor of one 

 of our mechanical journals actually defended the implied 

 statement on the ground that glass is very brittle! 



Hardness, as we all know, is a comparative term. Copper 

 is hard when compared with lead; it is soft when compared 

 with brass, and brass is soft when compared with common 

 iron. The latter is soft when compared with steel, and 

 steel itself is soft when compared with iridium or with the 

 diamond. 



Glass, however, according to all the scientific tests 

 used by the mineralogist and the physicist, is quite soft. 

 It is easily scratched by flint and by several minerals of 

 that grade, while flint is easily scratched by carborundum, 

 ruby, and some other substances the hardest material 

 known being the diamond. 



It is a curious fact that, next to the diamond, the hardest 

 substance should be an artificial product carborundum. 

 It readily cuts the hardest materials, and is invaluable as 

 an abrasive. 



Different kinds of glass vary greatly in hardness, but 

 they are all comparatively soft and may be cut by a good 

 steel tool. It is a common practice amongst amateur 



215 



