June 1, 1864] THE TECHNOLOGIST. 



ON GRANITE AND ITS USES. 503 



drinking glasses ; and lightning is an unmanageable servant. Our actual 

 glass is in larger part silica, that it may be a clear, bright, transparent, 

 insoluble, incorrodible, solid, enduring thing. But to secure its melting 

 under our ordinary furnace heats, alkali, namely, soda or potash, is 

 added, which renders it fusible and diminishes its tendency to crystallise ; 

 and to give back to it the solid enduringness and insolubility in liquids 

 which the alkali diminishes, infusible earths and heavy metallic oxides 

 are added. 



Thus window glass is made of the whitest sand melted with the 

 cheapest alkali, soda, and hardened by lime ; but as soda colours glass 

 green, flint-glass has potash instead of soda ; and as neither of these 

 alkalies nor lime confers the greatest brilliancy upon glass, oxide of lead 

 is added to the sand and potash, when the sparkle which we love to see 

 in decanters and lamp shades and lustres is desired ; and by largely 

 employing this lead oxide, the lenses and prisms suitable for the optician, 

 and passable imitations of the gems are produced, whilst small additions 

 of other metallic oxides give those beautiful colours which add such 

 glory to cathedral windows. According to some, we have lost the secret 

 of the ancient glass dyes ; but this is a mistake. Gold is as willing as 

 of old to stain glass ruby red, and so is the humbler copper, which can 

 also tincture it green. Silver secures a yellow or an orange, and iron 

 gives the same. Cobalt provides for blue, copper and chromium for 

 green, manganese for purple, and uranium for a topaz-like canary yellow. 

 Tin makes a white glass milky and opaque, such as we see in the dials 

 of watches ; and a black enamel is secured by the darker oxides of man- 

 ganese, iron, and cobalt. Bottle glass is the humblest product of the 

 glassy materials. Brown sand, spent lime, soapers' waste, clay, and 

 common salt, are resolved by the furnace into a dark glass, which, if 

 only cast into more graceful forms, would be as useful as it is without 

 offending the eye. 



The glass-maker's work, however, does not end when his vessels are 

 fashioned into shape by dexterous manipulation of its substance when 

 pliant and plastic. They are in the highest degree fragile as they first 

 leave his hands, so that they scarcely endure touching, and often fall to 

 pieces. This fragility is owing chiefly to the different amount of exten- 

 sion and contraction which different parts of the glass have undergone 

 whilst being fashioned into vessels, and to the unequal cooling of the 

 deeper, as compared with the more superficial layers of the substance 

 which have been more exposed to the cold external air in certain 

 manipulations, and to the hot air ol the furnace in others. A plastic 

 mass like glass contracts most where it is most cooled, and least where it 

 is least cooled. It thus resembles to some extent a web of woven tissue, 

 such as a sail-cloth where some of the threads are pulled so tight as to 

 be on the verge of breaking, whilst others are hanging in curves with 

 no strain upon them at all. Such a r piece of cloth is easily torn, for 

 when pulled or stretched it does not resist with the united tenacity of 



