MEANS OF ASCERTAINING THE QUALITJ OF GLASS. 143 



but no paper was sent ori/ the subject. This led Mr. Guyton unanswered, 

 to investiiJ^ate it himself. 



With respect to the general characters of glass he observes, Specific gra- 

 the specific gravity of different kinds of glass, all of which 

 may be good, is very various ; beside which, to ascertain it 

 requires nice instruments, and an expertness in their use not 

 commonly to be met with. 



The inspection of the fracture affords but a loose conjee- Fracture, 

 ture to the most experienced eye : though Mr. Ducloseau 

 asserts, that the friictm-e of good glass is always wavy, and its 

 angles more or less acute. 



The degree of hardness varies not only in different kinds Hardness. 

 of glass, but in glass of the same kind. Workmen used to 

 handle the diamond readily distinguish glass that cuts soft, 

 from that which cuts hard; so that this might seem a suffi- 

 cient indication of its good or bad quality. It is very difficult 

 however to discriminate degrees of difference in this respect, 

 neither is it always to be depended upon. 



Glasses that are called greasy [gras] are bad insulators of Bad glass not a 



electricity, and little capable of bein«j rendered electric by P^^'^f ^^ "°"" 

 '' . * ... "^ conductor of 



friction. Probably this, as well as their being liable to alter, electricity; 



depends on an excess of saline flux, or an imperfect refining, 



that has left sandiver in them. 



Bad glass placed on burning coals becomes dull and more and tamlshedl 

 or less meally on its surface. The same effect is more spec- ^ ^'^^^' 

 dily and evidently produced before the blowpipe. But in 

 all such trials we have to guard against the too rapid or too 

 unequal action of the heat. 



The experiments Mr. Guyton made with neutral salts Neutral salts 

 convinced him, that this mode of proof, which would have ^f ""'^ ^^^ °*' 

 been very convenient, was altogether inefficacious. Among 

 the specimens of the commonest window, glass however, 

 which is in general unalterable, he found one, that underwent 

 a perceptible change on its surface by merely boiling in a 

 solution of alum and mutiate of soda. 



There are few glasses, which^ when reduced to an impalp- Acids acton 



able powder) are not slightly acted upon by concentrated P°^^'<^^'-''^^'^ S^^^^ 



acids : though previously all well made glass resists sulphuric 



acid, the most powerful of all, even assisted by heat. On 3^^ gj^^^ ^^_ 



the contrary it very readily attacks puss of bad quality, at roded by sul-, 



., phuric acid. 



