1829.] On the Manufacture of Optical Glass. 283 



made with these heavy glasses, they have no sensible power of 

 discharging the electricity, but insulate as perfectly as sealing- 

 wax or gum-lac. If one of these plates of glass, without any 

 previous warming and drying, be lightly brushed or wiped with 

 flannel or silk, it instantly becomes strongly electrical, and 

 retains its electricity for a long time ; but it would be almost 

 impossible to develope electricity by such slight means with 

 flint or plate or even crown glass in a similar state. Hence 

 the glass makes as good an electrophorus as lac or resin, and 

 may probably be found hereafter to answer many useful elec- 

 trical purposes. But the great point at present in view, is the 

 proof which such electrical properties give of the absence of 

 that film of moisture which is so constant upon other glasses. 



113. All these circumstances are favourable to the opinion 

 that the heavy glass will not be found objectionable in the con- 

 struction of telescopes, because of any undue tendency to 

 tarnish, and especially when precautions are taken to protect 

 it from sulphuretted vapours in the manner before described 

 (107). No difficulty can be anticipated in preserving the air 

 within a limited and enclosed space free from such conta- 

 mination : to preserve it dry, if that had been necessary, under 

 the different circumstances of varying temperature and the 

 inevitable change of the air more or less frequently, would have 

 been a far more difficult task. 



114. The other kind of superficial change, i. e. the corrosion 

 or crystallization which takes place principally on plate glass, 

 is doubtless also due to the alkali present. Sometimes, indeed, 

 specimens of glass may be found where the alkali being too 

 abundant, a similar but more extensive action has taken place 

 over the whole of the surface, and the glass falls off in scales. 

 Whether the alteration be due to the action of the alkali on 

 the water only, or on the carbonic acid and other substances it 

 finds in the air, or to its united action on all together, is of little 

 consequence at present, as the substance on which it depends 

 is altogether absent from the glass under consideration. 



115. Among the great number of glasses made, there are 

 several of different composition, which have been, selected, 

 because of their general characters and properties, for more 

 extensive trial and investigation when time will permit. Of 

 these it would be useless to speak at present, as what might be 



