MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
49 
of pure water, strong indications of free alkali will be obtained. The same 
effect is produced by using plate glass ; and if the pulverization be very per- 
fect, the alkali can be detected in glasses containing far smaller quantities of 
that substance than either of those mentioned. This experiment, due to Mr. 
Griffiths, shows that in whatever state of combination the alkali may be, it can 
still act upon, and is subject to, the action of moisture ; and that flint glass is 
by no means a compound resulting from very strong chemical affinities, is also 
shown by an experiment which I made many years ago ; namely, that if flint 
glass be pulverised exceedingly fine, the powder will indicate the presence 
of sulphuretted hydrogen in air by becoming blackened, almost as readily as 
carbonate of lead. Glass may be considered rather as a solution of different 
substances one in another, than as a strong chemical compound ; and it owes 
its power of resisting agents generally to its perfectly compact state, and the 
existence of an insoluble and unchangeable film of silica or highly silicated 
matter upon its surface. 
111. The half-combined and hygrometric state of the alkali appears to 
be the cause of the deposited film of moisture which is well known to ad- 
here to ordinary glass when exposed to the atmosphere at common tempera- 
tures. This film is highly calculated to condense any portion of sulphuretted 
vapours which may be floating in the atmosphere, and thus bring them into 
contact with the oxide of lead under the most favourable conditions for the 
production of that action which is the direct cause of tarnish. Now from this 
cause of action the heavy glass is free ; and hence a satisfactory reason to me 
why the heavy glasses have suffered so little when left with common care in an 
usual atmosphere. 
112. An extraordinary difference exists between the electrical relations of 
this glass and other glasses, due principally to the same absence of alkali. 
Ordinary glasses, either flint, plate or crown, will, from the hygrometric film 
of moisture upon the surface, freely conduct electricity under common circum- 
stances. Thus if a gold-leaf electrometer be diverged, and then touched with 
them in their ordinary state, the electricity is instantly discharged, even though 
the hand be two or three feet from the part touching the instrument. If a 
similar experiment be made with these heavy glasses, they have no sensible 
power of discharging the electricity, but insulate as perfectly as sealing-wax 
MDCCCXXX. 
H 
