282 On the Manufacture of Optical Glass. [1829. 



in an agate mortar, then placed upon a piece of turmeric paper, 

 and moistened with a drop 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 perfect, the 

 alkali can be detected in glasses containing far smaller quantities 

 of that substance than either of those mentioned. This experi- 

 ment, 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 pulverized exceedingly fine, the 

 powder will indicate the presence of sulphuretted hydrogen in 

 the 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 unchange- 

 able 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 adhere to ordinary glass when exposed to the 

 atmosphere at common temperatures. 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 ordinary atmosphere. 



1 12. 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 tinder common circumstances. 

 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 



