64< DEVITRIFICATION OF GLASS. 



An exact analyfis made of a certain number of thefe cryfta!& 

 carefully detached from the mafs, might indicate their nature 

 and throw light on their formation. 



It often happens that thefe cryftallized nodules are enve- 

 loped in a cruft which feems foreign to their nature, and may 

 be compared to the cruft which envelopes flints in the middle 

 of the chalk-banks in which they feem to grow. 

 Speedy refrige- Thefe are the feries of phenomenon attending the devitrifica- 

 the°phenome- t ' on °* greenifli glafs when it happens (lowly ; but if the effect 

 non. is too rapidly produced, the appearances referable thofe ob- 



ferved in the devitrification of bottle glafs. The window glafs 

 here fpoken of, is of that kind which contains very little of 

 earthy matter, except wood afhes. It muft admit of man y 

 variations according to the differences in its compofition. 

 Clear and fimple White or colourlefs glalTes are very difficultly cryftallized 

 Lfil^dc vitrified. or devitrifieJ. When they are well made, we may affirm that 

 a long continued heat does not change them ; but for this pro- 

 perty it is neceffary that they fhould be compofed merely of 

 filex, with the precife quantity of flux for its faturation. In 

 this cafe, a very long continued fire will no otherwife change 

 them than by rendering them more yellow and hard. 



(The Conclufion in our next.) 



ADVERTISEMENT. 



Various articles of Philofophical News, Accounts of Books, 

 and other fubjects, have necejfarily been pqjiponed this month, on 

 account of illnefs with which the Editor has been afflicted. 



The fame caufe has alfo unfortunately prevented his Draughtf- 

 ■man from completing fome Dejigns, which would elfe have ap- 

 peared. 



Mr. Ezekiel Walker has been prevented^ by fever e indifpofition, 

 from anfwering the paper of C. L. in our lajl, upon the Horizon- 

 tal Moon. 



A note has alfo been received from C. L. in which he begs leave 

 to notice, that the large lens mentioned __ in his experiment I. has a 

 clear aperture of 6,1 inches in its frame, and was fo ufed in that 

 experiment. 



