GLASS 659 



floor of the vault, large iron trays are laid and hooked to each other in a series, which 

 are drawn from the fire-end towards the other by a chain, wound about a cylinder by 

 a winch-handle projecting through the side. The flint-glass articles are placed in their 

 hot state into the tray next the fire, which is moved onwards to a cooler station when- 

 ever it is filled, and an empty tray is set in its place. Thus, in the course of about 20 

 hours, the glass advances to the cool end thoroughly annealed. 



Besides colourless transparent glass, which forms the most important part of this 

 manufacture, various coloured glasses are made to suit the taste of the public. The 

 opaline crystal may be prepared by adding to the above composition (No. 2) phos- 

 phate of lime, or well-burnt bone-ash in fine powder, washed and dried. The article 

 must be as uniform in thickness as possible, and speedily worked into shape, with a 

 moderate heat. Oxide of tin, putty-powder, was formerly used for making opalescent 

 glass, but the lustre of the body was always impaired by its mea,ns. 



Crystal vessels are made of which the inner surface is colourless, and all the external 

 facets coloured. Such works are easily executed. The end of the blowing-rod must 

 be dipped first in the pot containing colourless glass, to form a bulb of a certain size, 

 which being cooled a little is then dipped for an instant into the pot of coloured glass. 

 The two layers are associated without intermixture ; and when the article is finished 

 in its form, it is white within and coloured without. Fluted lines somewhat deeply 

 cut, pass through the coloured coat, and enter the colourless one ; so that when they 

 cross, their ends alone are coloured. 



For some time past, likewise, various crystal articles have been exhibited in the 

 market with coloured enamel figures on their surface, or with white incrustations of a 

 silvery lustre in their interior. The former are prepared by placing the enamel object 

 in the brass mould, at the place where it is sought to be attached. The bulb of glass 

 being put into the mould, and blown while very hot, the small plate of enamel gets 

 cemented to the surface. For making the white argentine incrustations, small figures 

 are prepared with an impalpable powder of dry porcelain paste, cemented into a solid 

 by means of a little gypsum plaster. When these pieces are thoroughly dried, they 

 are laid on the glass while it is red hot, and a large patch of very liquid glass is placed, 

 above it, so as to encase it and form one body with the whole. In this way the in- 

 crustation is completely enclosed; and the polished surface of the crystal which 

 scarcely touches it, gives a brilliant aspect, pleasing to the eye. 



OPTICAL GLASS. An uniform flint-glass, free from striae, or wreath, is much in 

 demand for the optician. It would appear that such an article was much more com- 

 monly made by the English manufacturers many years ago than at present ; and that 

 in improving the brilliancy of crystal-glass they have injured its fitness for construct- 

 ing optical lenses, which depends, not so much on its whiteness and lustre, as on its 

 homogeneous character. Even a potful of pretty uniform glass, when it stands some- 

 time liquid, becomes eventually unequable by the subsidence of the denser portions ; 

 so that strise and gelatinous appearances begin to manifest themselves, and the glass 

 becomes of little value. Gas allowed to cool slowly in mass in the pot is particularly 

 full of wreath, and if quickly refrigerated, that is, in two or three hours, it is apt to 

 split into a multitude of minute splinters, of which no use can be made. For optical 

 purposes, the glass must be taken out in its liquid state, being gathered on the end of 

 the iron rod from the central portion of a recently-skimmed pot, after the upper layers 

 have been worked off in general articles. 



M. Guinand, of Brenets near Neufchatel, a workman in the watch and clock trade, 

 appears to have discovered processes that furnished almost certainly pieces of flint- 

 glass capable of forming good lenses of remarkable dimensions, even of 11 inches* 

 diameter, of adequate density and transparency, and nearly free from strise. Guinand'a 

 plan consisted mainly in thoroughly mixing the melted ' metal ' with an iron rod. 

 Guinand joined M. Frauenhofer, of Munich, and one of the largest of the lenses 

 produced by them, the diameter of which is 9 inches, is now in the observatory at 

 Dorpat. 



M. Cauchoix, the eminent French optician, says, that out of ten object-glasses, 

 4 inches in diameter, made with M. Guinand's flint-glass, eight or nine turned out 

 very good, while out of an equal number of object-glasses made of the flint-glass of 

 the English and French manufactories, only one, or two at most, were found serviceable. 



Guinand was long in communication with the Royal Astronomical Society of London ; 

 and he sent over some discs on flint-glass, of which Messrs. Dollond and Herschel 

 made a favourable report. A commission was formed, consisting of Herschel, Dollond, 

 Faraday, and Roget, but owing to the annoying interferences of the Excise officers, 

 notwithstanding the Government had made some special exceptions in favour of these 

 scientific experiments, the results were not practically of that high value which might 

 have been expected. Many of the observations however, were of great value. Amongst 

 other discoveries might be named the remarkable heavy glass, the Silico-borate of lead, 



UU 2 



