660 GLASS 



with which the discovery of tho 'so-called' magnetism of a ray of light was made. 

 M. Guinand died, and one of his sons worked with M. Bontemps, whilo tho widow 

 and another son set up works in Switzerland. From their manufactory some examples 

 of lenses were sent to the Great Exhibition of 1851. M. Bontemps was in 1848 pre- 

 vailed upon to accept the invitation of Messrs. Chance Brothers and Co. to unite with 

 them in attempts they were then making to improve the quality of glass. They suc- 

 ceeded in producing discs of extraordinary dimensions in flint of 29 inches diameter, 

 weighing two cwts. and of crown-glass up to 20 inches. Messrs. Chance, at the re- 

 commendation of the Jury, were induced to submit their disc of flint-glass to the 

 operation of grinding, finishing, and other processes necessary in order to ascertain the 

 uniformity of its density throughout, and its superior quality was fully established. 



M. Maes of Clichy, near Paris, proposes to manufacture optical glass, with the 

 addition of baryta, magnesia, and oxide of zinc, in combination with boracic acid. 

 The glass manufactured by M. Maes is exceedingly beautiful, but the boracic acid 

 renders it very expensive. 



An achromatic object-glass for telescopes and microscopes consists of at least two 

 lenses ; the one made with glass of lead, or flint-glass, and the other with crown-glass ; 

 the former possessing a power of dispersing the coloured rays relatively to its moan 

 refractive power much greater than tho latter ; upon which principle, the achromatism 

 of the image is produced, by re-uniting tho different coloured rays into one focus. 

 Three plans have been prescribed for obtaining homogeneous pieces of optical glass : 

 1, to lift a mass of it in large ladles, and let it cool in them; 2, to pour it out from 

 the^pots into moulds ; 3, to allow it to cool in the pots, and afterwards to cut it off in 

 horizontal strata. The last method seldom affords pieces of uniform density, unless 

 peculiar precautions have been adopted to settle the flint-glass in uniform strata ; 

 because its materials are of such unequal density, the oxide of lead having ;i spe- 

 cific gravity of 8, and silica of 27, that they are apt to stand at irregular heights in 

 the pots. 



One main cause of these inequalities lies in the construction of the furnace, whereby 

 the bottom of the pot is usually much less heated than the upper part. In a plate-glass 

 furnace the temperature of the top of the pot has been found to be 130 Wedgw., while 

 that of the bottom was only 110, constituting a difference of no less than 2160 V. 

 The necessary consequence is that the denser particles which subside to tho bottom 

 during the fusion of the materials, and after the first extrication of the gases, must 

 remain there not ; being duly agitated by the expansive force of heat, acting from 

 below upwards. 



The following suggestions, deduced from a consideration of principles, may pro- 

 bably lead to some improvements, if judiciously applied. The great object is to 

 counteract the tendency of the glass of lead to distribute itself into strata of different 

 densities ; which may be effected either by mechanical agitation or by applying the 

 greatest heat to the bottom of the pot. But however homogeneous the glass may bo 

 thereby made, its subsequent separation into strata of different densities must be pre- 

 vented by rapid cooling and solidification. As the deeper the pots, the greater is tho 

 chance of unequal specific gravity in their contents, it would be advisable to make them 

 wider and shallower than those in use for making ordinary glass. The intermixture 

 may be effected either by lading the glass out of one pot into another in tho furnace, 

 and back again, with copper ladles, or by stirring it up with a rousor, and then allowing 

 it to settle for a short time, till it becomes clear and free from air-bubbles. The pot 

 may now bo removed from the furnace, in order to solidify its contents in their ho- 

 mogeneous state ; after which the glass may be broken in pieces, and be perfected by 

 subjecting it to a second fusion ; or what is easier and quicker, we may form suitable 

 discs of glass without breaking down tho potful, by lifting it out in flat copper ladles 

 with iron shanks and transferring the lumps after a little whilo into tho annealing 

 leer. 



To render a potful of glass homogeneous by agitation is a most difficult task as an 

 iron rod would discolour it, and a copper rod would bo apt to melt. An iron rod 

 sheathed in laminated platinum would answer well, but for its expense. A stone- 

 ware tube supported within by a rod of iron might also bo employed for the purpose 

 in careful hands; the stirring being repeated several times, till at least the ^ 

 suffered to stiffen a little by decrease of temperature. It must then bo allowed to settle 

 and cool, after which the pot, being of small dimensions, may be drawn out of the 

 fire. 



The second method of producing the d< .sired uniformity of mixture, consists in 

 applying a greater heat to tho bottom than to the upper part of the melt i; 

 Fig. 1101 represents in section a furnace contrived to effect this object. ]t is cylin- 

 drical, and of a diameter no greater than to allow the flames to play round the pot, 

 containing from three to four cwts. of vitreous materials. A is the pot resting upon 



