GLASS 21 the most perfect portions of the comparatively homogeneous mass thus obtained, the lenses were cut out by a process similar to that of sawing blocks of stone. By one of the sons of Guinand the secret was imparted to M. Bontemps; and in 1828 lenses were made in France of 12 to 14 in. diameter. In 1848 Bon- temps went to England, and in conjunction with the Messrs. Chance and co. made disks of flint and of crown glass larger than any be- fore produced. At the exhibition in London in 1851, a disk of flint glass was exhibited by Messrs. Chance and co. 29 in. in diameter and weighing 2 cwt. ; and at the Paris exposition in 1855 they exhibited one of the same diam- eter made of crown glass. One of these was afterward sold to the French government for 1,000. They are of pure color, and of such homogeneous structure that the light is trans- mitted without polarization. Prof. Faraday, one of a committee appointed by the astro- nomical society of London to experiment upon the means of producing optical lenses, while Guinand's secret method of making these 6 in. in diameter was exciting the admiration of the scientific world, discovered the heavy glass called by his name (composed of protox- ide of lead 104 Ibs., silicate of lead 24, and dry boracic acid 25), which has proved of con- siderable importance in investigations connect- ed with the polarization of light ; but its lia- bility to change unfits it for general optical uses. Lenses both of flint and of crown glass are used in the object glasses of achromatic tele- scopes, serving by their combination to coun- teract the unequal tendency of each to dis- perse the rays of light. It seems to be con- ceded by scientific men that the glass best adapted to achromatism would be a flint glass possessing a smaller refractive power and a larger dispersive index, and a crown glass hav- ing, conversely, a greater refractive power and a less dispersive index. The annual pro- duction of plate glass in Europe may be stated in round numbers at upward of 10,000,000 sq. ft., of which about 4,000,000 sq. ft., valued at about 28,000,000 francs, is produced in France, 8,750,000 in England, 1,500,000 in Germany, and 1,000,000 in Belgium. The industry is limited to a few large establishments, there being six each in France and England, and two each in Germany and Belgium. In addition to the above, large quantities of rough plate glass are made in England for horticultural and other cheap purposes. About 15,000,000 sq. ft. of window glass, of the value of about 15,- 000,000 francs, is produced annually in France, and about 100,000,000 bottles, valued at about 20,000,000 francs; the production of flint glass amounts to about 15,000,000 francs, and of ordinary table glass about the same. The en- tire production of the country exceeds 75,000,- 000 francs. The exports of glass from Eng- land in 1872 were 2,131,924 sq. ft. of plate glass, valued at 243,780; 113,004 cwt. of flint, valued at 300,484; 760,836 cwt. of common KINDS. No. of establish- ment!. Hands em- ployed. Capital. Annual pro- duct*. Cut 29 5 18 114 85 285 200 170 12,308 2,859 $136,700 195,700 148,800 10,385,882 8,244,560 $470,875 355,250 297,480 14,300,949 3,811,308 Plate Stained Ware, not specified. Window Total 201 15,822 $14,111,642 $19,246,862 bottles, valued at 373,138; and other kinds of glass to the value of 204,593. The latest statistics on the manufacture of glass in the United States are afforded by the census of 1870, as follows : The establishments were chiefly in Pennsyl- vania, New York, New Jersey, and Ohio. Of the five manufactories of plate glass, three were in Ohio and one each in New York and New Hampshire. Not included in this statement is the Lenox rough plate glass company at Lenox, Mass. The importations of glass and glass ware into the United States for the year ending June 30, 1873, amounted to $5,834,712, including cylinder, crown, and common win- dow, $2,759,728 ; cylinder and crown polished, $21,217; fluted, rolled, or rough plate, $34,- 180 ; cast polished plate not silvered (2,482,- 359 sq. ft.), $1,550,857; cast polished plate sil- vered (2,392,274 sq. ft), $823,076; other man- ufactures, $2,230,986. Of the cylinder, crown, or common window, $2,181,044 worth came from Belgium and $451,223 from England; of the cast polished plate not silvered, 1,955,666 sq. ft, valued at $1,252,991 , from England, 246,- 698 sq. ft, valued at $155,450, from Belgium, and 39,047 sq. ft., worth $22,963, from France ; of the silvered plate, 2,297,049 sq. ft, valued at $764,913, was the production of England. COLORED AND ORNAMENTED GLASS. Mould- ed or pressed glass never exhibits its full lustre or the clearly cut configurations of the mould. This defect is remedied by the process called cutting glass, which is in reality grinding and afterward polishing it. It is easily effected upon the soft flint glass by applying the sur- faces to be cut to the face of revolving disks of iron or copper fed with emery, or, for coarse grinding, with sand and water. Stones are also used instead of the metallic disks. The marks of the rough grinding are removed by a smooth grindstone, and the polishing is then completed by wooden disks, to which pumice or rotten stone, and finally the preparation of tin and lead called putty powder, are applied. The fine polishing of chandelier drops is effected by a lead wheel supplied with fine rotten stone and water. Glass globes and lamp shades ac- quire their interior ground surface by the wear- ing action of sand placed within them, the globes being themselves introduced into the interior of a drum which is caused to rotate rapidly. Letters and designs are engraved on glass by the use of small disks of copper set in rapid revolution by means of a lathe operated by the
