540 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Though gliss was known in ancient times, it was exceedingly 

 rare and costly, and, even among the Greeks and Romans, it was 

 a luxury. 



The manufacture of glass Avas introduced into Euroj)e by the 

 Crusaders ; and, for a long time, Venice had the monopoly of 

 this industry. In the latter part of the seventeenth century, in 

 France, the manufacture was monopolized by the nobility, all 

 other persons being excluded from it by law. Though the art 

 of making glass is so old, its chemistry was first understood by 

 a chemist who died in 1S47. It is claimed for Berzelius that he 

 first showed the relations of the composition of glass to chemis- 

 tr3^ He ascertained that silica is an acid — silicic acid — and 

 combines with bases to form salts — glass is a double or triple 

 silicate of soda, or potassa, of some familiar oxide of metal. 

 Bv the combination of silicic acid with different bases — either 

 one or more in the same compound — a great variety of glasses 

 may be made, but there^ are only eight of these varieties that are 

 of any considerable importance. The simplest of these is soiuble 

 glass, which is a single silicate of potassa or soda. This article 

 is attracting much attention at the present time, and there is a 

 practical maker of it in the room, who, I hope, will give us an 

 account of the process of its manufacture. Bohemian or crown 

 glass is a silicate of potassa and lime. This variety is used for 

 the convex portion of achromatic lenses. Window glass is a sil- 

 icate of soda and lime. Bottle glass is made of very coarse 

 materials ; it is a silicate of soda, lime, iron and alumina. In 

 this phial are specimens of the materials used in making this 

 bottle, just as as they are mixed at the manufactory. Crystal 

 glass is a silicate of potassa and lead. Flint glass is the same, 

 with a larger proportion of lead. This is the glass used for the 

 concave portion of achromatic lenses. Enamel is a silicate and 

 stannate of soda and lead ; or the soda may be replaced by pot- 

 assa. The presence of iron in glass gives it a green or red color. 

 This color may be removed by the binoxide of manganese, but 

 in this case it is liable to return in the course of time by the 

 reoxidatioa of the iron. In Bond street, in this city, there are 

 old windows in which the glass has a reddish hue, obtained in 

 this way. Will Dr. Stevens give us the geology of this subject ? 



Dr. Stevens. — The silex used in making glass is obtained 

 usually in the form of sand. Formerly all the sand used in this 

 country in glass making was collected on the sea shore. The 

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