650 



GLASS 



arch of the oven and the tops of the pots. This is best prevented by introducing at 

 first into the pots the whole of the salt mixed with the charcoal, the lime, and one- 

 fourth part of the sand; fusing this mixture at a moderate heat, and adding gradually 

 afterwards the remainder of the sand, increasing the temperature at the same time. 

 If wo put in the whole ingredients together, as is done with potash-glass, the sand and 

 lime soon fall to the bottom, while the salt rises to the surface, and the combination 

 becomes difficult and unequal. 



Sulphate of potash acts in the same way as sulphate of soda. 



Chloride of sodium also, according to Kirn, may be used as a glass-flux with advan- 

 tage. The most suitable proportions are 4 parts of potash, 2 of common salt, and 3 

 of lime, agreeably to the following compositions : 



1. 2. 



Quartz-sand 60-0 57'1 



Calcined carbonate of potash . . . 17 '8 19*1 



Common salt 8-9 9'5 



Lime 13'3 14'3 



For No. 1, the melting heat must be 10 hours, which turns out a very pure, solid, 

 good glass; for No. 2, 23 hours of the furnace are required. Instead of the potash, 

 Glauber salt may be substituted ; the proportions being then 19-1 Glauber suit, 9'5 

 chloride of sodium, 14'3 lime, 57'1 sand, and 1'3 charcoal. 



.The oxide of lead is an essential constituent of the denser glasses, and may bo 

 regarded as replacing the lime, so as to form with the quartz-sand a silicate of lead. 

 It assimilates best with purified pearlash, on account of the freedom of this alkali 

 from iron, which is present in most sodas. 



Its atomic constitution may be represented as follows : 



The above analysis by Berthier relates to a specimen of the best English crystal- 

 glass, perfectly colourless and free from air-bubbles. This kind of glass may, how- 

 over, take several different proportions of potash and silica to the oxide of lead. 



The composition of mirror-plate, as made on the Continent, is as follows : 



White quartz-sand 300 Ibs. 



Dry carbonate of soda 100 



Lime slaked in the air 43 



Gullet, or old glass 300 



The manganese should not exceed one-half per cent, of the weight of soda. 



Optical glass requires to be made with very peculiar care. It is of two different 

 kinds : namely, crown-glass and flint-glass. The latter contains a considerable pro- 

 portion of load, in order to give it an increased dispersive power upon the rays of 

 light, in proportion to its mean refractive power. 



Optical crown-glass should be perfectly limpid, and have so little colour, that a 

 pretty thick piece of it may give no appreciable tingo to the rays of light. It should 

 bo exempt from striae or veins as well as air-bubbles, and have not the slightest 

 degree of milkiness. It should, moreover, preserve these qualities when worked in 

 considerable quantities. Potash is preferable to soda for making optical crown-glass, 

 because the latter alkali is apt to make a glass which dovitrifies and becomes opal- 

 escent, by long exposure to heat in the annealing process. A simple potash-si lii-ato 

 would be free from this defect, but it would bo too attractive of moisture, and apt to 

 decompose eventually by the humidity of the atmosphere. It should, therefore, 

 contain a small quantity of lime, and as little potash HS suffices for making a per- 

 fect glass at a pretty high temperature. It is probably owing to the high heats 

 used in the English crown-glass works, and the moderate quantity of alkali 

 whirh is employed, that our crown-glass has been found to answer so well for optical 

 purposes. 



