142 MODERN SCIENCE READER 



nearly 700 C. without devitrifying. The significance of 

 this curve is obvious when we consider that in working 

 the glass it must be brought down to a temperature at 

 which it possesses a sufficient degree of tenacity. The 

 dotted line on Fig. 1 indicates points of equal tenacity. 

 It will be seen that at temperatures corresponding to this 

 particular tenacity, glasses of certain compositions are be- 

 low the temperature required to keep them from devitrify- 

 ing. They cannot therefore safely be cooled down to this 

 degree of tenacity. Others, in a small region extending 

 about from seventy-one to seventy-six per cent. Si0 2 , are 

 above that temperature, and will therefore remain glassy 

 if cooled till they have the tenacity corresponding to the 

 dotted line. 



The pure soda glasses cannot be used for the general 

 purposes to which "glass" is put, as they are water- 

 soluble. The second diagram shows some of the properties 

 of a lime-soda glass, such as we have for example in 

 window glass. In this diagram the ordinates represent 

 the proportion of soda (Na 2 0), and the abscissae proportion 

 of lime (CaO). It is understood throughout that the 

 proportion of silica (Si0 2 ) is 100. The lines drawn out in 

 full are lines of equal stability, the numerals 2, 3, 5 repre- 

 senting increasing degrees of instability. The dotted lines 

 correspond to equal temperatures of devitrification. The 

 compositions suitable for window glass are those comprised 

 within the field bounded below by one of these dotted lines, 

 and above by one of the full lines. For if we step outside 

 this region, either the temperature of devitrification is too 

 high, so that the glass cannot be cooled down to a working 

 temperature, or else the glass is too unstable. 



In addition to lime and soda, many glasses contain 

 also alumina or boron trioxide. The effect of these con- 

 stituents is to increase the stability of the glass. In the 

 case of alumina this is probably due to the formation of 

 double silicates of the type of feldspar. The beneficial 

 influence of the boric oxide is probably due to another 



