146 MODERN SCIENCE READER 



Another property of glass which is of great importance 

 in connection with certain of its applications is its behavior 

 toward changes of temperature. If a hot glass article, 

 while still soft, is allowed to cool quickly, inequalities in 

 density are produced within the mass of glass, giving rise 

 to internal strains. A glass tube, for this reason, is always 

 under a peripheral compressing strain, while the inside is 

 under tension. The consequence of this is a high resistance 

 to mechanical injury on the outside, while any scratch on 

 the inside, especially in the case of thick walled tubes, al- 

 most inevitably leads to the cracking of the tube. Such 

 tubes in which the outer wall is under a compressing strain 

 and the inner wall under tension nevertheless possess cer- 

 tain advantages over tubes free from strain. For if such 

 a tube is heated from within to a temperature considerably 

 exceeding that of its surroundings, the thermal expansion 

 of the inner layers must first neutralize the existing tension 

 before the tube can acquire any tangential expansion strain 

 tending to burst the tube. Such a tube can therefore with- 

 stand considerably higher differences of temperature be- 

 tween its inner and outer surface than a tube initially free 

 from strain. The same advantage is gained where the 

 tube is to be subjected to pressure from within, as in water 

 gages for boilers. For this purpose tubes free from strain 

 cannot be used at all. The sensitiveness of the inner wall 

 may be prevented by making the tube of two layers having 

 a different coefficient of expansion with temperature. 

 Tubes of this kind are largely employed for boiler gages. 



A glass having a very low coefficient of expansion will 

 cool without acquiring any appreciable strain. Such a 

 glass is particularly well adapted to resist sudden temper- 

 ature changes and is used for example in making chimneys 

 for incandescent gaslight. Another purpose for which 

 glass possessing special properties is required is the con- 

 struction of thermometers. One of the errors to which 

 thermometers are subject is the apparent lowering of the 

 freezing point if the latter is observed soon after the 



