CHAPTER XI. 



THE CHANGE OF THE REFRACTION OF AIR WITH TEMPERATURE. 



78. Apparatus. In the earlier report (Carnegie Inst. Wash. Pub. 149, III, 

 Chap. 15, p. 223) I began some experiments on the change of the refrac- 

 tive index of air with rise of temperature. The question is interesting, inas- 

 much as the temperature coefficient has not in most investigations been 

 found identical with the coefficient of expansion of air, as Lorentz had obtained 

 it and as would otherwise be anticipated; but a value, over 3 per cent larger, 

 first put forward by Mascart, seems preferable. My earlier work was left 

 unfinished, however, because the design of the apparatus, in which the refrac- 

 tion tube was heated in an independent annular steam-bath, was unsatis- 

 factory. It seemed to be impossible to reach the temperature of the steam 

 in that way, even after half a day's waiting. In the present work, therefore, 

 the apparatus is modified, so that the steam may play directly on the long 

 refraction tube. In this way the temperature difficulty was quite eliminated. 



93 



J 



s 



,a 



w 



^3 



rt: 



s^Jt'* '' 



^a 



The tube containing the air column was made of inch brass gas-pipe, 71.7 

 cm. long (between windows) and 2.5 cm. in internal diameter (A, fig. 93, 

 which shows one end of the apparatus) . The ends were closed with the usual 

 brass caps a, in which round windows, about 2 cm. in diameter, had been cut 

 on the lathe. The ends were closed by plates of glass g, secured between two 

 jackets of rubber and " vulcanized" fiber. L shows the axis of the beam of 

 light. 



BB is the steam chamber, steam entering at S and leaving by a similar tube 

 at the other end of the apparatus. Steam is thus directly in contact with the 

 tube. The projecting end of A is inclosed by a recess packed with wadding, 



133 



