A Compensated Interference Dilatometer. 211 



meter, bent so that the bulb lies within the chamber in contact 

 with the tripod itself ; this has been found to be a point of the 

 first importance. With the aid of the thermostat and a graduated 

 gas tap a constant temperature can readily be attained. 



The author determines the position of the bands at about 10 C., 

 again near 70, and once more about 120, in order that not only 

 the mean coefficient, but the absolute coefficient at any temperature 

 and the increment per degree may be ascertained. The transit of 

 each band is followed and recorded permanently by means of a 

 specially constructed tape-puncturing recorder. This method is 

 found more satisfactory than relying exclusively on the Abbe 

 method of mere observation of the initial and final positions of the 

 bands for light of two wave-lengths. 



The results of numerous determinations of the expansion of the 

 platinum-iridium of the tripod are given, carried out with the sur- 

 face of the tripod table and the cover-wedge separated at the long 

 interval of 12 mm., by the aid of green mercury light. The mean 

 value is very similar to that of Benoit, and is 



a = 10 



The result of several determinations in red hydrogen light of the 

 expansion of the pure aluminium used for the series of compensa- 

 tors, carried out by the Fizeau relative method with a block 12 mm. 

 thick, is 



Similar determinations for the black glass of the crystal-covering 

 plates afford the value : 



In a subsequent memoir the author intends to present the resiilta 

 of determinations of the expansion of the sulphates and selenates of 

 potassium, rubidium, and caesium. 



