360 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 

 The three results for aluminium are collated below : 



Mean values, a = O'OOO 022 041. b = O'OOO 000 010 599. 



The agreement of the results is very gratifying, and leaves no doubt as to the 

 accuracy of the observations. The final result may be expressed as follows : 



The mean coefficient of expansion a + bt of the pure aluminium of the compen- 

 sators, between and t, is 



O'OOO 022 04 + O'OOO 000 010 Qt, or 10" 8 (2204 + 1'06). 



The true coefficient of expansion at t, or the mean coefficient between any two 

 temperatures whose mean is t, a. = a + 26^, is 



a = O'OOO 022 04 + O'OOO 000 021 2t, or = 10~ 8 (2204 + 2'12). 



The value given by FIZEAU for aluminium for 40 (' Compt. Kend.,' vol. 68, p. 1125 ; 

 and ' Ann. Chim. Phys.,' [4], vol. 8, p. 335), is O'OOO 023 13 ; he further gives for the 

 increment, Aa/Ai = 2fc, the value 2'29. This increment agrees fairly well with the 

 author's value of 2 '12 ; and if the value for is calculated by diminishing 

 O'OOO 023 13 by 40 times the increment 2'29, the number O'OOO 022 21 is obtained, a 

 figure which also agrees satisfactorily with the author's constant a. The coefficient 

 for 40, calculated from the author's formula, is O'OOO 022 89. 



DETERMINATION OF THE EXPANSION OF THE CRYSTAL-COVEKING-GLASSES. 



For the purpose of determining the expansion of the black glass of the covering- 

 glasses employed with crystals whose surfaces cannot be made to take a polish equal 

 to that of glass, a block of the same glass, 13 millims. thick, was procured. The 

 mode of adjustment and observation was precisely similar to that for the aluminium 

 block. Two series of determinations were made, employing red hydrogen light. In 

 each case the platinum-indium screws were adjusted so as to leave an air-layer about 

 a quarter of a millimetre thick between the upper black glass surface and the lower 

 surface of the large cover -wedge. 



This glass proved to have an expansion coefficient slightly less than that of 

 platinum-iridium, so that the converse was observed of what happens in the case of 

 aluminium ; that is to say, instead of a reduction of the thickness of the air-layer by 



