114 Messrs. Nichols and Wheeler on the Coefficient 



(1) the volume of the dilatometer at some known tempe- 

 rature ; 



(2) the coefficient of expansion of the glass of the dilato- 

 meter ; 



(3) the relative volumes of the immersed and exposed por- 

 tions of the dilatometer ; 



(4) the exact internal area of the neck at every point ; 



(5) the height of the mercury column in the neck ; 



(6) the temperature of the bath and of the surrounding 

 atmosphere. 



We made use of two dilatometers, identical in principle and 

 differing but slightly in form, viz. : — 



cubic centim. 



Dilatometer a, capacity at 15° 0. . . 286*3425 



„ b, „ „ ... 282-6184 



Coefficient of expansion of the glass . '000024 



The calibration of the necks by the method usually employed 

 in calibrating thermometer-tubes was impracticable on account 

 of the large diameter of the tubes. The method resorted to 

 was as follows: — The dilatometer having been inverted and 

 the neck at the bent portion stopped up, equal amounts of 

 mercury, carefully weighed, were poured down the neck; and 

 the height of the mercury column, after the addition of each 

 such portion, was measured with the cathetometer. In this 

 way a very satisfactory curve of cross-section areas was ob- 

 tained and the capacity of each millimetre of the neck tabu- 

 lated. 



The solution within the dilatometer during each experiment 

 was kept under constant pressure by means of the adjustable 

 mercury-tube T, which was pushed up as fast as the mercury 

 rose within the neck. The liberation of gas from the solution, 

 which would inevitably have followed any considerable de- 

 crease of pressure, was by this means prevented. 



The thermometers used were four in number : — 



Baudin 7310, 7313, and 7315 ; 

 and 



Golaz. 



The variation of each of them from the air-thermometer was 

 determined, and is given in our tables of observations. The 

 Golaz thermometer showed an almost constant difference from 

 the air-thermometer, while in the case of the Baudin thermo- 

 meters this difference varied for different parts of the scale. 

 In the bath, water was used for ordinary temperatures ; and 

 between 0° C. and —20° C. we employed the usual freezing- 

 mixture of common salt and ice. 



