54 THE ROYAL SOCIETY OF CANADA 



a plan of which is shown. A water vacuum pump was connected to M» 

 in order to cause the liquid air to flow through the lead tube. By 

 adjusting the third -way of the tap by means of a pinch cock, so as 

 to admit more or less air, the quantity of liquid air drawn through 

 could be regulated to any degree of fineness. 



As a fluid for the bath we used methylated spirits. This enabled 

 us to obtain steady temperatures as low as -90°C. without any serious 

 increase n the viscosity of the spirits. A water motor, run at a high 

 speed, caused excellent circulation of the spirits in the bath around 

 the bulb of the dilatometer D. 



A platinum resistance thermometer T, enabled us to measure 

 the temperature of the bath. The bath could be maintained constant 

 to 1/100 of a degree at any desired temperature down to the freezing 

 point of the mercury. 



Theory of the Method. 



The theory of the weight dilatometer may be stated as follows: — 

 Let V = volume of fluid and dilatometer at 0°C. 

 V t = volume of dilatometer at t°C. 

 a = absolute expansion of the fluid. 



g = absolute expansion of them aterial of the dilatometer^ 

 P = density of the fluid at 0°C. 

 then 



V t = V (1 + g t) 

 Po = Pt (1 + a t) 

 We have 



V t V = M t (1) 



VoPo = M t + m (2) 



where m = the mass of fluid expelled at a temp. t°C. 

 From 1 and 2 we have 



1 + a t M t + m 



M t t M t 



which is the formula used for calculation. 



