292 J. Dewar — Boiling Point of Liquid Hydrogen. 



stand and to read the barometer placed alongside simultan- 

 eously. 



* The formula of reduction used was that given by Chappuis 

 in the Travaux et Memoires du Bureau International des 

 Poids et Mesures, vol. vi, p. 53, namely, 



where Y is volume of reservoir at 0° C. 



T, temperature of reservoir, measured from 0° C. 



v, volume of " outside" space at the temperature of the 

 room. 



t, temperature of the room. 



a, coefficient of expansion of the thermometric gas. 



/3, coefficient of alteration of volume of reservoir, due to 

 change of pressure. 



S, coefficient of expansion of substance of reservoir. 



H , initial pressure (in these experiments always reduced to 

 0° C). 



H + A, pressure at temperature T, after all corrections have 

 been made. 



On putting /3 = as already explained, equation (1), by 

 algebraic transformation and without any approximation, was 

 altered into the form. 



Where T = T - 273 + (-? ' (Say) = T '" (2) 



in which P and P replace H and H 4-A and x = 



V (l + a«) 



The gases used as thermometric substances were hydrogen, 

 oxygen, helium, and carbonic acid. The values of a adopted 

 in equation (3) were taken from Chappuis' memoir, and were 

 0-00366254 for the first three and 0-00371634 for carbonic 

 acid. The reciprocals of these coefficients are 273-035 and 

 269-083. The number " 273 " which appears in 6 is so nearly 

 equal to the reciprocal of the former value for a that it was 

 allowed to remain for the first three gases ; but in dealing with 

 carbonic acid it was replaced by 269-083. 



In these experiments T x is always negative, and numerically 

 less than 273, so that the value of 6 is always greater than 

 unity; nevertheless it differs from it but slightly, its value 

 being unity when T 1 = —273° C, and rising" to 1'02 when 

 Tj = 0° C. in the case of thermometer No. 1, where x — 1/50. 

 It may be noted that when h is neglected, T 1 is the usual value 



