414 Prof. Masson on the Boiling-points, Molecular 



vapour-pressure being equal. Here also the deviations are 

 small but real. 



If these two expressions be taken together, we get the 

 following rider to the first law : — 



Different compounds of the same group are expanded equally 

 in changing from liquid at the boiling-point to vapour at the 

 boiling-point ; for 



vt Vt v t c' /D v 

 \p) 



T " T 



c 



If we neglect deviations from normal vapour-density, and 

 if all measurements be expressed in grammes and cubic centi- 

 metres, the actual value of vt for any vapour at atmospheric 



pressure is — ^^ — . Hence c'= 81*7766, and 



and this ratio is of constant value in each group of com- 

 pounds. It may be called the coefficient of ebullition expan- 

 sion. (Compare J. A. Groshans, Phil. Mag. 1885, xx. p. 197.) 



The chief evidence in support of the law is afforded by the 

 compounds of the hydrocarbon radicals with the halogens. 

 The data they afford are brought together in Table I., where 

 they are divided into eleven groups according to the con- 

 stitution of the radical. It will be seen that the examples are 

 of various kinds, monohalides of different homologous series, 

 both saturated and unsaturated, being represented, and also 

 dihalides and trihalides. The molecular volumes are calcu- 

 lated from the densities recorded by various observers, but as 

 far as possible the same observer has been quoted for the com- 

 pounds of the same group, so as to ensure comparable results 

 within that group. 



The different columns of Table I. give (1) the name of the 

 compound, (2) its formula, (3) the molecular weight, M, (4) 

 the liquid density, Dt, at the boiling-point under atmospheric 

 pressure, (5) the observer's name, (6) the found molecular 

 volume, Vt, (7) the boiling-point on the absolute scale, T, (8) 



V T 



the value of the ratio -^, and (9) the number 81*7766 ^-, 

 -L Vt 



which is, as already explained, the measure of what may be 



called the ebullition expansion. 



