THE MOLECULAR WEIGHT OF LIQUIDS. 267 



as unit of pressure, an atmosphere, then the volume energy 

 of a gas is equal to a constant multiplied by the absolute 

 temperature, thus : pv = RT. The constant R, for all gases 

 which do not dissociate, is 0*0819. If, then, in investigat- 

 ing an unknown gas, we happened to find that its volume 

 energy increased proportionately to the absolute tempera- 

 ture, and that the proportionality factor was 0*0819, we 

 should be justified in concluding that the molecular weight 

 chosen in expressing v was the correct one. But, if the 

 proportionality factor were 2 x 0*0819 = 0*1638 we should 

 then conclude that the number chosen for the molecular 

 weight, and consequently the volume of the gas, was twice 

 as great as it should have been. 



If the factor had a value intermediate between 0*0819 

 and 0*1638 the deduction would follow that we were dealing 

 with a mixture, and, finally, if the value of the factor were 

 found to vary with rise of temperature, we should conclude 

 that the composition of the mixture was altering. The 

 volume energy would then be no longer a rectilinear, but a 

 curvilinear function of the temperature, and the composition 

 of the mixture could be deduced at any given temperature 

 by a comparison of the slope of the tangent to the curve at 

 that temperature with the slope of the normal line, expressed 

 by the number 0*0819. 



This method offers no advantage over the ordinary 

 method for determining the molecular weights of gases, but 

 an account of it has been given here, as an analogous 

 method has been employed to determine the molecular 

 weight of liquid substances. 



To the Hungarian physicist Eotvos belongs the credit 

 of having first shown (1886) that what has since been 

 termed the molecular surface energy by Ostwald, is, between 

 certain limits, a linear function of the temperature. Eotvos 

 also observed that there were certain exceptions to the rule, 

 and he pointed out that these exceptions could be made to 

 disappear by choosing appropriate molecular weights which 

 were higher than those ordinarily accepted. 



Reference has already been made to the gas equation pv 

 = RT, where/ is the pressure of the gas, v the volume contain- 



