34 



NEW METHOD FOR DETERMINING COMPRESSIBILITY 



The Probable Compressibility of Chlorine. 



It has already been suggested that there is good reason for believing 

 that chlorine is much more compressible than bromine. There are 

 indeed at least three reasons for this belief, namely : first, the fact that 

 chloroform and carbon tetrachloride are both much more compres- 

 sible than bromine, although their boiling points are no lower; sec- 

 ondly, because bromoform is so much less compressible than the 

 analogous chloroform ; and thirdly, because bromine is so much more 

 compressible than iodine, a fact which leads one to infer that chlorine 

 would be yet more so, because of the well known periodicity of these 

 elements. 



There are at least two ways in which a quantitative estimate may be 

 formed of the compressibility of chlorine. Fortunately the differences 

 between the boiling points of chloroform and bromoform (152 

 6i = 9i) is almost exactly the same as the difference between the 

 boiling points of chlorine and bromine (6o ( 32)= 92 ) ; or, to 

 express the same relation in another way, the boiling point of chlorine 

 is as much below that of chloroform as that of bromine is below that 

 of bromoform. 1 Thus the effect of the internal pressure of cohesion, as 

 indicated by the different boiling point, upon the compressibility in the 

 former case may be assumed to be the same as the known effect in the 

 latter case. Overlooking other minor differences, such as the absolute 

 temperature and the possibly varying effect of the hydrogen and carbon 

 (which however probably constitute only a small proportion of the 

 volume of chloroform and bromoform), it maybe supposed that the 

 following equation holds approximately true : 



Pc\ Pchcij a 



^CH Br 3 



From the figures already given for the variables in the right hand 

 member of this equation, it is easy to compute the following values 



1 The boiling points of the halide substitution products of methane are inter- 

 esting in their regularity. Each substitution of bromine for chlorine involves a 

 rise of boiling point of about 28. This regularity, which is shown in the table 

 below, supports the method of approach used above. 



