Determination of the Critical Density. 507 



Fluorbenzone 3*79 Stannic Chloride. . . . 3-75 



Chlorobenzeno 3*78 Ether 3-82 



Bromobenzene .... 3*81 Methyl Alcohol .... 4*52 



Iodobenzene 3*79 Ethyl Alcohol 4-02 



Benzene 3'71 Propyl Alcohol .... 4-02 



Carbon Tetrachloride. 3*66 Acetic Acid 5*00 



The first eight numbers agree fairly r well; and it may be 

 stated that for many substances the ratio of the actual critical 

 density to the theoretical density (for a perfect gas) is about 3* 8. 

 The alcohols, notably methyl alcohol, and acetic acid, form 

 exceptions to this law, and in the case of the esters the ratios 

 (3-89 to 3*95) are rather high. 



LYTII. On the Determination of the Critical Density. By 

 Sydney Young, D.Sc, and G. L. Thomas, B.Sc, Uni- 

 versity College, Bristol*. 



IT has been shown by M. E. Mathias (Compt. Bend. cxv. 

 p. 35) that the method of determining the Critical 

 Density proposed by MM. Cailletet and Mathias {Compt. 

 Rend. cii. p. 1202, civ. p. 1563) gives good results in a con- 

 siderable number of cases. We have recently investigated 

 some of the thermal properties of five of the lower esters, 

 and find that these substances afford further proof of the 

 truth of the " Law of the Diameter " described by these 

 authors. We therefore think it may be well to give the 

 data illustrating this law, without waiting for the com- 

 pletion of the work on the remaining esters. 



The law of the diameter may be briefly explained as 

 follows : — It is well known that if the densities of any sub- 

 stance, both as liquid and as saturated vapour, be mapped as 

 ordinates against the temperatures as abscissae, they fall on 

 a continuous curve which passes through the critical point. 

 This curve possesses, according to MM. Cailletet and Mathias, 

 the characteristic property that the locus of the middle points 

 of chords parallel to the axis of ordinates is a straight line. 

 If this is so, it follows that the ordinate of the point on the 

 diameter corresponding to the critical temperature gives the 

 critical density. 



That the law holds very closely for the esters so far in- 

 vestigated is shown by the results tabulated below. The 

 densities of each substance, both in the state of liquid and 

 of saturated vapour, together with the. means of these values, 



* Communicated by the Physical Society: read October 28, 1892.' - 



