INTRODUCTION. 71 



The first, called the "method of corresponding states" was developed 

 for this purpose by Leduc. 1 It depends on the assumption of van der 

 Waals that two gases are in corresponding states and deviate equally from 

 the hypothetical perfect gas if their temperatures and pressures are equal 

 multiples or sub-multiples of their critical temperatures and pressures, 

 respectively. In calculating the correcting factor, the compressibility and 

 critical constants are used. Leduc 2 by this method obtained 64.056 

 as the molecular weight of sulphur dioxide, and 34.071 as the molecular 

 \veight of hydrogen sulphide, and hence from both S == 32.056. 



The second method is the "method of critical constants" as developed 

 by Guye. 3 The correction is calculated by the use of van der Waals's equa- 

 tion, the quantities a and b being calculated from the critical constants of 

 the gases under consideration. In a complicated manner Guye applied a 

 correction to the quantities a and b of van der Waals's equation, because 

 they appear to vary slightly with the temperature and the pressure, 4 and 

 in this way was obtained the value 64.065 as the molecular weight of sul- 

 phur dioxide; hence S = 32.065. 



The third method is called the "method of limiting density." It was 

 originated by Daniel Berthelot in 1898 5 and has been used by Rayleigh 6 

 and Jaquerod. 7 It depends on the experimental determination of the com- 

 pressibilities of gases at pressures in the neighborhood of one atmosphere. 

 By an extrapolation, the ''limiting ratio" of the densities at very low pres- 

 sures can be calculated. This method has reached its greatest perfection 

 in the hands of Lord Rayleigh, who has shown that for the permanent 

 gases the deviation from Boyle's law only varies slightly with the pressure 

 and therefore the extrapolation is fairly safe. This physical method has 

 been applied with the greatest success to the cases of hydrogen, carbon, 

 and, especially, nitrogen, because in these cases the correction is compar- 

 atively small. Here also, however, the application to sulphur is far less 

 satisfactory. It is to be regretted that Lord Rayleigh's work did not 

 include compounds of this element. 



Jaquerod and Pintza determined the density of sulphur dioxide at 760 

 mm., 570 mm. and 380 mm. pressure, and from these results calculated the 

 compressibility. The results were extrapolated to zero pressure on the 



'Leduc, Ann. de Chim. et Phys. [7], 15, 5 (1898). 



2 Leduc, Ann. Chim. et Phys. [?]. 15, 94 (1898). 



3 Guye, C. R.. 138, 1215 (1904) ; Bull. Soc. Chim. [3], 5 Aout (1905) ; Jour. Chim. 

 Phys., 3, 321 (1905). 



4 Guye, Bull. Soc. Chim. [3], 5 Aout, p. xn (1905). 



5 Berthelot, C. R., 126, 954, 1030, 1415, 1501 (1S98) ; Jour, de Phys. [3], 8, 263 

 (1899). 



Rayleigh, Phil. Trans. A., 204, 352 (1905) ; A., 196, 205 (1901), and A., 198, 417 

 (1902). 



7 Jaquerod and Pintza, C. R., 139, 129 (1904) ; Jaquerod and Scheuer, C. R., 

 140, 1384 (1905). 



