152 Mi*. Curl Barus on the Pressure- Variations of 



Inferences. 



12. Equations expressing tension of saturated vapour in 

 terms of temperature have been invented in great number. 

 Among them the simple form of Magnus and the more elaborate 

 exponential due to Biot, and applied by Regnault, are given 

 in most text-books. A remarkably close-fitting exponential 

 is investigated and tested by Bertrand*. Quite recently 

 M. Oh. Antoinef has devised and applied a new form. 

 Following the early suggestions of Bertrand J and of J. J. 

 Thomson §, I have used the equation of Dupre ||, 



log p = A-BI6- Clog 0, . . . . (1) 



where p and 6 are corresponding values of pressure and 

 boiling-point in absolute degrees Centigrade, and A, B, and 

 G are constants. Equation (1) has been independently 

 interpreted by Thomson. Its immediate meaning appears 

 more clearly in the differential form 



or 



d P /dd=-p(c/e-B / i0 2 ), 

 deidp = (e>i P yj(ifW(i - 0C/B')) ■ 



with reference to which it is to be observed that C/B' is a 

 small quantity. 



Applying equation (1) to the results of the Tables I. to V., 

 I found the following set of constants by direct computation. 



Table VI. — Constants for Boiling-point and Pressure. 



Metal. A. 



B. 



C. 



Sulphur* 



-35969 

 -30567 

 +42-265 



- 3661 

 + 1391 

 + 11435 



-13-066 

 -11-180 

 +10-022 



Cadmium 



Zinc 



* When these constants were computed, the data of Table I. only were in 

 hand. 



The results are irregular, even as to sign. Other direct 

 methods gave similarly irregular constants, which need not 



* Thermodynamique, pp. 154 to 162 (Paris : Gauthier-Villars, 1887) 

 t C. R. cvii. pp. 681, 778, 836, 1888. 



X L. c. pp. 90 to 102. 



§ ' Application of Dynamics, &c., ? pp. 158 to 161 (Macmillan, 1888). 

 || Theorie mecanique de la chaleur, pp. 96 to 120 (Paris : Gauthier- 

 Villars, 1869). 



