of Superheated Vapours, 42 1 



periments on steam*, adhesion always comes into play. Fair- 

 bairn and Tate's result is therefore itself quite intelligible, but 

 throws no light upon the exact behaviour of steam. Moreover 

 these experimenters would not possibly, by their method, have 

 attained any results even for not adhering vapours ; for they fall 

 into the oft committed and as often exposed error of having the 

 upper part of the pressure-measuring mercury column in the 

 heated bath, and the lower part in the temperature of the room. 



Confining ourselves, then, to the consideration of the values of 



■~ found for sulphide of carbon and chloroform, these hold, as 



we have said, for an approximately constant vapour-space. Now 



with low pressures the quotient —^ appears =0; that is, there 



the superheated vapour experiences exactly the same pressure- 

 expansion for constant volume as does the dry air with which 

 it was compared. Hence the question mentioned at the com- 

 mencement, and started by my earlier experiments, is now de- 

 cided thus — that the superheated vapour under small pres- 

 sures can certainly possess smaller coefficients of expansion for 

 constant volume than 0*003663, so far as such smaller coefficients 

 can belong also to dry air, but that a still smaller expansion of 

 vapour is not to be admitted. According to Regnaultf, how- 

 ever, dry air under these small pressures shows decidedly 

 smaller coefficients than 0*003663 — for example, with an initial 

 pressure of 110 millims. the value 0*003648; so that my 

 former results (which were directed only to comparison with 

 the number 0*003663) are certainly hereby confirmed, and at 

 the same time have the limits of their validity prescribed in the 

 behaviour of dry air. 



For higher pressures (that is, where at the boundary of satu- 

 ration, according to my former experiments, vapour shows a 

 considerable departure from the gaseous state) the quotient 



~- obtains truly a small, but yet an undeniable positive value. 



Consequently the pressure-expansion of vapour contained within 

 a constant volume is there greater than that of dry air; and 

 indeed we shall not be wrong if we suppose that, the greater 

 the vapour-pressure becomes, so much greater is the positive 



value of -~* This is sufficiently indicated by Tables XX. and 



* Pogg. Ann. vol. cxxxvii. p. 602. 

 t Mem. de V Acad. vol. xxi. p. 110. 



