478 M. R. Pictet on the Application of the Mechanical 



volatilization ; and finally the volume v of the liquid becomes 

 volume V of the vapour. During the passage from the liquid 

 to the gaseous state the temperature t° has been kept constant, 

 and the tension of the vapour, P, has been likewise constant. 



Now, if all known liquids be taken and made to undergo 

 the same transformation at the same temperature t°, we find 

 for each different numeric values of \, P, and V. For each 

 liquid the maximum tension P, the volume V, and the latent 

 heat A, vary ; and the deviations may be very considerable. 

 We may therefore inquire whether these three quantities are 

 independent variables, to be determined empirically for each 

 liquid, or if any relation permits them to be expressed in terms 

 of one another. 



M. Regnault, in his large and splendid work on the maxi- 

 mum tensions of volatile liquids and their latent heats of 

 volatilization, has considered the question only from an ex- 

 perimental point of view, and has given interpolation-formulas 

 for the tensions and latent heats, without seeking to establish 

 any theoretic ratio between these two factors. The interpo- 

 lation-formulae are often most complex, and do not enable 

 us to distinguish any thing general when the different liquids 

 are compared. His copious statement, however, of the num- 

 bers obtained by accurate experiments will serve as a control 

 in the following theoretical discussion. 



In order to put the problem in a complete, definite, and 

 readily comprehensible form, let us suppose we have a re- 

 servoir, A (PI. IV. fig. 1), containing any volatile liquid. A 

 pump, B, draws off the vapour, which forms in the reservoir 

 A at a constant temperature t° and tension P, and forces it 

 into the receiver C at a temperature t' and under a constant 

 pressure P'. The volatile liquid will pass from the liquid into 

 the gaseous state in the reservoir A, afterwards from the 

 gaseous into the liquid state in the reservoir C. To maintain 

 the operation and complete the cycle, the liquid is to be 

 brought back to its initial temperature ; and the conduit tube 

 D permits the liquid accumulated in C to return into the first 

 reservoir, A. We suppose in the above operation t f > t°, and 

 consequently P' > P. 



Thus, for example, we take 1 kilogramme of ether at 

 t° =x 0° C. ; its vapour is drawn off under the tension P , and 

 forced into C at a temperature tf = 20° and under a pressure 

 P^o ; and then 1 kilogramme of liquid ether is reduced from 

 the temperature 20° to. 0° in passing through the tube D. 

 The cycle is complete, since the ether has returned to 0° under 

 the initial pressure P , which was the point from which it 

 started. 



