538 



Prof. J. Dewar. 



[June 17, 



This method worked exceedingly well, giving exactly the same results 

 as before, the only drawback being that the action was very sluggish, 

 the galvanometer taking about two minutes to come to rest after each 

 addition of pressure. The above results seem to prove that the calcu- 

 lated value of the variation of the freezing-point of water, deduced 

 from the observed difference of volume of ice and water, and the 

 latent heat of fluidity at the melting-point under one atmosphere of 

 pressure, is identical with the mean experimental value obtained from 

 a series of observations extending to 700 atmospheres. From Cla- 



TV 



peyron's formula we are thus entitled to infer that = constant, 



L 



where T is absolute temperature, Y the difference of volume of the 

 two states, and L the latent heat of fluidity. If Y is assumed to be 

 approximately constant, then T varies as L, Thus the latent heat of 

 ice diminishes as the freezing-point is lowered by pressure. This is in 

 accordance with the deductions of Clausius* from other considerations. 



III. " On the Critical Point of Mixed Vapours." By James 

 Dewar, M.A., F.R.S., Jacksonian Professor of Natural Ex- 

 perimental Philosophy in the University of Cambridge. 

 Received June 10, 1880. 



The following experiments regarding the behaviour of carbonic 

 acid in presence of different vapours above the temperature of the 

 critical point of the pure gas, were undertaken to ascertain if any 

 optical discontinuity could be observed in such mixtures. As the 

 object was intentionally a qualitative investigation, many of the 

 pressure observations have been taken with the metallic manometer. 



The Liquefaction of Carbonic Acid in presence of other Bodies. 

 1. Carbonic Acid and Bisulphide of Carbon. 



Carbonic acid liquefied in presence of a small quantity of bisulphide 

 of carbon at a pressure of 49 atmospheres and a temperature of 19° 0. 

 It floated on the convex surface of the bisulphide, the line of separation 

 being sharp and well defined. 



At 35° C. liquid condensed on the surface of the bisulphide in the 

 same way at a pressure of 78 atmospheres, at 40° C. it still appeared 

 at 85 atmospheres, at 55° C. there seemed to be a distinct appearance 

 of two liquids, and at 58° C. there was still the same apparent separa- 

 tion under a pressure of 110 atmospheres. Observed at 47° C. and a 



* "Phil. Mag.," 1851. 



