450 Drs. Ramsay and Young on 



it will be seen that he compared the relation of increase of 

 pressure to temperature-difference at constant volumes, and 

 came to a conclusion opposed to ours. This is owing to his 

 having made very few observations, and having accidentally 

 chosen those which support his statement. If the coefficients of 

 increase of pressure for unit rise of temperature be calculated 

 by means of Table V., it may be noticed that, although irre- 

 gular, there is no tendency towards a rise or fall of the 

 coefficient. 



Regnault has measured the rise of pressure of gaseous car- 

 bonic anhydride at constant volume. He gives the results of 

 four experiments, none of which are available for our purpose, 

 inasmuch as the volumes of a gram are too large. 



Reverting to the behaviour of etber, as shown on Plate VII., 

 it will be seen that the curves have been drawn in the region 

 where measurements are impossible. These curves have all 

 the same general form. After rise of pressure and decrease of 

 volume have proceeded for some distance, the curves bend 

 downward, presenting the abnormal feature of decrease of 

 volume with fall of pressure. The pressure continues thus to 

 fall, and at 160° the isothermal touches zero-pressure. At 

 lower temperatures, with small volumes, the pressure becomes 

 negative, and may even represent an enormous tension. At 

 0° the isothermal at vol. 1*85 cub. centim. per gram reaches 

 the almost incredibly great tension of —271,700 millim. ; 

 and it has at that volume (the smallest our results allow us 

 to calculate) by no means reached its limit. At still smaller 

 volumes the tension would doubtless still increase, until the 

 curve turned, and further decrease of volume would be repre- 

 sented, as it is at higher temperatures, by increase of pressure. 

 The existence of these unrealizable portions of such iso- 

 thermal curves was, we believe, first suggested by Prof. James 

 Thomson, in a paper in the ' Proceedings of the Belfast 

 Natural History and Philosophical Society/ Nov. 29, 1871. 

 Since that time attempts have been made to express relations 

 between the pressure, temperature, and volume of gases and 

 liquids by Yan der Waals and by Clausius ; and the formulas 

 which they propose, and which we hope to consider in a sub- 

 sequent paper, give isothermals of similar form. Portions of 

 these curves have, indeed, been experimentally verified. In 

 Professor Thomson's paper, above referred to, he points out 

 that Donny, Dufour, and others have observed the phenomenon 

 commonly alluded to as " boiling with bumping/' This is 

 usually the effect of a rise of temperature at constant pressure. 

 But it may equally well be produced, as we have frequently 

 had occasion to remark, at constant temperature by lowering 



