Experiments with Soap-bubbles. 409 



solid cryohydrate must melt into an unstable solution. The 

 improbability of this event leads us to conclude that h cannot 

 increase with p. Similarly we may reason that h cannot 

 decjease as p increases. A cryohydrate must therefore be 

 regarded as a body of definite chemical composition, and not as 

 a mere solution in which the proportions of salt and ice depend 

 upon various accidents. 



This property of a cryohydrate has already been noticed by 

 Dr. Guthrie, but he has not examined the effects of pressure. 



Again, Duhem has shown that when a dilute solution, 

 formed by dissolving any salt in water, is reduced in tempera- 

 ture until the pure ice appears, then increase of pressure 

 causes some of the ice to melt, or, in analytical language, 



-~ is negative for the weakest stable solution. 



If, however, the solution is formed by dissolving the salt 

 in a liquid, like acetic acid, which contracts whilst freezing, 



then -4- is positive. 

 dp 



As h is constant along the freezing line of the cryohydrate, 



it follows that in the former case ^- is negative, and in the 



latter case positive. 



It is now evident that when a salt is dissolved in water, the 

 cryohydrate expands in the act of freezing, and its melting- 

 point is lowered by pressure, whereas, when a salt is dissolved 

 in a liquid, like acetic acid, which contracts whilst freezing, 

 the cryohydrate also contracts whilst freezing, and its melting- 

 point is raised by pressure. 



LI. Experiments with Soap-bubbles. By C. V. Boys, 

 A.M-S.M., Demonstrator of Physics at the Science Schools, 

 South Kensington*. 



[Plate II.] 



THOUGH none of the experiments I am about to describe 

 depend upon any property of a soap -film which is not 

 perfectly well known and understood, yet they serve to illus- 

 trate in a striking and beautiful manner the behaviour of 

 bubbles under special circumstances, and so as lecture-experi- 

 ments simply I hope they may be considered worthy of the 

 attention of the Physical Society. 



* Communicated by the Physical Society : read April 14, 1888. 



