484 



Mr. J. B. Hannay. 



[June 10, 



the melting points of solids, which, depend almost solely npon tempera- 

 ture, a very great amount of pressure being required to alter them. 



The difference between the fluid and gaseous states is not then 

 entirely dependent upon the length of the mean free path, but also 

 upon the mean velocity of the molecule. 



June 14, 1880. 



IV. " On the Solubility of Solids in Gases. II." By J. B. 

 Hannay, F.R.S.E., F.C.S. Communicated by Professor G. 

 G. Stokes, D.C.L., &c, Sec. R.S. Received May 24, 1880: 



In a former paper* it was pointed out that a solid dissolved im 

 a liquid was not deposited from solution when the liquid passed 

 through the critical point into the gaseous state, and the conclusion 

 drawn that solids are soluble in gases. I have shown in another 

 paper f that it is probable that after a liquid passes its critical point it 

 becomes a vapour or gas, and this quite independent of pressure, and 

 that the cohesion limit for any one substance lies more in the direction 

 of an isothermal line than a continuation of the boiling line. Before,, 

 however, I had carried out that work, I determined to make sure that 

 a truly gaseous state had been reached, and the fluid containing the- 

 solid was raised to 150° above its critical point, and the gaseous solu- 

 tion expanded to nearly twice its liquid volume, and still the solid 

 remained in solution. In these circumstances it was found that non- 

 volatile solids were retained in solution in truly gaseous menstrua. In 

 the second paper referred to, I have pointed out some of the difficulties 

 which beset such an investigation ; and when it is remembered that only 

 air-baths can be used, the temperature being over 200° and sometimes 

 over 300°, and that very many liquids dissociate under these tempera- 

 tures, it will be seen that conclusions should be drawn only from sub- 

 stances perfectly free from suspicion. All the experiments, therefore, 

 which I have performed in iron tubes cannot bear on a discussion im 

 this matter ; and I shall defer going into the natural minerals formed 

 artificially from aqueous solution at a red-heat till another paper. For 

 high temperatures glass does not do, as every variety of it is easily 

 decomposed by water ; but by the use of rock crystal blocks I have 

 obtained some results of interest where the action of the water can be 

 seen. However, I intend in this paper to treat in a fuller manner of 

 work touched upon in former papers. One of the first pieces of work I 

 set about to finish was to determine whether or not the critical points 

 were altered by the introduction of a solid into solution. 



In a former paper it was thought that the critical point of alcohol 



* " Solubility of Solids in Grases," "Proc. Roy. Soc," ante, p. 178. 



f " On the State of Fluids at their Critical Temperatures," ante, p. 478. 



