APPENDIX E. 



ON THE RELATIONS BETWEEN LIQUID AND VAPOUR 



In connection with the present research a number of side issues have presented 

 themselves, some of which come fairly within the scope of the Eeport. I commence by 

 reprinting two Notes, read on January 19 and February 2, 1885, to the Royal 

 Society of Edinburgh : ' — 



ON THE NECESSITY FOR A CONDENSATION-NUCLEUS. 



" The magnificent researches of Andrews on the isothermals of carbonic acid formed, 

 as it were, a nucleus in a supersaturated solution, round which an immediate crystal- 

 lization started, and has since been rapidly increasing. 



"They gave the clue to the explanation of the paradoxical result of Regnault,that 

 hydrogen is less compressible and other gases more compressible, under moderate , 

 pressure, than Boyle's Law indicates ; and to that of the companion result of Natterer 

 that, at very high pressures, all gases are less compressible than that law requires. 

 Thus they furnished the materials for an immense step in connection with the 

 behaviour of fluids above their critical points. 



" But they threw at least an equal amount of light on the liquid-vapour question, 

 i.e. the behaviour of fluids at temperatures under their critical points. In Andrews' 

 experiments there was a commencement, and a completion, of liquefaction ; each at a 

 common definite pressure, but of course at very different volumes, for each particular 

 temperature. 



" In 1871 Professor J. Thomson communicated to the Royal Society a remarkable 

 paper on the abrupt change from vapour to liquid, or the opposite, indicated by these 

 experiments. He called special attention to the necessity for a ' start,' as it were, in 

 order that these changes might be effected. [It is to this point that the present Note is 

 mainly directed, but I go on with a brief analysis of Thomson's work.] He pointed out 

 that there were numerous experiments proving that water could be heated, under 

 certain conditions, far above its boiling point without evaporating ; and that, probably, 



1 Proc. Boy. Soc. Edin., vol. xiii. pp. 78 and 91, 1885. 



