EXPEraMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 119 



method. Thus the analogy is complete between the volatilisation of ice 

 against external pressure and the boiling of a liquid against external 

 pressure. 



Similar experiments were made bj Ramsay and Young with acetic 

 acid (melting 16'4°), with naphthalene (melting 79'2°), with camphor 

 (melting 175°) ; the results by heating the bulb containing solid camphor 

 adhering to the thermometer were confirmed by the pressures obtained 

 with solid camphor over mercury at different temperatures below 175°, 

 the results agreeing very nearly, as shown on a diagram. The general 

 conclusion to be drawn from this paper is that corresponding to boiling- 

 points of liquids there are similar temperatures for solid bodies volatilising 

 without liquefying — viz., temperatures which are constant while the solid 

 volatilises at a constant pressure, but which are different for different 

 pressures, the volatilising point of a solid rising with rise of pressure, and 

 being lower with lessened pressure, as is the case with the boiling-point 

 of a liquid ; and moreover that the volatilising point (for any pi-essiire) is 

 the same as the temperature at which the solid over a mercury-vacuum 

 has the same pressure, or sensibly the same ; the second method giving 

 the true vapour-pressure, while the former method gives a temperature 

 not absoluteh/ identical with that observed for the same pressure over a 

 mercury-vacuum, though the difference is extremely minute. 



It was not superfluous to prove by direct experiment the deductions 

 made by Professor James Thomson from Regnault's numbers as to the 

 discontinuity of the curve for the ice-vapour pressure with the curve for 

 water-vapour pressure, and to show by dii-ect experiment that at tem- 

 peratures common to the two (below the freezing-point), the curves of 

 vapour-pressure are distinct, the one for water-vapour pressure being 

 ■continuous with the curve for water-vapour pressure above 0° ; and it 

 was important to prove that these propositions, mutatis mutandis, apply to 

 other substances. This task, for water, acetic acid, benzene, and camphor, 

 was undertaken and successfully accomplished by Ramsay and Young, 

 and is pubhshed in ' Phil. Trans.' Part II. 1884. 



Vapour-pressures from Solids and Liquids — Banisay and, Young. 



In Naumann's ' Thermochemie ' (Brunswick, 1882), at p. 178 is a 

 passage, quoted by Ramsay and Young, showing that Naumann had con- 

 vinced himself that from naphthalene (melting 79"5°), the same vapour- 

 pressure is produced either from liquid or solid at the same temperature ; 

 and he alludes to former experiments (Regnault's, no doubt), which 

 yielded similar conclusions in the cases of water, benzene, ethene bromide, 

 acetic acid, cyanogen chloride, and carbon tetrachloride. 



It will be remembered that no satisfactory results were obtained by 

 Regnault with the substances he tried. In the paper referred to ' Ramsay 

 and Young first give results for solid camphor, the pressures being found 

 for many temperatures up to the melting-point 175°, and for liquid 

 camphor up to 198° ; and it is very evident on the curve of pressures 

 plotted out that the curves for liquid and for solid camphor meet at a 

 re-entering angle near 175°, which is the melting-point for a pressure of 

 one atmosphere. For camphor the operations were conducted by jacket- 

 ing a barometer-tube, very carefully ensured from the danger of entrance 



» mil. Trans. 1884, Part II. p. 461. 



