112 REPORT — 1886. 



less completely, in the space in which the vapour has to contend now 

 only against the pressure of the atmosphere acting on it.' 



Regnault finds that for substances of moderate volatility, for which 

 both methods can be applied, the two methods give identical results, pro- 

 vided the bodies are 'perfectly pure, but not otherwise ; he found that tbe 

 addition to alcohol, or to carbon bisulphide, of xwo- of ^ volatile substance 

 was in each case sufficient to disturb the two curves of either and to 

 destroy their identity. Among the subjects used by Regnault, and which 

 led to this conclusion, were ethyl alcohol, ethyl oxide, carbon bisulphide, 

 cbloroform, benzene, carbon tetrachloride, ethyl chloride, ethyl bromide, 

 ethyl iodide, methyl alcohol, acetone, phosphorus terchloride ; these being- 

 some of the substances for which Regnault found (' Memoires,' t. xxvi.) 

 curves of relation of temperature and vapour-pressure by methods and 

 formulas similar to those used for water (' Memoires,' t. xxi.). 



When a liquid boils with bumping, as in the case of methyl alcohol, 

 liquid 80-2, liquid NHj, it may be heated with the vapour over it 

 to temperatures considerably higher than the temperature at which the 

 vapour in each case is, in the static method, in equilibrium with the at- 

 mospheric pressure (t. xxvi. p. 645). 



Vapour-pressures from Solid and Liquid — Regnault. 



Regnault's experiments on the vapour-pressure on water included pres- 

 sures for low temperatures down to — 32° ; i.e. for a long range of tern- 

 perature during which the water is solid — ice. Regnault found that the 

 whole curve was continuous, including this portion of it, and inferred 

 that the solidification of water made no break in the curve of vapour- 

 pressui-es; in regard to water he says (t. xxi. p. 609) it would be necessary 

 to make corrections of 3 or 4 hundredths of a millimetre, a quantity almost 

 inappreciable to observation, to bring about a complete coincidence 

 between the curve given by the formula log F = a — ba^ — c/>, in which 

 x=zt + 20° and the graphic curve ; on p. 599, alluding to a formula 

 which applies very exactly to all observations of his between 0° and 100°, 

 loo- F = a + &a* — c/?*, he says that the values of the vapour-pressure for 

 temperatures below 0° are constantly veiy slightly gi-eater than those 

 given by observation, and he therefore does not apply this formula to 

 temperatures under 0°. The fact is that the methods which were em- 

 ployed were not of a nature to show at once that the vapour-pressure 

 from solid water helow 0° and liquid water ahove 0° formed two curves 

 meeting at an angle at 0° or one curve. Regnault himself ' attacks this 

 question directly but unsuccessfully, being unable to prove, to his satis- 

 faction, that the state, solid or hquid, of a body exerts an influence on the 

 elastic force of its vapour at a given temperature in the barometric vacuum. 



On p. 751 he says in reference to water, alluding to the experiments 

 and results obtained for it in t. xxi. : ' I have proved that the curve con- 

 structed on the expei'iments [for ice below 0°] presented a perfect con- 

 tinuity with the curve which is given by the elastic forces of the vapours 

 furnished by liquid water at tempei-atures above 0°.' 



Regnault in this part of this mcmoire takes other easily frozen bodies 

 to examine ; ethylene dihromide, benzene, glacial acetic acid ; and comes to 

 the conclusion in all these cases that (p. 759) his experiments prove that 



• Memoires, t. xxvi. pp. 751-7G0. 



