522 Drs. Ramsay and Young on 



numbers the value of -~ at any pressure could easily be calcu- 

 lated with very small error. The time required by this pro- 

 cess is very great ; and it was only adopted in the case of 

 water, for which the pressures for each degree between — 32° 

 and 280° have already been calculated (Balfour Stewart, 

 ' Treatise on Heat '), and of carbon bisulphide, alcohol, chloro- 

 benzene, bromobenzene, aniline, methyl salicylate, bromonaph- 

 thalene, and mercury between certain limits of pressure, 

 generally 150 or 200 to 700 millim. (Ramsay and Young, 

 Chem. Soc. Journ. 1885, p. 640). 



In all other cases tangents were drawn to touch the vapour- 

 pressure curves at points corresponding to definite pressures, 

 several curves being required for each substance on different 

 scales to admit of this being done. With care, this method 

 yields fairly satisfactory results. 



It has been mentioned that the identity of the reduced 

 numbers for any one substance holds at pressures ranging 

 between about 150 and 1500 to 2000 millim. At lower pres- 

 sures it is much more difficult to determine the values of -£ 



at 



with accuracy, and the influence of experimental errors is 

 greater. Moreover, it has been necessary provisionally to 

 adopt the method of tangents for all substances except water 

 and carbon bisulphide at low pressures ; and it is doubtful, 

 therefore, whether much confidence is to be placed in the 

 values at pressures below 150 millim. 



As regards high pressures, with the exception of alcohol all 

 the substances have been investigated by Regnault ; and it may 

 be worth while to mention one or two facts which appear to 

 throw some little doubt on the accuracy of some of his determi- 

 nations at pressures above 2000 or 3000 millim. In vol. xxvi. 

 of the Mdmoires de V Academie Regnault gives the vapour- 

 pressures of a large number of substances for each 5°, calcu- 

 lated from his empirical formulae. Many of these series have 

 been examined by the method of differences ; and it has been 

 found that in several cases the numbers forming the third set 

 of differences increase slowly up to pressures of from 2000 to 

 3000 millim., but decrease at higher pressures. This is 

 notably the case with alcohol ; and it has been shown by 

 Ramsay and Young, in a paper read before the Royal Society 

 in May 1885, that the higher vapour-pressures of this sub- 

 stance as determined by Regnault are too low. It has also 

 been pointed out by Vincent and Chappuis (Compt. Rend. 

 vol. c. p. 1216) that the vapour-pressures of methyl chloride 



