EXPEEIMENTAL KNOWLEDGE OF THE PEOPERTIES OF MATTER. 125 



between 0° and 46°, or between 0° and temperatures higher than 46°, 

 e.g. between 0° and 50°, by using constant pressures over 760 mm. 



This in fact is what'' Ramsay and Young have made possible by 

 determining the pressures of the vapour of carbon bisulphide for every 

 degree from 0° to 50° inclusive. 



Thus instead of only one we have fifty constant temperatures, being 

 boiling-points for sixty known pressures from 127'9 mm. to 857'1 mm.; 

 the temperatures being air-thermometer temperatures ; to say that fifty 

 constant temperatures are available is sufficient perhaps ; but in fact 

 other temperatures are obtainable by interpolation between two succes- 

 sive degrees (up to 50°). 



Similar tables have been prepared by the authors for the other sub- 

 stances for every degree centigrade ; thus for the eight diff'erent substances 

 there are eight tables, giving in the case of — 



Carbon bisulphide vapour pressures from 



Ethyl alcohol „ ,, „ 



Chlorobenzene ,, „ „ 



Bromobenzene „ „ ,, 



• Aniline _ „ „ „ 



Methyl salicylate „ ,, „ 



Bromonaphth aline „ ,, „ 

 Mercury 



for each degree centigrade in each table the vapour pressure of the 

 substance. 



These valuable tables are founded on the definiteness and constancy of 

 the relationship between vapour-pressure and temperature for each pres- 

 sure for each substance ; and as most of these results have been obtained 

 by the dynamical method they assume that this method is as trust- 

 worthy as the statical and gives, when properly applied, as definite and 

 constant results. 



Ramsat/ and Young's Formula B'=:B + c (f — t). — Galczclated and olserved 

 Tables of the Absolute Temperatures and Vapour-pressures of a Substance 

 compared. 



In the brief sketch, p. 123, of Ramsay and Young's method of deter- 

 mining the vapour-pressures of mercury for various temperatures, it 

 was stated that by certain experiments the relation between temperature 

 and vapour-pressure of mercury was determined at about the tempera- 

 ture of boiling sulphur, and that, from this and from three or four data at 

 lower temperatures, a series of pressures for a long range of temperatures 

 was deduced from Regnault's series for ivater by the use of an equation 

 of tbe form R'=R-Hc (t'—t) ■ in this R is the ratio of the absolute tem- 

 perature of two bodies corresponding to any the same vapour-pressure ; R' 

 the ratio at any other pressure the same for both ; t' and t are the tem- 

 peratures of one of the bodies corresponding to the two vapour-pressures • 

 and c a small constant. 



What Ramsay and Young have proved - is that for the substances of 



widely different kind examined by them, c is very small, that it can be 



accurately determined, and that it is constant for any pair of substances ; 



that when either water, ethyl alcohol, carbon bisulphide, or sulphur 



' J.C.S. September 1885, p. 640. ^ p;,,^ ;y^^ January 1886. 



