EXPEEIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 123 



for ordinary temperatures) very doubtful. It is described in the Britisb 

 Association volume for 1883. A shallow glass tube 14 mm. diameter 

 containing freshly distilled mercury was suspended near the bottom of 

 a closed flask of about 1'9 litre capacity, for nine days ; the tube was 

 then removed and boiling nitric acid poured into the flask and allowed 

 to stand some time, the nitric acid neutralised with ammonia, the solution 

 washed out of the flask, acidified with HCl, and treated with H2S. By 

 comparison with the result of operating similarly with solutions of mer- 

 cury of known strengths, the mercury was found to be between '00006 gms. 

 and -00012 gms. 



The flask contained, therefore, as vapour about "00009 gms. of mercury. 



A second similar experiment gave '00012 gms., therefore at ordinary 

 temperature 1 litre of Hg-vapour contains '00006 gms., which would 

 correspond to pressure of mercury='00574 mm. ; and McLeod says this 

 number may be too large, for probably some mercury condensed on the 

 inside of the flask. 



In "Wiedemann's 'Ann.' 1882, xvii. p. 177, is an elaborate investigation 

 of the evaporation of fluids, especially mercury, by Hertz. His vapour- 

 pressures of mercury are very different indeed from Regnault's from 0° to 

 100°, as will be seen : — 



Regnault giving at 0° vapour-pressure 0'02 mm. 

 Hertz „ 0° „ 0-00019 mm. 



and Regnault „ 100° ,, 0'7455 mm. 



Hertz 



100° 



0'285 mm. 



Ramsay and Young • determine, by a neat and accurate method, in 

 which mercury in a small bulb of glass at one end of a narrow graduated 

 tube is heated to the boiling-point of sulphur, the vapour-pressure of 

 mercury at that temperature ; and deduce, from this and a few data for 

 lower temperatures, by means of the formula'^ R'=R4-c(i'- f)— see 

 further on (p. 12-3) — the vapour- pressures for temperatures from 360° to 

 130° from Regnault's vapour-pressures of water ; and the vapour-pressures 

 of mercury between 130° and 40° were calculated by ' extrapolation ' {loc. 

 at. p. 48), by means of the known vapour-pressures of mercury at tempera- 

 tures 160°, 220°, and 280° — which pressures and temperatures sufiice to 

 find the three constants in the formula log. p=^a + ba\ which was then 

 applied to find j) at lower temperatures. 



Let us compare some of Hertz' values with those of Ramsay and 

 Young : — 



J.C.S., January 1886, p. 37. 



- Phil. Mag. January 1886. 



