1833.] Mountains from the boiling point of water. 195 
Perhaps it is necessary to explain, that the boiling point is that degree 
of heat at which the elastic force of aqueous vapour is just capable of 
counterpoising the pressure of the atmosphere, or the weight of the co- 
lumn of mercury in a barometer. The method then of discovering the 
law of progression of the tensions has generally had for its basis the ex- 
posure to heat of a portion of water in a closed vessel, such as a glass tube 
or a small boiler, under the pressure of a column of mercury, measuring 
the height to which the latter is raised at different temperatures. 
Betancourt, Scumipt, Datton, Watt, Creicuton, SourHern, 
Taytor, and more recently Ure, Arsspercer, Perkins, and DuLone 
(assisted by a commission of the French Academie), are some of the 
illustrious names which are connected with these researches experimen- 
tally ; while Ropison, Youne, Ivory, Lapiacz, Prony, Trepcoxp, 
Cortouis, Larocue and others have attempted to construct mathema- 
tical formule, capable of embracing the range of their experiments from 
the freezing point up to 500° Farh.* _It is quite unnecessary for me to 
enter into any lengthened history of this branch of physics, which the 
reader will find ably discussed in Robison’s Mech. Phil., Biot, Tredgold 
on the Steam Engine, Daniell’s Meteorology, and in the report of 
Dutone to the Academie on the experiments made by order of the 
French Government to determine the elastic force of aqueous vapour at 
high temperatures. (An. Chim. xliii.] 
All the experiments agree in proving the elastic force of steam to 
follow a geometrical ratio with arithmetical increments of heat. The 
index of the power representing the law of variation was assumed as 
5.13 by Sourusern, 6 by Creicuton,7 by Youne, by Coriotts 5,355, 
and by Dutone 5. But the formula of Trepcotp is acknowledged to agree 
more closely with experiments below 300° than any other :—his ex- 
ponent is also 6, with a different co-efficient ; if f = elastic force, and 
¢ temperature, then by his formula 
6 
i tt, 100 ; ort = 177 f 1 —100 
( 177 ) t 
in logarithms 
log. f = 6 (log. (¢+ 100) — 2.247968) 
* The experiments of the French Academicians Baron de Prony, Arago, Gerard 
and Dulong, in 1829, extend to the temperature of 435° Fahrenheit, or apressure of 
24 atmospheres, which they measured by absolute pressure of a column of mercury 
sixty feet high ina glass tube attached to the tower of the Old Church of Sainte Ge- 
neviéve:—they were afraid of passing this limit, as the least explosion would have 
brought down the tottering fabric. Their glass tube was jointed and ingeniously 
supported: Mr. Daniell has however since worked with single glass tubes of 40 feet 
long, in his water barometer experiments. We deal now-a-days boldly with feet, 
where inches were formerly thought sufficient ! 
cc2 
