Chemistry and Physics. 238 
ner endeavors to show* that the mean path of the molecules of the 
residual gas in the bulb of the ee when it is in the condi- 
sitiveness is reached, and si on further exhaustion the sensitive- 
ness rapidly diminishes, seems to us,—especially when viewed in 
connection with the Seiticionie just referred to—fatal to the 
emission theor 
tion of the ne mena is said to have been first suggested by 
Mr. Johnstone Stone Similar views were al ry early ex- 
- pressed by Professors Tait and Dewar.{ More recently they have 
been adopted by Mr. Pa and are now very generally ac- 
cepted by physicists. 
n 
pail I 
fa indies i for exaienie, with the adjacent molecules, 
ae pene of one system, and, by a well known mechanical law, n 
wall then of course a force is exerted between the opposing ne 
esata is then simply a heat engine in which the action sakes 
place between two surfaces of different temperature, the heater and 
the cooler of the engine. As such a difference of temperature may 
be maintained in a great variety of ways, so the apparatus admits 
of a great number of modifications. Moreover, it is obvious that 
* Pogg. Ann., clx, 305. + Beiblitter, i, 167 i 330. a July 15th, 1875. 
§ Philosophical Transactions, vol. clxvi, p 5, in postscript dated June ot 1877. 
See also Mr. Stoney’s papers. Phil. Mag., aa t pp. 177-182, oe (18 
Am. Jour. Sct.—Turrp Series, Vou. XIV, No. 81.—Sept, 1 
16 
