10 PRESSURE OF GASES 19 



the kinetic energy of all the molecules into three parts 

 which in the mean are all of equal magnitude, one of them 

 being the energy of a motion at right angles to the wall, and 

 the others corresponding to motions which are parallel to 

 the wall and at right angles to each other. 



Only the first of these components of the energy comes 

 into account in the estimation of the pressure on the vessel, 

 and we therefore find the correct value of the pressure by 

 ascribing to all the molecules a velocity perpendicular to the 

 wall and a kinetic energy equal to one-third of the total 

 mean kinetic energy of a molecule. 



This result of our investigation is identical with the 

 assumption with which Joule l and Kronig 2 started in 

 their calculations, as they assumed a gas, enclosed in a cube, * 

 to press as strongly against the faces as if one- third of the 

 molecules moved parallel to each of the three directions of 

 the edges, so that each face was impinged upon by only 

 one-third of all the molecules. 



11. Calculation of the Pressure 3 



With this simplified assumption it is easy to calculate 

 the value of the pressure from the resultant action of the 

 impacts which the surface undergoing pressure receives 

 from the molecules that meet it. 



This surface, which we will call the stressed surface, 

 may be taken either as a mathematical plane or surface 

 inside the space filled with gas, or as a wall of the 

 containing vessel. The former assumption has the 

 advantage of allowing the calculation to proceed without 



1 Mem. of the Manchester Lit, and Phil Soc. [2] ix. 1851, p. 107; Phil. 

 Mag. [4] xiv. 1857, p. 211. 



2 Berlin 1856 ; afterwards reprinted in Pogg. Ann. xcix. 1856, p. 315, and 

 in many other periodicals. 



3 Other calculations beside those of Joule and Kronig: Clausius, Pogg. 

 Ann. c. 1857, p. 353; Maxwell, Phil. Mag. [4] xix. 1860, p. 29, xxxv. 1868, 

 p. 195 ; Stefan, Wiener Sitzungsberichte, xlvii. 1863, p. 91 ; 0. E. Meyer, De 

 Gasorum Theoria, Vratisl. 1866 ; Pfaundler, Wien. Sitzungsber. Ixiii. 1871, 

 p. 159; v.Lang, ibid. ixiv. 1871, p. 485, Pogg. Ann. cxlv. 1872, p. 290; 

 Saalschiitz, Schr. d. phys.-okon. Ges. zu Konigsberg, 19. Jahrg. 1878, 

 Sitzungsber. p. 45. 



c 2 



