208 PHENOMENA DEPENDENT ON MOLECULAR PATHS 82 



sum of the momenta which the impinging particles lose in a 

 second gives the amount of the friction which the surface- 

 layer of the gas has experienced at the wall. 



But to form this sum we have to make a hypothetical 

 assumption as to the magnitude of the loss which a single 

 particle suffers on impact at the wall. In the case carried 

 out in 11 we supposed that no kinetic energy was lost on 

 the collision, but that the particle was reflected from the 

 wall with the same speed as that with which it struck it. 

 We could not now consistently suppose that the particles of 

 gas lose any of their speed ; but we might assert that of the 

 forward motion, which all the molecules in common possess, 

 a part must be transformed into heat-motion in consequence 

 of the divergency of the directions in which the impinging 

 particles are reflected. How large a part this will be 

 depends on the degree of unevenness of the surface, with 

 respect to which, therefore, we have to form a definite 

 conception. 



The fixed wall on which the particles impinge does not 

 form a plane or continuously curved surface at all ; it is 

 made up itself of molecules which leave spaces between each 

 other of sufficient size to allow other molecules to penetrate 

 into them. On the breadth of these molecular pores rests 

 the capacity of solid bodies for condensing 1 on their surfaces 

 considerable quantities of gases and vapours, that is, for 

 depriving them of the mobility proper to their state of 

 aggregation. The gaseous molecules penetrate thereby 

 deeply into the interior of the solid body, so that they are 

 able to pass through the walls of glowing tubes 2 whose 

 briskly moving molecules may often leave wide interspaces, 

 and also, when helped by the force of electrolysis, through 

 platinum foil. 3 



From such observations we are forced to look upon the 

 surfaces of solid bodies, even if most excellently polished, as 



1 Or adsorbing them, according to the modern nomenclature. Compare the 

 observations of B u n s e n and Kayser, Wied. A nn. 1883-5. 



2 H. Sainte Claire-Deville and Troost, Comptes Rendus, 1863, Ivii. 

 p. 965 ; Pogg. Ann. 1864, cxxii. p. 331. 



3 Helmholtz, 'Bericht iiber Versuche des Herrn Dr. Elihu Root,' 

 Monatsber. d. BerL Akad. 1876, p. 217. 



