332 POPULAR SCIENCE MONTHLY. 



sure in these metals must amount to many atmospheres. These cor- 

 puscles increase the specific heat of a metal and the specific heat gives 

 a superior limit to the number of them in the metal. 



An interesting application of this theor}^ is to the conduction of 

 electricity through thin films of metal. Longden has recently shown 

 that when the thickness of the film falls below a certain value, the 

 specific resistance of the film increases rapidly as the thickness of 

 the film diminishes. This result is readily explained by this theory 

 of metallic conduction, for when the film gets so thin that its thick- 

 ness is comparable with the mean free path of a corpuscle the num- 

 ber of collisions made by a corj^uscle in a film will be greater than in 

 the metal in bulk, thus the mobility of the particles in the film will 

 be less and the electrical resistance consequently greater. 



The corpuscles disseminated through the metal will do more than 

 carry the electric current, they will also carry heat from one part to 

 another of an unequally heated piece of metal. For if the corpuscles 

 in one part of the metal have more kinetic energy than those in 

 another, then, in consequence of the collisions of the corpuscles with 

 each other and with the atoms, the kinetic energy will tend to pass 

 from those places where it is greater to those where it is less, and in 

 tliis way heat will flow from the hot to the cold parts of the metal, as 

 the rate with which the heat is carried will increase with the number 

 of corpuscles and with their mobility, it will be influenced by the same 

 circumstances as the conduction of electricity, so that good conductors 

 of electricity should also be good conductors of heat. If we calculate 

 the ratio of the thermal to the electric conductivity on the assumption 

 that the whole of the heat is carried by the corpuscles we obtain a 

 value which is of the same order as that found by experiment. 



Weber many years ago suggested that the electrical conductivity of 

 metals was due to the motion through them of positively and nega- 

 tively electrified particles, and this view has recently been greatly 

 extended and developed by Eiecke and by Drude, the objection to any 

 electrolytic view of the conduction through metals is that, as in elec- 

 trolysis, the transport of electricity involves the transport of matter, 

 and no evidence of this has been detected, this objection does not apply 

 to the theory sketched above, as on this view it is the corpuscles which 

 carry the current, these are not atoms of the metal, but very much 

 smaller bodies which are the same for all metals. 



It may be asked if the corpuscles are disseminated through the 

 metal and moving about in it with an average velocity of about 10' 

 centimeters per second, how is it that some of them do not escape from 

 the metal into the surrounding air ? We must remember, however, that 

 these negatively electrified corpuscles are attracted by the positively 

 electrified atoms and in all probability by the neutral atoms as well, so 



