440 SCIENCE PROGRESS 



two plates taken at an interval of about twenty years, and 

 rapidly cutting them alternately out of sight. When the 

 plates are adjusted so that the background of stars appears 

 at rest, the large proper-motion stars are evidenced by their 

 motion. It is to be hoped that this method will become more 

 widely used. 



PHYSICS. By James Rice, M.A., University, Liverpool. 



A very notable contribution to the theory of metallic con- 

 duction has been recently made by Sir J. J. Thomson. The 

 paper is published in the Proc. Phys. Soc. London for August, 

 and also in the Phil. Mag. for July. 



It is now generally accepted that the conductivity of a 

 substance both for electricity and heat is dependent on the 

 properties of the electrons in it ; but in order to relate the 

 conductivity to these properties, some assumption must be 

 made concerning the motion of the electrons. The assumption 

 which has had the greatest vogue hitherto is originally due to 

 Drude and J.J. Thomson, and is known as the " free electron " 

 hypothesis. Intermolecular space in a metal is supposed to 

 contain electrons which have escaped from molecules and are 

 free for a time sufficient to allow them to acquire on the average 

 the energy possessed by gas molecules on the average at the 

 same temperature, an energy which is in fact proportional to 

 that temperature on the absolute scale. The electrons are 

 assumed to have their motions distributed about the average 

 according to the usual Maxwell law which holds in the kinetic 

 theory of gases, and, on account of their comparatively large 

 mass and volume, the molecules play much the same role as 

 the walls of the containing vessel in gas theory. 



This electronic motion produces no motion of electrons as a 

 whole in one direction in a metal unless a potential difference is 

 established between two surfaces in it. In that case the 

 electrons have combined with their ordinary free path motion 

 between collisions a drift in the direction of the electric force 

 exerted on them, which, as they are negatively charged, is 

 opposite to the electric field, and this drift constitutes the 

 electric current. The hypothesis leads directly to Ohm's Law, 

 and determines the conductivity in terms of the charge, mass, 

 density, and free path of the electrons and the temperature. 



