L. Page — A Century's Progress in Physics. 327 



to the formation of ions. Zeleny has found that ions of 

 opposite sign have somewhat different mobilities in an 

 electric field, and experiments of Wellisch (39, 583, 1915) 

 show that at low pressures some of the negative ions are 

 electrons. T. S. Taylor (26, 169, 1908 el seq.) and Duane 

 (26, 464, 1908) have investigated the ionization produced 

 by alpha particles, and Bumstead (32, 403, 1911 et seq.) 

 has studied the emission of electrons from metals which 

 are bombarded by these rays. The investigations of 

 Franck and Hertz, and McLennan and Henderson, show 

 a significant relation between the ionizing potential 

 (energy which must be possessed by an electron in order 

 to produce an ion on colliding with an atom) and a quan- 

 tity, to be considered later in more detail, which has been 

 introduced by Planck into the theory of radiation. 



Methods of Science. — Scientific progress seems to fol- 

 low a more or less clearly defined path. Experimenta- 

 tion brings to light the hidden processes of nature, and 

 hypotheses are advanced to correlate the facts discov- 

 ered. As more and more phenomena are found to fit into 

 the same scheme, the hypotheses at first proposed tenta- 

 tively, although often only after extensive alterations, 

 become firmly established as theories. Finally there may 

 appear a fundamental clash between two theories, each of 

 which in its respective domain seems to represent the only 

 possible manner in which a large group of phenomena 

 can be correlated. The maze becomes more perplexing 

 at every step. At last a genius appears on the scene, 

 approaches the problem from a new and unsuspected 

 point of view, and the paradox vanishes. Such changes 

 in point of view are the milestones which mark the 

 progress of science. That science is stagnant whose 

 only function is to collect, classify and correlate vast 

 stores of experimental data. The sign of vitality is the 

 existence of clearly defined and fundamental problems 

 any possible solution of which seems irreconcilable with 

 the most basic truths of the science in question. The 

 greater the paradox grows, the more certain the advent 

 of a new point of view which will bring one step nearer 

 the comprehensive picture of nature which is the goal of 

 natural philosophy. 



The Ether. — From the earliest times philosophers have 

 been attracted by the possibility of explaining physical 

 phenomena in terms of an all-pervading medium. So 



