408 Conduction in Solutions and Gases, 



ions have been abundantly confirmed. It appears certain that 

 electric charge exists in discrete units, vitreous and resinous, 

 each of magnitude 1*5 x 10~ 19 coulombs approximately. Each 

 ion, whether in an electrolytic liquid or in a gas, carries one 

 (or an integral number) of these charges. An electrolytic ion 

 also contains one or more atoms of matter; and a positive 

 gaseous ion has a mass of the same order of magnitude as that 

 of an atom of matter. But it is possible in many ways to 

 produce in a gas negative ions which are not attached to atoms 

 of matter ; for these the inertia is only about one- thousandth 

 of the inertia of an atom; and there is reason for believing 

 that even this apparent mass is in its origin purely electrical.* 



The closing years of the nineteenth century saw the founda- 

 tion of another branch of experimental science which is closely 

 related to the study of conduction in gases. When Rontgen 

 announced his discovery of the X-rays, and described their 

 power of exciting phosphorescence, a number of other workers 

 commenced to investigate this property more completely. In 

 particular, Henri Becquerel resolved to examine the radiations 

 which are emitted by the phosphorescent double sulphate of 

 uranium and potassium after exposure to the sun. The result 

 was communicated to the French Academy on February 24th, 

 1896.f " Let a photographic plate," he said, " be wrapped in 

 two sheets of very thick black paper, such that the plate is not 

 affected by exposure to the sun for a day. Outside the paper 

 place a quantity of the phosphorescent substance, and expose 

 the whole to the sun for several hours. When the plate is 

 developed, it displays a silhouette of the phosphorescent 

 substance. So the latter must emit radiations which are 

 capable of passing through paper opaque to ordinary light, and 

 of reducing salts of silver." 



At this time Becquerel supposed the radiation to have been 

 excited by the exposure of the phosphorescent substance to the 

 sun ; but a week later he announced^ that it persisted for an 



* Cf. p. 343. f Comptes Rendus, cxxii (1890), p. 420. 



I Ibid., cxxii (March 2nd, 1896), p. 501. 



