Sbptejibee 7, 1900. ] 



SCIENCE. 



369 



proportion as its atoms are heavier than 

 those of hydrogen. 



Thomson has undertaken to find the 

 charge carried by the gaseous ion as fol- 

 lows : "When the discharge of an induction 

 coil is sent through a vacuum tube, there 

 is seen a luminous glow, stretching in a 

 straight line from the electrode to the wall 

 of the tube. This glow, called the ' cath- 

 ode ray' would seem to be a stream of nega- 

 tively charged particles, from the cathode, 

 or negative terminal in the tube, projected 

 in a straight line until some solid obstacle 

 is encountered. This cathode ray, when 

 it meets the tube, or any body in its path, 

 may produce fluorescence ; it always pro- 

 duces heating, it also excites the vibrations 

 called by Rontgen the X-ray. 



A magnet held near the cathode ray 

 draws it to one side, as if it were a conduc- 

 tor carrying an electric current. Professor 

 Thomson has made use of this property to 

 determine the ratio e/m for the electrified 

 particles. Of course the more strongly the 

 flying particles are charged, the more they 

 will be drawn aside from their rectilinear 

 path, while the heavier the particles, the 

 more nearly would their inertia keep them in 

 a straight line. The ratio of the charge to 

 the mass of a particle determines its ve- 

 locity at right angles to the original direc- 

 tion. 



Again, the flying stream may be drawn 

 aside from its course by an electrified plate 

 at the side of the stream, by which it will 

 be attracted or repelled according as the 

 plate has a positive or negative charge. 



Both these methods for deflecting the ray 

 were employed. The energy of the flying 

 particles was also determined from the heat 

 which they produced when directed upon a 

 thermopile; and the ratio of the charge 

 upon the particles to their mass was thus 

 found to be about 10', or nearly 1000 times 

 as large as for the hydrogen atom in the 

 electrolysis of solutions. 



Again, when ultra-violet light falls upon 

 an amalgamated zinc plate, the gas near the 

 plate becomes conducting. Here again if a 

 magnetic field is produced near the plate, 

 the path of the charged particles is changed. 

 This path can no longer be seen, as in the 

 cathode ray ; it may, however, be inferred 

 from the change of conduction, when the 

 distance between the electrodes is varied. 

 The ratio of the charge to the mass of the 

 particles is, in this case, the same as in the 

 cathode ray, as above determined. 



If, as is believed, the electric current in 

 these cases consists of a stream of charged 

 particles, we are apparently shut up to the 

 alternative that the charge of each ion is 

 1000 times as great as is found in solutions, 

 or that the mass of the ions is ywoo ^^ 

 great as that of the hydrogen atom. Prob- 

 ably the former supposition seems much less 

 opposed to our preconceived ideas than the 

 latter, but it is a question to be decided by 

 experiment rather than by preconceived 

 ideas. 



To make a direct measurement of the 

 mass of the single ions, or particles taking 

 part in electric conduction, Thomson ex- 

 amined air which had been rendered con- 

 ducting by exposure to Eontgen rays. The 

 quantity of electricity carried by such air 

 is measured without special difficulty. To 

 count the number of ions taking part 

 in the conduction is quite another matter. 

 This counting has, however, been actually 

 accomplished in the following manner : 

 Damp air, which has been freed from dust 

 by filtering, is exposed to the Eontgen rays 

 and its conductivity determined ; it is then 

 suddenly expanded to li times its volume. 

 The expansion and consequent cooling, 

 causes a fine fog or mist to form. It has 

 been found that when such a mist is formed, 

 there is at the center of each drop, a minute 

 particle of dust, or other substance, upon 

 which condensation has taken place. In 

 this case, all the dust had been filtered out, 



