REVIEWS 775 



in the cathode, to the joint action of codirectional electric and magnetic fields ; 

 the trace of the deflected rays on a plane perpendicular to the undeflected rays 



e 



will then be a straight line if the velocity of the rays is constant, and the ratio — 



variable, and a parabola if the velocity is variable, and the ratio — constant, 



m 



different values of the ratio giving different parabolas (e is the charge on the 



carrier of mass m). The trace of the rays is detected by means of a photographic 



plate, and from its nature Sir J. J. Thomson is able to make a series of striking 



deductions. 



The nature of the curves on the plate depends upon the pressure of the gas. 

 When this is not very low (relatively speaking), these are straight lines, generally 

 only two, corresponding to the atom and molecule of hydrogen ; Wien, how- 

 ever, also obtained evidence of the existence of positively changed oxygen 

 atoms. For these pressures the effect is complicated by the fact that, owing 

 to collisions with the gas molecules, these positive rays are not positively changed 

 all the time, but alternate this condition with the neutral and negative state, as 

 Wien showed. The effective charge, e, thus varies according to the fraction 

 of the time during which a carrier is charged with electricity of one kind. In 

 Sir J. J. Thomson's recent series of experiments this effect was avoided by using 

 very high vacua. 



In these high vacua experiments, to which a large part of the book is devoted, 

 the main curves are parabolic, to each parabola corresponding a definite value of 



— for the rays producing it. Sir J. J. Thomson has from the different parabolic 



traces photographed shown the presence of atoms of various elements with 

 positive charges of one or more units, of neutral atoms, and of negatively 

 charged atoms in the "positive" rays in different gases. In addition 



he has found values of — which indicate molecules of various sorts with one 



m 



positive charge : the number of molecules of different kinds present in the case 



of a complex gas may be very large ; for instance, in the case of phosgene gas, 



COCL, molecules of the composition CO, CI,, CC1 and COCL, were found. 



Molecules with a multiple positive charge have never been detected. A strong 



confirmation of the monatomic nature of helium, argon, and the other inert 



gases is afforded by the fact that, for them, only curves corresponding to single 



charged atoms have been observed, while for oxygen and hydrogen, for 



instance, diatomic molecules are easily detected. Another striking result from 



the photographs is that all the atoms except hydrogen can acquire multiple positive 



charges, which agrees with Prof. Rutherford's theory that the hydrogen atom 



consists of a positive nucleus and only one electron. Mercury, the heaviest 



atom investigated, acquires from one to eight positive charges ; the maximum 



number of charges possible appears to depend, not on the valency of the 



atom, but on the atomic weight. 



As the photographs afford no indication of the relative proportions in which 



the different and differently charged atoms and molecules are present, Sir J. J. 



Thomson has measured the number of electrified particles of the various kinds 



present by an electrostatic method, isolating them by a parabolic slit, through 



which the rays of different kinds are brought to pass by altering the strength of 



the magnetic field. In this way estimates of the number of positively and 



negatively charged atoms have been made ; consideration of these results shows 



50 



