November 6, 1919] 



NATURE 



23: 



Even in the present year a further significant 

 advance in this direction has been made. For 

 it appears, from the latest results of Sir Ernest 

 Rutherford on the passage of a-particles through 

 nitrogen, as though the nuclei of an exceedingly 

 minute proportion of the nitrogen atoms struck by 



the a-particle were shattered by the collision. If 

 this is so, artificial transmutation on an infini- 

 tesimal scale has already been accomplished, 

 though, it is true, only by the aid of a previous 

 natural transmutation, still impossible artificially 

 to imitate. 



lONISATION OF GASES. 



By Prof. J. S. Townsend, F.R.S. 



DURING the last fifty years many physicists 

 have been occupied in studying problems 

 connected with electric currents in gases. The 

 earlier work was principally confined to experi- 

 mental investigations of the general outlines of 

 the phenomena which occur in discharges obtained 

 with high potentials. The large number of com- 

 plicated and surprising properties of gases which 

 were thus discovered naturally attracted much 

 attention, and it is very interesting to read the 

 accounts of the first experiments of the discharges 

 in air and through vacuum tubes which were 

 written before any special investigations of the 

 theory of the conductivity were undertaken. 



It would clearly have been extremely difficult 

 to obtain from these experiments any general 

 theory of electricity to explain what was taking 

 place, as such a large number of different pheno- 

 mena seemed to occur simultaneously. From the 

 first, some physicists maintained that the currents 

 through gases were carried by means of ions, as 

 in liquids, although there were peculiar differences 

 between the two cases, and it was not evident 

 why under a given force a gas might act either 

 as an insulator or as a conductor. 



The greatest success in advancing the theory 

 of electricity was obtained from careful studies of 

 the discharges at very low pressures. In this 

 direction some remarkable experiments on the 

 cathode rays were made by Hittorf in i86g. He 

 found that the rays travelled in straight lines 

 from the cathode ; when they fell on glass they 

 caused the surface to fluoresce, and an obstacle 

 in the path of the rays cast a shadow on the glass. 

 He also found that the rays were deflected by 

 a magnet into circles, or more generally into 

 spirals, which were described in the direction 

 which would be taken by negatively charged 

 particles moving from the cathode. Notwith- 

 standing these results, and further experiments 

 made by Crookes, the projected particle theory 

 of the rays was not at first universally accepted, 

 and some physicists maintained that the rays 

 were an undulatory motion of the ether. This 

 question was decided by Perrin in 1895. He 

 showed by direct experiment that the rays* carried 

 j a negative charge, but thus far the origin of the 

 , rays, their velocity, and the mass and charge of 

 each particle were unknown. 



The question of the ratio of the charge to the 



mass was studied by Schuster in 1890, and he 



concluded that in gases it was of the same order 



as in liquids, but for negative ions it was larger 



NO. 2610, VOL. 104] 



than for positive. This was the first indication 

 of the characteristic difference between positive 

 and negative ions in gases. 



A direct method of finding the ratio of the 

 charge e to the mass m of the cathode particles, 

 and the velocity of the particles, was devised by 

 Wiechert, and in 1897 he described the experi- 

 ments which showed that in some cases the velo- 

 city of the rays was about one-tenth of the velo- 

 city of light, and that the ratio e/m for the 

 cathode rays was between 4000 and 2000 times as 

 great as the corresponding quantity for a hydro- 

 gen atom. Thus, assuming the charges to be the 

 same in the two cases, the experiments showed 

 that the mass of a cathode particle is very small 

 compared with the mass of an atom of hydrogen. 

 This small cathode particle has been called the 

 electron. Further experiments show that currents 

 of negative electricity obtained from metal sur- 

 faces by other methods also consist of streams of 

 electrons. Thus Sir J. J. Thomson investigated 

 the charged particles set free from hot wires or 

 from a metallic surface by the action of ultra- 

 violet light, and found that in both cases the ratio 

 efm was the same as for cathode rays. The 

 values of e/m afterwards found by various 

 methods show that the ratio of the mass of the 

 electron to the mass of an atom of hydrogen is 

 I : 1830. This value of e/}7i is constant provided 

 the velocity is small compared with the velocity of 

 light, but with velocities of this order the effective 

 mass of the electron increases, and Kaufmann 

 found that the value of e'lm diminishes in accord- 

 ance with Lorentz's theory as the velocity ap- 

 proaches the velocity of light. 



During the earlier part of this period some in- 

 vestigations were made of the currents that can be 

 obtained with forces smaller than those required 

 to produce discharges. The positive and negative 

 ions produced in air at atmospheric pressure 

 at the surface of incandescent metals, the con- 

 ductivity of flames, and the charges obtained in 

 newly prepared gases, or by bubbling air through 

 water, were examined. In these cases the mass 

 associated with the ions is comparatively large, 

 and varies rather irregularly over wide ranges, so 

 that it was difficult to formulate precise theories 

 from the results of the experiments. These large 

 ions have the property of condensing water 

 vapour, and in a moist atmosphere small drops 

 are easily obtained which form a visible cloud. 

 This phenomenon led to the method of estimating 

 the charge on each particle. The number of 



