NATURE 
611 

THURSDAY, AUGUST 5, 1915. 


A CHAPTER OF BRITISH SCIENCE. 
Electricity in Gases. By Prof. J. S. Townsend. 
Pp. xv+496. (Oxford: Clarendon Press, 
TOUS.) nice 145. met: 
HE present book was written to appear as 
the first of six volumes of a handbook on 
Radiology, to be published in German under the 
editorship of Prof. Erich Marx. The translation 
of (Rutherford’s ‘“‘Radio-active Substances and 
their Radiations ”’ appeared nearly two years ago 
as the second volume of the series. The other 
volumes in preparation deal with kathode rays, 
X-rays, photoelectric effects, and the theory of 
electrons and X-rays. It was intended that the 
last two subjects should be treated by a number 
of well-known physicists, including Profs. H. A. 
Lorentz, P. Zeemann, Langevin, Einstein, Riecke 
and Sommerfeld. No doubt the publication of 
the later volumes has been delayed by the out- 
break of war. 
The best-known work on the passage of elec- 
tricity through gases is Sir J. J. Thomson’s 
treatise, of which a second edition was published 
in 1906. This gave a very complete account of 
the work up to that date in this new department 
of physics. Since that time the growth of ex- 
perimental facts has been so rapid that it would 
be difficult to treat the whole subject adequately 
within the compass of a single volume. For- 
tunately a number of small books have been 
published from time to time dealing with special 
topics, e.g. H. A. Wilson’s “The Electrical 
Properties of Flames and Incandescent Solids,” 
Townsend’s ‘“‘Ionisation by Collision,” Sir J. J. 
Thomson’s “Positive Rays,” Kaye’s ‘‘ X-Rays,” 
W. H. and W. L. Brage’s “X-Rays and 
Crystals,” Crowther’s “Molecular Physics,” and 
Soddy’s “Chemistry of the Radio-Elements.” 
These have proved of great value and interest 
in giving a first-hand account of recent advances 
of our knowledge in these special departments. 
The present volume is mainly confined to a dis- 
cussion of the properties of ions and the mode 
of conveyance of electricity through gases. The 
allied subjects of radio-activity, characteristic X- 
radiations, wave-lengths of X-rays, and therm- 
ionics, receive only passing mention, as_ they 
were to be treated in detail by other writers in 
subsequent volumes. After a preliminary dis- 
cussion of the methods of production of ions in 
gases by different agencies, there follows a de- 
tailed discussion of the motion of ions in the 
electric field, the laws of their diffusion and re- 
combination, and the methods of determining the 
NO. 2388, VOL. 95] 
ionic charge. The latter includes an interesting 
account of the experiments of Perrin. Then 
follows a very complete discussion of ionisation 
by collision—a subject which the author has made 
peculiarly his own. Other chapters are devoted 
to the discussion of the conditions and types of 
discharge between conductors of various shapes, 
and the passage of electricity through vacuum 
tubes. The final chapter gives an interesting re- 
view of the pioneer researches on the nature of 
the kathode and positive rays. 
Our information on the subject of ionisation 
and the passage of electricity through gases has 
grown so rapidly, and is in many respects so de- 
finite and complete, that the reader is liable to 
forget how modern the subject is, for the whole 
advance is included within a period of less than 
twenty years. It is instructive to note what a 
prominent part British science has taken in its 
development. Practically all the pioneer dis- 
coveries in this subject were made in the Caven- 
dish laboratory, and it is of interest to recall 
briefly the more important features in this ad- 
vance. Shortly after the discovery that X-rays 
imparted a transient electrical conductivity to 
gases, in 1896 J. J. Thomson and Rutherford 
showed that the results were simply explained on 
the assumption that the current through the gas 
was due to the movement of charged ions produced 
throughout the volume of the gas by the radia- 
tion. This theory was soon generally accepted, 
and has formed the starting point of, subsequent 
advances. Then followed the work of Ruther- 
ford on the recombination of the velocity of ions 
and the application of the ionisation theory to 
radio-activity. Zeleny discovered the difference 
in velocity between the positive and negative 
ion, and made the first accurate measurements 
of the mobilities. To this period belong the 
classical researches of C. T. R. Wilson on the 
ions as nuclei of condensation—investigations 
which have had such a remarkable development 
in recent years in making visible the processes 
occurring in the ionisation of gases by different 
types of radiation. Then followed the application 
of the ionisation theory to explain the conduc- 
tivity of flame gases and flames by J. McClelland 
and H. A. Wilson, and later the investigations 
of H. A. Wilson and O. W. Richardson on the 
electricity from glowing 
escape of negative 
bodies. 
We can only refer in passing to the pioneer 
experiments of Townsend, J. J. Thomson, and 
H. A. Wilson on the magnitude of the charge 
carried by the gaseous ions. In 1898 Townsend 
began his well-known researches on the diffusion 
| of ions, and on the ionisation of gases by collision 
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