146 
NATURE 
[OcToBER 26, 1916 
by chemical interaction between ‘certain of the 
constituent minerals. 
At the close the author appends a useful table 
for discriminating the metallic minerals occurring 
in meteorites, and has added two indices, one 
general, and the other giving the falls mentioned 
in the book. 
ESCAPE OF ELECTRONS FROM HOT 
BODIES. 
The Emission of Electricity from Hot Bodies. 
By Prof. O. H. Richardson. Pp. vii+304. 
(London: Longmans, Green and Co., 1916:) 
Price gs. net. 
HIS work, which is one of the series of mono- 
graphs in~ physics published under the 
editorship of Sir J. J.. Thomson and_ Prof. 
Horton, deals with the emission of positive and 
negative electricity from hot bodies. The closely 
related subject of the conductivity of flames is not 
included. It will be remembered that a volume 
dealing with this part of the subject was published 
a few years ago by Prof. H. A. Wilson. 
While it had long been known that hot bodies 
discharge both positive and negative electricity, 
the detailed investigation of this important sub- 
ject was an immediate consequence of the develop- 
ment of the ionisation theory of gases. The 
author was one of the first workers in this new 
field of work begun at Cambridge, and with the 
help of numerous students he vigorously con- 
tinued his investigations at Princeton University, 
and more recently at King’s College, London. A 
large part of our knowledge of this subject is due 
to his investigations, 
As a consequence, we have a first-hand account 
of this interesting subject, written by one who 
has a full appreciation of the experimental diffi- 
culties and the adequacy of the theories proposed. 
A large part of the volume is devoted to the study 
of the emission of negative electrons from heated 
filaments at low gas-pressures, its variation with 
temperature, the effect of residual gases, and the 
treatment of the metal surfaces. Although the 
electronic current is usually large and easily 
measured, there are in many cases wide dis- 
crepancies in the magnitude of the current ob- 
tained under similar conditions by different 
observers. This lack of definiteness in the data 
seems to result mainly from the part played in 
the electronic emission by the condition of the 
surface, and the presence of gaseous or other 
impurities. A general theory is advanced on the 
supposition that a metal contains free electrons 
which obey the gas laws. At high temperature 
some of the electrons acquire sufficient energy to 
escape from the metal surface. The theory of the 
escape of electrons from a hot metal is thus 
analogous to that of the evaporation of a liquid. 
This theory seems to explain satisfactorily the 
rapid variation of the electronic current with the 
temperature. The author has shown experiment- 
ally that energy is expended in the escape of elec- 
NO. 2452, VOL. 98] 
trons from the metal surface, and that heat is 
given up to the metal surface when electrons pass 
into it. The connection of these effects with the 
contact difference of potential has been carefully 
examined. 
As is well known, there has been considerable. 
difference of opinion in the past as to the origin 
of the large electronic emission from incandescent 
bodies. Its susceptibility to conditions, and 
especially to the presence of impurities, has led 
some to suppose that part, if not all, of the elec- 
tronic emission is the result of chemical action 
between the heated metal and the gaseous or other 
impurities. The author evidently considers that 
there is a true electronic emission depending only 
on the metal, and his contention is certainly 
strongly supported by the observed fact that a 
tungsten filament in the highest possible vacuum 
continues its emission of electrons unchanged with 
time. On the other hand, the electron current 
from most metals is very markedly influenced by 
the previous history of the wire, and is extra-* 
ordinarily susceptible to the presence of a minute 
amount of impurity. For example, Langmuir in 
a recent paper directs attention to the striking fact 
that the heating of a filament containing thorium 
in the neighbourhood of the tungsten wire increases. 
the thermionic current of the latter by a million 
times. While great progress has been made in 
the last decade in extending our knowledge of this 
subject, there is no doubt that much work still 
remains to be done to clear up many outsisaaae 
difficulties. 
In the last chapter the author discusses the 
experimental evidence of the emission of positive 
electricity from heated metals and salts, and 
describes the ingenious method developed by him 
to determine the mass of the carriers. The sur- 
prising fact is brought out that in the great 
majority of cases the carriers of positive elec- 
tricity are atoms of potassium. The reason why 
potassium, which, even when present only in 
minute quantities, is an impurity, should be the. 
active element in this emission 
explanation. 
In the preface the author mentions that he has 
not thought it desirable to include an account of 
{ 
4 
is difficult of — 
the practical applications of thermionic emission, — 
but mentions some of the more important papers 
in which this side has been discussed. It is to 
be hoped, however, that when a second edition 
is called for, the author will devote a chapter to 
the very interesting application of the thermionic 
emission to the rectification of alternating 
currents, and to the magnification of small cur- 
rents in radio-telephony and _ radio-telegraphy. 
1 
It is of great value that students should appre- — 
ciate the striking way in which the pure science 
researches in this subject have proved of great 
technical value. 
This book can be strongly recommended to all 
those who are interested in modern physics as a 
clear and up-to-date account of our knowledge of 
an . important department of modern scientific 
research. 
