JULY 30, 1914] 
NATURE 
Eiesae 
the use of members. The Federal 
gives general accounts of the scientifically impor- 
tant aspects of the country, and it was some time 
ago placed in the hands of prospective visitors. 
It is the intention of the Commonwealth Govern- 
ment also to present a copy to each member of the 
general committee and to each foreign corre- 
sponding member. An admirable supplement to 
this work is given by the handbooks issued by the 
respective States, and by booklets which deal 
specifically with the localities chosen as objects of 
excursions. 
the time which members can spare from their 
work in Great Britain must be spent on the ocean ; 
but it may be conceded that the proposed arrange- 
ments in Australia, if successfully carried through, 
will have offered to the visiting party as full and 
jects of a colonial meeting of the British Associa- 
tion as it is possible to obtain in the short space 
of three and a half weeks, devoted to visiting four 
cities distributed along a stretch of railway line 
1800 miles in length. 
THE ELECTRIC EMISSIVITY AND THE 
DIRECTIVE DISINTEGRATION OF HOT 
BODIES, 
aoe department of physics which concerns 
itself with the electrical and kindred pro- 
Handbook ; 
thermionic currents were due to a kind of “elec- 
tric evaporation” of the unattached electrons ir 
the solid. 
Treating the problem from this point of view, 
Richardson derived his well-known expression :— 
J =abhe- 4%, 
where I is the saturation current, @ the absolute 
temperature, and a and b are constants for any 
particular material. As will be seen, the formula 
(which is identical in type with that of Kirchoff 
for vapour-pressure, and shares its peculiar 
ae PAN a a 
It is a misfortune that so great a proportion of | elasticity "} contemplates the electric emission 
_ as due solely to the effect of temperature. 
It was not until 1912 that doubts were first cast 
on the adequacy of this explanation. Pring and 
Parker (Phil. Mag., January, 1912), working in Sit 
Ernest Rutherford’s laboratory at Manchester, 
well-chosen an opportunity for achieving the ob- | took especial pains to purify and free from gas a 
| sample of carbon rod, and in consequence of these 
precautions found that the thermionic emission 
from the rod in a high vacuum was reduced to a 
value of the order of a million times smaller than 
was indicated by Richardson’s formula. Later, 
| Pring (Proc. Roy. Soc., November, 1913) con- 
_ tinued and refined these experiments, and suc- 
ceeded in further reducing the ionisation currents. 
Clearly, Richardson’s constants for carbon 
_ needed amending, as he himself readily recog: 
_nised, though the two investigators still find them- 
perties of incandescent bodies has recently at- | 
tracted a good deal of controversial attention. 
From the outset, the subject aroused great in- 
terest, possibly in view of its speculative possi- 
bilities for solar physicists and others, and 
numerous workers have carried its development 
into great detail andsome complexity. In a short 
résumé of the two phenomena specifically men- 
tioned in the title of this article, we can do no 
more than touch on general outlines. Of the 
selves in disagreement (Richardson, Proc. Roy. 
Soc., May, 1914) as to the extent of the correc- 
tion which is afforded by Pring’s experiments. 
This point remains at issue, but the further 
observations of Pring on the effect of introducing 
traces of different gases into the vessel containing 
the carbon strongly favour his contention that 
| the thermionic currents owe their origin, at any 
several aspects, the electrical one, now generally | 
” 
known as ‘‘thermionics,” calls for first mention. 
) 
Eleciric Emissivity.—It was Guthrie in 1873, at | 
the Royal School of Mines, who did pioneer work | 
on what we now know to be a characteristic | 
feature of incandescent bodies, and that is, to put 
it simply, their property of emitting an excess 
of positive electricity at a red heat, and at higher | 
| Soc., February, 1912), at the National Physical 
_ Laboratory, had approached the question from 
temperatures, a much larger excess of negative. 
The positive electrification, we have reason to 
believe, is carried chiefly by atoms or molecules 
of either occluded gas (such as CO), or impurities 
such as the alkali metals. 
The degree of the negative emissivity at high | 
| emissions of unparalleled magnitude from carbon 
temperatures was shown by subsequent workers, 
Elster and Geitel among others, to depend not | 
only on the body and its temperature, but also on | 
the nature and pressure of the surrounding gas. 
Sir J. J. Thomson established the fact that no 
matter what the nature of the hot body, the elec- 
tron was the prime agent in the transport of the 
negative electricity; and later, Prof. O. W. 
Richardson (then a student at the Cavendish 
Laboratory), in his early work on a subject now 
associated with his name, concluded that these 
NO. 2425, VOL. 93| 
rate in great part, to an interaction, probably of 
a cyclic character, between the carbon and the 
surrounding gas. Pring tried a number of gases 
—helium, argon, nitrogen, carbon dioxide, etc.— 
and found that the ionisation effects at very low 
pressures followed closely the order of the known 
chemical activities. It may be noted that Freden- 
hagen, working about this time on sodium and 
potassium in a high vacuum, also concluded that 
the thermionic effect was a chemical one. 
In the meantime, Harker and Kaye (Proc. Roy. 
quite another point of view. With the object of 
accentuating these high-temperature currents, they 
had recourse to the carbon-tube resistance furnace. 
Under these conditions they obtained electrical 
at atmospheric pressure, no electromotive force 
being applied. At temperatures approaching 
3000°C. the thermionic current attained a 
value of several amperes, and readily lit up a nest 
of small glow-lamps; it was no longer a question 
for electrometer or sensitive galvanometer. These 
investigators concluded that the effect in their 
experiments was largely conditioned by the 
furnace-gas and the expulsion of the impurities 
present in the carbon. In more recent work, 
