4.4.0) TRANSACTIONS OF SECTION A. 
The following Papers and Report were read :— 
1. On Variability in the Products resulting from Changes in Radium 
Emanation. By Sir Witti1amM Ramsay, K.C.B., FBS. 
2. Pseudo-high Vacua. By F. Soppy and T. D. MACKENZIE. 
The electrical characteristics of a high vacuum occur in helium, purified by 
calcium, and subjected to further purification by the passage of the discharge at 
pressures between 3 and 3 mm. of mercury, while in hydrogen the same holds at 
about s+ mm. These pressures are far higher than is commonly supposed. Pure 
helium conducts at pressures higher than { mm. of mercury in the same way as 
common gases. In brief, the behaviour of helium is precisely similar to that of 
argon.ora common gas like nitrogen at about one-tenth the pressure. This explains 
not only the extraordinary resistance at low pressure, but also the extraordinary 
conductivity of the gas at atmospheric pressure. The absorption of helium, argon, 
and neon in spectrum tubes after continuous running occurs mainly in the volatilised 
film of aluminium deposited fr. m the electrodes, The gascan be mainly recovered 
by dissolving the film in mercury or heating the tube. In the latter case it is 
reabsorbed on running with extraordinary rapidity. The portion causing the 
Campbell-Swinton effect is only a small fraction of the total. 
3. On the Range of Freedom of Electrons in Metals.' 
By Professor J. Larmor, Sec. RS. 
It was remarked that perhaps the most obscure present problem in abstract 
physics is the mechanism of the transfer of electricity (the electron) from molecule 
to molecule. A hopeful plan is to study it in its time-relations; the optical phe- 
nomena of metals introduce times, the periodic times of the vibrations, that are 
small enough for this purpose. 
The experiments of Hagen aud Rubens show that the behaviour of metals to 
long infra-red radiation depends on their steady okimic resistance alone. Thus the 
time required to establish conduction completely is a small fraction of the period 
of such waves. If the same free electrons to which conduction is due have velocity 
of mean square determined by the gas-laws, this restricts their range of freedom 
almost to the interspace between the molecules. On the other hand, the fact that 
the square of the guasi-index of refraction of light for the nobler metals is not far 
removed from being a real negative*quantity, indicates that the number of such 
free electrons is of about the same order of magnitude as the number of the mole- 
cules. This again recalls the electrochemical principle that the number of trans- 
Jerable electrons in an atom represents its valency. 
4. Report of the Committee on Electrical Standards.—See Reports, p. 73. 
MONDAY, AUGUST 5. 
The following Papers were read :— 
1. Optical Pyrometry. By Dr. L. Houporn. 
If one wishes by means of the radiation from a body to obtain its temperature, 
it is necessary to recognise that the radiation depends, not only upon the tempera- 
ture, but also upon the emissive power of the radiating body. We deal first, 
therefore, with the radiation of a black body which we take as thestandard, For 
each temperature and wave-length it emits the maximum possible amount of 
radiation, and can be realised by means of a uniformly heated inclosure from 
' For details see Phil. Mag., August 1907. 
