SECTIONAL TRANSACTIONS, 
SECTION A.—MATHEMATICAL AND PHYSICAL 
SCIENCE. 
(For references to the publication elsewhere of communications entered in 
the following list of transactions, see p. 464.) 
Thursday, September 8. 
1, Prof. J. C. McLennan, F.R.S.—Radiation and Absorption by 
Atoms with Modified Systems of Extra-nuclear Electrons. 
2. Prof. R. W. Woop, For. Mem. R.S.—The Spectra of Hydrogen 
from Long Vacuum Tubes. 
The spectrum of hydrogen obtained in the laboratory exhibits a Balmer 
series of lines, of which only twelve members are found under ordinary con- 
ditions. ‘Thirty of the lines have been recorded in the solar spectrum. The 
impossibility of obtaining the higher members of the series in the laboratory 
results, in part at least, from the presence of a continuous spectrum and the 
so-called secondary spectrum. 
By employing vacuum tubes of unusual length it has been found that the 
secondary spectrum appears only at the ends, the central portion radiating a 
very pure Balmer spectrum. By this means it has been possible to record 
the series down to the twentieth line. Very remarkable phenomena have been 
observed with tubes of this type. Starting with a very feeble current, the 
secondary spectrum only is in the tube, the Balmer lines being absent. As the 
current is increased the Balmer lines appear and increase in intensity, while 
the secondary spectrum fades away, passing through a minimum value of 
about one-fiftieth of the intensity shown at the ends of the tube, after which 
it slowly increases in intensity. 
If a heavy current is employed, there is an explosive flash of the secondary 
spectrum at the moment of closing the switch, the Balmer lines being feeble. 
In about ,4, sec. the secondary spectrum has nearly disappeared (reduced to 
one-fiftieth of its initial value) and the Balmer lines have risen to full intensity. 
These phenomena occur only when a trace of oxygen or water vapour 
is present. With pure hydrogen, continued operation of the tube eventually 
causes the complete disappearance of the Balmer series, the secondary spectrum 
remaining, and the colour changing from fiery purple to white. A new spectrum 
has been found which is much more complicated in structure than the secondary 
spectrum. This appears when the tube is in the condition best suited to the 
exhibition of the Balmer series. 
3. Prof. R. W. Woop, For. Mem. R.S.—The Time Interval between 
the Absorption and Emission of Light in cases of Fluorescence. 
In the case of mercury vapour, illuminated by the instantaneous flash of 
an aluminium spark, it has been found that the vapour remains non-luminous 
during the period of excitation and for about ;:;4,5 sec. after, subsequently 
bursting out in a flash of green fluorescent light. This appears to be the 
first observation of a fluorescent or phosphorescent body remaining dark during 
the period of illumination. Other substances have been observed with a new 
type of phosphoroscope which records the phenomena of phosphorescence to 
4p0005 sec. Nothing analogous in its behaviour to mercury vapour has been 
found up to the present time, however. 
It is only freshly formed mercury vapour which exhibits the phenomenon 
of fluorescence. No trace of visible luminescence is shown by mercury vapour 
at any density, or by any light stimulation, wnless metallic mercury is present 
and liberating nascent molecules. It is believed that these are diatomic when 
they first leave the metal, subsequently breaking up into monatomic molecules. 
