AA NATURE 
light of frequency equal to that of the absorption line. 
This condition was perfectly fulfilled by the vapour 
of mercury, which has an absorption line at A=2536 
in the ultra-violet. 
If a beam of monochromatic light of this wave- 
length was focussed at the centre of an exhausted 
quartz bulb containing a drop of mercury at atmo- 
spheric temperature, it was found that the light was 
powerfully scattered by the vapour, photographs of 
the bulb made with a quartz lens showing the cone 
of rays much as if the bulb were filled with smoke. 
The scattered light is invariably much more homo- 
geneous than the incident beam, in which the ‘‘line”’ 
has a finite width, whereas the scattered light corre- 
sponds only with the centre of this line. The rest 
gets through the vapour unaffected. With the light 
thus scattered—the resonance radiation—a photograph 
was made of a quartz bulb containing a minute drop 
of mercury at room temperature. The bulb appeared 
as if filled with ink owing to the opacity of the vapour 
for the rays. 
These phenomena, visible only to the camera, can 
be visually reproduced in the case of sodium vapour 
excited by the light from a sodium flame. If the 
density of the vapour is increased by warming it, the 
distance which the light can penetrate into the bulb 
is diminished and eventually the resonance radiation 
is all emitted from a region so close to the surface 
that it appears as a bright yellow patch on the inner 
surface of the glass. 
If this patch is now used as a lamp, and focussed 
by a concave mirror on the surface of the same globe 
(or another in which the vapour is of sufficient density 
to give the patch effect) so as to fall partly on a sur- 
face whitened by deposited magnesia and partly on 
the enclosed vapour, the brightness of the two con- 
tiguous patches thus formed is practically equal. 
This proves that, under those conditions, at com- 
paratively low densities, true absorption does not exist, 
the light abstracted from the incident beam being re- 
emitted as light of the same wave-length but in all 
directions. 
The factor of true absorption makes itself manifest 
as soon as we admit air or some other foreign gas. 
Even if the pressure is only a millimetre or two the 
effect is very marked. 
Another point which can be brought out by this 
method of attack is whether or not the mechanisms 
the vibration frequencies of which correspond to the 
various lines in a spectrum are independent of each 
other or are interconnected. 
An ingenious method was described whereby a beam 
of considerable intensity, consisting, however, of only 
D, or D, light, could be obtained, and if the sodium 
vapour excited by either of these was examined spectro- 
scopically the emitted light contained only that one of 
the lines which was used to excite it. This shows 
that the D, and D, mechanisms are quite independent. 
In other cases, however, vapours excited by light of 
any one line of their spectrum gave out a resonance 
spectrum of that line and one or more others showing 
that some groups of mechanisms were interdependent 
and could not be excited separately. 
Stimulation by Waves of Very Short Wave-Length. 
—Experiments were then described in which air, 
nitrogen, etc., had been caused to emit ultra-violet 
light when exposed to the action of radiation of wave- 
length less than the Schumann rays, the smallest 
waves hitherto known. Schumann rays were com- 
pletely absorbed by quartz, but would pass through a 
considerable thickness of fluorite, but the rays to 
which he referred could be reduced in intensity by 
98 per cent. by a plate of fluorite 1 mm. thick. : 
Nitrogen was more actively stimulated than air by 
NO, 2305,0ViOL, 93) 
[Marcu 12, 1914 
these rays, as oxygen seemed to have a destructive 
effect on the phenomena. Thus iodine vapour, if 
mixed with nitrogen, emitted a green light under the 
action of the rays, while remaining dark if mixed 
with oxygen. 
He urged the necessity of an exact mathematical 
treatment of the phenomenon of a molecule of vapour 
re-emitting radiation which it has abstracted from an 
incident beam, true absorption being absent. 
At the conclusion of the lecture a number of in- 
teresting experiments illustrative of the subject of the 
lecture were shown. These included the resonance 
radiation of sodium stimulated by D light, of iodine 
vapour stimulated by the light from a quartz-mercury 
lamp, and of the author’s method of extinguishing 
one of the D lines from the light from a sodium flame. 
STRUCTURAL ANALOGIES BETWEEN 
IGNEOUS ROCKS AND METATS: 
T was in Sheffield that the late Dr. H. C. Sorby 
lived and worked. It was to the Sheffield Literary 
and Philosophical Society that, in 1864, Sorby pre- 
sented the first account of his microscopical examina- 
tion of the structures of commercial steel. In Sheffield 
the worth of Sorby’s work is now being recognised, 
and during the presidency of Mr. Arthur Balfour the 
Sheffield Society of Engineers and Metallurgists, an 
active and growing society closely associated with the 
industries of the city, has founded the ‘‘Sorby Lec- 
ture,” to ‘‘mark its progress,” and to perpetuate the 
memory of its late honorary member. 
The first Sorby Lecture, on February 28, was the 
occasion for a large gathering of Sheffield’s leading 
manufacturers and citizens at the Cutlers’ Hall. The 
lecture was delivered by Prof. W. G. Fearnsides, the 
occupant of the Sorby chair of geology at Sheffield 
University, ‘‘On Some Structural Analogies between 
Igneous Rocks and Metais.”’ 
In the first part of the lecture Prof. Fearnsides 
traced the progressive development of the research by 
which Sorby, already trained to a knowledge of optics 
and of chemistry, learned from Williamson the art of 
making transparent sections of hard objects, and 
applied it (1849) to the study of rocks. Limestones 
were the first rocks to claim his attention (1851), then 
slates (1856), and then igneous rocks (1857), and from 
these, through meteorites (1862), he was led to study 
irons (1863-4). The difficulties which Sorby encoun- 
tered and his patient toil, continued in defiance of 
indifference and ridicule, were discussed, and it was 
conjectured that the apathy with which his results 
were received was due to his own inability to appre- 
ciate the difficulties which his refined technique and 
the vector variations of the optical properties of 
minerals presented to other people. 
The recognition of the value of Sorby’s petrographic 
methods grew gradually through the sixties and seven- 
ties of last century, but it was not until after his 
announcement to the Iron and Steel Institute in 
1886, that in the previous year a new microscope had 
enabled him to see the true composite nature of the 
“pearly constituent’’ of steel, that his pioneer work 
on metals attracted any attention. ; 
It was by a fortunate but unforeseen coincidence that 
the first Sorby Lecture was delivered within a few 
days of the fiftieth anniversary of the day on which 
Sorby read the first of all papers dealing with the 
micro-structure of commercial metals, and the sub- 
ject for the lecture was chosen accordingly. 
The second part of the lecture dealt with the modern 
view that igneous rocks and metals are alike products 
derived by progressive partition of components during 
the crystallisation of mixed solutions. Being thus 
4 
