j FEBRUARY 19, 1914] 
Boltzmann law of total radiation already men- 
tioned. 
Planck’s ‘‘action constant” has turned out a 
most useful quantity in all sorts of investigations, 
and although its actual nature is somewhat doubt- 
ful,’ it may yet turn out to be, like the velocity 
of light, one of the fundamental constants of 
nature. 
But before any quantum theory of radiant 
energy can be accepted, it must make its peace 
with those phenomena (chiefly diffraction and inter- 
ference) which overthrew Newton’s emission 
theory, and established the wave theory of light. 
That has not yet been done, or even attempted, 
so there is but little prospect as yet of a decisive 
battle. E. E. Fournier pD’ALBE. 
TRANSPARENCE OR TRANSLUCENCE OF 
THE SURFACE FILM PRODUCED IN 
POLISHING METALS.1 
N a communication to the British Association 
(B.A. Report, 1901, p. 604) it was suggested 
that all smooth metal surfaces are covered with 
an enamel-like transparent layer. In a subse- 
quent communication to the Royal Society (vol. 
Ixxii A, p. 218) the actual formation of a surface 
layer or skin by polishing was demonstrated. Two 
of the photo-micrographs in the latter paper, 
Figs. 5 and 6, plate 9, showed that minute pits 
on a polished surface of antimony had been 
covered over by a film of this description. It was 
suggested that the diminished reflecting power of 
the film covering the pits probably indicated that 
it had become translucent, but no direct evidence 
of this translucence was afforded by these par- 
ticular observations. It was also suggested that 
the film might have been carried across the pits on 
a support provided by small granules or flakes 
which had filled up the pit to the level of the 
general surface. The purpose of the present com- 
munication is to record and illustrate certain re- 
cent observations which show :— 
(x) That the film which covers the pits is trans- 
parent, or at any rate highly translucent, and 
(2) That in the case of the smaller pits the 
mobile film has been carried across the empty 
pit without any support from below. 
In the casting and working of copper, unless 
certain precautions are taken, the metal is always 
more or less spongy owing to the presence of gas 
bubbles. When the surface of this metal is 
ground and polished some of the gas bubbles are 
laid open and appear on the surface as tiny pits. 
If the cast metal has been subjected to cold work- 
ing, by rolling or otherwise, the larger bubbles 
are distorted and take elongated and other varied 
forms. 
By any method of polishing which will give 
a fair surface the pits are flowed over and 
obliterated, but by lightly etching the surface with 
a solvent the surface skin can be removed, and 
the pits are again disclosed. By careful regula- 
3 It is an energy divided by a frequency, but has also been regarded as 
an angular momentum. 
1 Paper read before the Royal Society on February 12 by Dr G. T. 
Beilby, F.R.S. 
NO. 2312, VOL. 92| 
NATURE 
691 
tion of the action of the solvent it is possible 
to remove the surface layer step by step, and the 
film covering the pits can be reduced to extreme 
thinness. Through this thin film one seems to be 
looking right into the pit. In polishing metal sur- 
faces the amount of the metal which is removed 
by the polishing agent can be varied through 
wide limits under conditions which need not be 
specified here. It is sufficient for the present pur- 
pose to state that by suitable methods the skin 
developed on the surface may be raised to a maxi- 
mum thickness or reduced to a minimum. For 
the present inquiry it was desirable that the film 
produced should be as thin as possible. The 
copper used in these experiments received its final 
polishing on fine linen stretched over a hard, flat 
surface, and moistened with one of the ordinary 
commercial brass polishing liquids. On _ the 
copper surface prepared in this way the pits, as 
seen under high magnification, appear as blue 
spots on the pale rose-coloured ground of the solid 
metal. While some of the film-covered pits 
appear uniformly blue, others show patches of red 
at various parts of their surface. When these 
red patches were first noticed it was supposed that 
they indicated a thickening of the film at these 
points to the extent necessary for normal reflec- 
tion. More careful study has shown that the red 
patches are due to reflections from the inner con- 
cave surface of the pit. The beam of light from 
the vertical illuminator behind the back lens of 
the object glass of the microscope passes through 
the film covering the pit, strikes the concave 
metallic surface, and is reflected back through the 
film to the object glass and thence to the eye- 
piece. The reflecting surface of the pits is 
evidently far from optical perfection, and the re- 
flected beam is therefore more or less broken up 
by irregularities of the reflecting surface. 
By the use of autochrome plates it has been 
possible to obtain high power photo-micrographs 
in natural colours of pits on a copper surface. 
Four of these transparencies have been reproduced 
by the three colour process, and are shown on the 
plate issued as a supplement to this week’s 
Nature. Figs. 1 and 2 are at a magnification of 
800 diameters, and 3 and 4 at 1800 diameters. 
In Figs. 1 and 3, the pits are covered by a blue 
film, but show patches of red on the blue. Figs. 2 
and 4 show the same pits after the film has been 
dissolved and removed by a to per cent. solution 
of ammonium persulphate acting for 20 to 30 
seconds. On comparing the members of each 
pair, 1 with 2, and 3 with 4, it is seen that the 
red patches in 1 and 3 correspond with the spots 
of light reflected from the concave surfaces of the 
uncovered pits as shown in 2 and 4. 
It is clear that the pits which show these re- 
flections from the under surface must have been 
practically empty when they were covered by the 
film, so that the film during its flow was quite 
unsupported from below. 
The thickness of the films covering the pits is 
probably of the order of 10 to 20 micro- 
millimetres. 
