160 
The generally-accepted ideas as to the limits of magnetisation 
and magnetic induction therefore need modification. 
TABLE (Abstract) 
H Ww I k | m B 
3°9 2210 | 587 I51'0 1899'I 7390 
See 3460 735 128°9 1621°3 9240 
17°7 7530 1083 612 770'2 13630 
30°'2 9215 1197 39°7 500'0 15100 
78 11550 337 D7 216°5 16880 
208 13810 1452 70 88'S 18470 
427 15130 1504 3°5 45°3 19330 
585 15905 1530 2°6 33°9 19820 
May 27.—‘‘On the Relation between the Thickness and the 
Surface-tension of Liquid Films.” By A. W. Reinold, M.A., 
F.R.S., Professor of Physics in the Royal Naval College, 
Greenwich, and A. W. Riicker, M.A., F.R.S. 
Plateau, Liidtge, and van der Mensbrugghe have investigated 
experimentally the relation between the thickness and surface- 
tension of thin films. None of these observers, however, have 
used films thin enough to show the black of the first order of 
Newton’s colours. The authors have therefore made a careful 
comparison of the surface-tension of black films with that of 
coloured films, the thickness of which was from 10 to 100 times 
greater. The principle of the method is the same as that 
utilised in Liidtge’s experiments. The interiors of the films to 
be compared are connected, and the relation between their 
surface-tensions is deduced from measurements by which their 
curvature is determined. In the authors’ experiments a cylin- 
drical film was thus balanced against another, which, though 
sometimes cylindrical and sometimes spherical, was initially of 
the same curvature as itself. The necessity for this arrangement 
arises from the fact that the authors’ previous observations have 
shown that a cylindrical film thins to the black of the first order 
more readily than one of any other form. The fact that small 
changes in the forms of cylindrical and spherical films, attached 
to two circular rings, convert them into unduloids or nodoids, 
renders the mathematical theory somewhat complicated, but 
other considerations have been made to give way to the necessity 
of obtaining films which readily yield the black. 
Preliminary experiments were instituted to test the observa- 
tions of Liidtge and van der Mensbrugghe as to difference of 
surface-tension between two films, one of which had been 
formed more recently than the other. These experiments 
showed that when one of the films was kept thick by supplying 
liquid to its upper support (flooding), while the other was 
allowed to thin, a considerable apparent difference of surface- 
tension was obtained. Before, however, this could be accepted 
as a trustworthy determination of an actual difference of sur- 
face-tension, several possible sources of error had to be con- 
sidered. Thus, experiment shows (1) that the fact that 
the thicker film displays the greater surface-tension cannot 
be attributed to any peculiarity of the apparatus or mode 
of thickening adopted ; (2) that it is not due to the weight of 
the thicker film ; and (3) that only a small part of the observed 
difference can be ascribed to any slipping of the film over the 
liquid attachments to the solid supports. 
The cause of the phenomenon cannot at present be assigned 
with certainty. Perhaps many causes are at work. Reasons 
are given for the conclusion that it is merely an instance of the 
difficulty which many observers have found in preserving a liquid 
surface pure. 
On the assumption that the rapid change in the surface-tension 
of a newly-formed film is not due to its thinning, but to a dis- 
turbing cause, attempts were then made to eliminate this cause, 
or reduce it so as to compare films of very different thicknesses. 
Two methods of attacking the problem were carried out. In 
the first the procedure was as follows:—The diameters of two 
cylindrical films were measured when they were in the same 
state ; an electric current was passed up one of them in order to 
thicken it; and then, after a sufficient length of time had elapsed 
for the direct effect due to the disturbance produced by the 
current to pass off, the diameters were again measured. By this 
means it was possible to compare two films, one of which was 
NATURE 
| une 17, 1886 
nearly all black, while the other displayed a little black and the 
colours of the first and second orders. Both films were then 
allowed to thin, and assuming (in accordance with previous 
observations of the authors) that that which was already black 
remained in a constant state, any change of diameter which took 
place, as the coloured film became black, could be observed. 
In a second group of experiments a cylinder was balanced 
against a sphere. As a spherical film thins more slowly than a 
cylinder, a comparison between a thick film (sphere) and a 
black or partially black film (cylinder) could be made without 
having recourse to an electric current, and greater differences of 
thickness were obtained than in the earlier observations. 
The differences of surface-tension measured in these observa- 
tions were very small. They never exceeded 1°5 per cent., and 
the black films were sometimes more and sometimes less curved 
than the thicker films with which they were compared. There 
was no evidence of any regular change in the surface-tension as 
the thickness diminished, and the average difference between 
the tension of the black and coloured films as deduced from 
fifteen experiments was only 0°13 per cent. 
The general result of the inquiry, therefore, appears to be 
that when the black part of a soap-film forms in the normal way, 
spreading slowly over the surface, no evidence of any change in 
surface-tenston dependent on the thickness of the film is furnished 
by a direct comparison of the tensions of thin and thick films over 
a range of thickness extending from 1350 to 12 millionths of a 
millimetre. 
This conclusion is based upon a method of experiment by 
which a change of 4 per cent. in the value of the tension must 
have been detected, had it existed, and upon fifteen independent 
comparisons of the tensions of black and coloured films. 
The authors next discuss the bearing of their observations 
upon the question of the magnitude of the so-called ‘‘ radius of 
molecular attraction.”” They point out that if the mere equality 
in the surface-tensions of thick and thin films is to be considered 
conclusive, they have accumulated much stronger evidence for 
the statement that the radius of molecular attraction is less than 
half the thickness of a black film, 7.e. <6 x ro-&'mm. than 
Plateau produced for the assertion that 59 X 10 &mm. is a 
superior limit to its magnitude. They are, however, unwilling 
to draw this conclusion from their experiments until an explana- 
tion is forthcoming in harmony with it, of the apparent dis- 
continuity in the thickness of the film which always (except 
under very special circumstances) occurs at the edge of the 
black. 
They are themselves inclined to look upon the sharp edge of 
the black as evidence of a change in surface-tension due to the 
tenuity of the film, and to regard the result of tbeir experiments 
as fixing asuperior limit (0°5 per cent.) to the difference of the 
tension of the black and coloured parts. 
As no explanation of the discontinuity at the edge of the 
black has (as far as the authors are aware) ever been put for- 
ward, they conclude by a suggestion which, though no doubt of - _ 
a speculative character, may serve to draw attention to a subject 
which is they believe of considerable interest. 
They show that the main facts to be accounted for, viz. the 
discontinuity, the uniform thickness of the black, the wide 
variations in the thickness of the part of the coloured film which 
is in contact with the black, and thé equality in the surface- 
tensions of the black and coloured films, could be explained if it 
were supposed that the surface-tension has a critical value when 
the thickness is somewhat greater than 12 X 107° mm. 
The possibility of the existence of such a critical value has 
been pointed out by Maxwell (Zucyel. Brit., art. “ Capillarity ”). 
It would be explained by the assumption frequently made in 
discussions on the nature of molecular forces, that as the distance 
between two molecules diminishes, the mutual force between 
them is alternatively attractive and repulsive. 
June 10.—‘‘On some New Elements in Gadolinite and 
Samarskite, Detected Spectroscopically.” By William Crookes, 
Deh Woe CRS, 
The recent discovery by my distinguished friend M. de Bois- 
baudran (Comptes rendus, cii., p. 1003, May 3, 1886) of the 
existence of a new element which he calls Dysprosium, makes it 
unadvisable on my part, as a fellow investigator in spectroscopic 
research, to delay any longer the announcement of some of the 
| results I have obtained during the fractionations of the samarskite 
and gadolinite earths. 
PE Oy Oe ie a) OR ge at ey 
