210 
NATURE 
[Fuly 2, 1885 
as the products of climate, by A. Woeikof, and a map of Paul 
Acherson’s journey in the Libyan desert, with the accompanying 
descriptive account of the journey. 
A LONG-DELAYED letter from the Bishop of Central Oceania 
gives, Scéence states, details of the honours rendered by the civil and 
religious authorities to the relics of the companions of La Pérouse. 
These last survivors of that unfortunate expedition were massacred 
by the Samoans on the Islet of Tutuila on December 11, 1787. 
Father Vidal, of the mission, had been searching twelve years 
for the remains, which were finally identified in October, 1882. 
The authorities in France, on being notified, caused a beautiful 
mortuary tablet to be prepared, and forwarded to the admiral 
on duty at that station. A monument was erected, upon which 
the tablet was fixed, and a small chapel built near it. The 
whole was dedicated by Bishop Lamaze and Commandant 
Fournier, of the French Navy, with solemn ceremonial and 
minute-guns on the ninety-seventh anniversary of the event. 
LIQUID FILMS* 
HE molecules in the interior of a liquid are surrounded on 
all sides by others which they attract, and by which they 
are themselves attracted, while those on the surface have neigh- 
bours on one side only. In consequence of this difference in 
their surroundings there is in all probability a difference in the 
grouping of the interior and exterior molecules which is attended 
by corresponding variations in the physical properties of the 
liquid of which they are constituent parts. Thus it was shown 
by M. Plateau that the viscosity of the surface of a liquid is in 
general different from that of its interior. The most striking 
example of this phenomenon is afforded by a solution of saponine. 
Two per cent. of this substance dissolved in water does not 
effect any marked change in the properties of the great mass of 
the liquid, but produces a most remarkable increase in the 
surface viscosity, so that forces which suffice to create rapid 
motion in bodies which are completely immersed, fail to produce 
any appreciable movement if they lie in the exterior surface. 
The first attempt to obtain a numerical estimate of the difference 
of the resistances experienced by a body oscillating in turn in 
the interior and in the surface of the liquid was made about two 
years ago by Messrs. Stables and Wilson, students in the York- 
shire College. In the case of a horizontal disc suspended in 
water, the logarithmic decrement diminishes to about one half 
as the surface is approached. In a saponine solution, on the 
other hand, it is 125 times greater in the surface than in the 
interior, and about 38 1imes greater in the surface than at a 
depth of o-1 mm. below it. Even in the latter case the greater 
part of the resistance is due, not to the friction between the 
disc and the liquid, but to that experienced by the supporting 
rod in the surface, so that in ail probability the surface viscosity 
is more than 600 times greater than that of the mass of the 
liquid. 
The immense change in the resistanceswhich takes place when 
the disc is immersed to a depth of 0-1 mm. only confirms the 
general opinion that any peculiarity of grouping or arrangement 
due to proximity to the surface extends to a very small depth. 
A liquid must thus be conceived as surrounded by a yery thin 
layer or skin, the properties of which are different from that of 
the liquid in the interior, and to which rather than to any ideal 
geometrical boundary the term ‘‘surface” might be applied. It 
may, however, prevent confusion if it is called the serfuce-layer. 
Many attempts have been made to measure the thickness of 
the surface-layer. In particular, M. Plateau studied a thinning 
soap film with a view of determining whether or no the pressure 
exerted on the enclosed air by the film when very thin is the 
same as when it is comparatively thick. Had any such difference 
been observed it might have been taken as prima facie 
evidence that the tenuity was so great that all the interior 
portions of the film had drained away, and that the thickness 
did not exceed that of the two surface-layers. 
This experiment has been criticised by Prof. Reinold and 
myself, but it is not intended in this lecture to enter upon the 
general question of the thickness of the surface-layer, or the 
interesting theoretical problems which are closely connected 
with it, as we are at present engaged in an investigation which 
we hope may throw further light upon the subject. There are, 
however, two preliminary questions on which we have arrived 
at definite conclusions. 
* Lecture at the Royal Institution by Prof. A. W. Riicker, M.A., F.R.S. 
In any experiments which have for their object the detection 
of small changes in the properties of a soap film as it becomes 
thinner, it is essential that we should be able to assert with 
certainty that no causes other than the increasing tenuity have 
been in play, by which the effect looked for might either be 
produced or masked. Changes in the temperature or composi- 
tion of the film must especially be prevented. 
The liquid ordinarily employed for such investigations is the 
“liquide glycérique” of M. Plateau. In dry air some of the 
water of which it is in part composed would evaporate, while in 
moist air, in consequence of the hygroscopic properties of the 
glycerine, additional water would be absorbed. ‘Though these 
facts were well known, and though they are evidently possible 
sources of error, no attempt (as far as [am aware) had been 
made before our own to determine what precautions it was 
necessary to take to prevent the results of experiments such as 
M. Plateau’s being affected by them. The first question then 
that we set ourselves to answer, was—to what extent is the 
composition of a soap film altered by changes in the temperature 
or hygroscopic state of the air which surrounds it? 
The method adopted in answering this inquiry was to measure 
the electrical resistance of soap films formed in an inclosed 
space containing a thermometer and hair hygrometer. If the 
observations led to the conclusion that the resistance of film 
varied inversely as its thickness, they would prove that no change 
in composition had taken place, and that the film at the thinnest 
had afforded no evidence of an approach to a thickness equal to 
that of the surface-layers. If the specific resistance was found 
to vary according to some regular law as the thickness altered, 
there would be a strong presumption that the thickness was not 
much greater than, and was possibly even less than that of the 
two surface-layers. If, lastly, the changes were irregular, they 
might safely be accribed to alterations in temperature or 
constitution, 
To obtain the desired facts it was necessary (1) to devise a 
method of forming the films in a closed chamber, (2) to measure 
their thickness, and (3) to determine their electrical resistance. 
The films were formed in a glass box at the lower extremity of 
a platinum ring which communicated by means of a tube with 
the outside. In the earlier experiments a cup of the liquid was 
raised by rackwork to the ring and then withdrawn, leaving a 
film behind it. The latter was blown out by air which had been 
dried and passed through tubes containing “‘liqnide glycérique.” 
When large enough it adhered to a second platinum ring placed 
vertically below the first, and on some of the air being with- 
drawn it assumed the cylindrical form. / 
The thickness was measured by means of the colours dis- 
played, two independent determinations being obtained by two 
beams of light incident at different angles. Newton’s Table of 
Colours was revised, and it was found that the differences 
between the thicknesses given by him and those determined by 
new experiment were far greater than the error of experiment of 
a single observer. Hence, if accurate measurements are required 
by means of Newton’s scale, every experimenter must reconstruct 
thot scale for himself. 
At first the electrical resistance was determined by means of 
Wheatstone’s bridge. The edges of the film where it is close 
to its solid supports are often, however, the seat of phenomena 
which might affect the results. Thin rings of white or black 
appear which alter the resistance considerably, and which intro- 
duce errors for which it is almost impossible to make any 
accurate allowance. This fact, combined with the advantage of 
avoiding errors due to polarisation, and of being able to select 
any particular part of the film for examination instead of the 
whole, led us to adopt a different method. Gold wires attached 
toa movable support were thrust into the film, and the differ- 
ence of potential between these when a current was ‘passing 
through the film was compared with that between the extremities 
of a known resistance included in the same circuit. } 
The result of these observations was to prove that the specific 
resistance of the films altered in an irregular manner, varying 
between 200 and 137 ohms per cubic c.m, A closer inspection 
showed that abnormal results were always accompanied by 
abnormal variations in the thermometer or hygrometer. When 
those films were selected which had been observed when such 
variations were especially small, it was found that the range of 
variation of the specific resistances was only between 137 and 
146, and that the mean yalue was 143, that of the liquid in mass 
being 140°5 (at the same temperature). It was also proved that 
between thicknesses varying from 1370 to 374 millionths of a 
