448 
PROFESSORS A. W. REINOLD AND A. W. RUCKER ON 
of molecular attraction. This conclusion, if correct, would show that Quincke’s 
number is at least ten times too small. 
The values derived from the dynamical theory of gases are, however, much smaller. 
Van der Waals* finds 0'23xl0 -/ centims. to be an inferior limit, but considers 
Quincke’s value too high. 
0. E. Meyer,! on the contrary, thinks that the cause of the discrepancy is to be 
found in the fact that the expression “ radius of molecular attraction ” ( WirTcungssphdre) 
is used in the theory of capillarity in a sense different from that in which it is employed 
in the theory of gases. He believes that, if we possessed a kinetic theory of liquids, 
the magnitude investigated by Quincke and Plateau would be found to be that of 
the free path of the molecules in the liquid, or would acquire some similar meaning. 
Plateau’s experiments consisted in measuring the pressure of the air confined in a 
soap bubble by means of a water manometer. A decrease in that pressure might be 
expected if the thickness of the film became less than twice the radius of molecular 
attraction. His conclusion is based on the fact that no regular change was observed in 
the case of a bubble which lasted for three days, and thinned to the pale yellow of the 
first order of Newton’s scale. 
Ludtge formed a liquid film at one end of a cylindrical tube, and, when it had 
become thin, closed the other end by a newly formed film. A small quantity of air 
was then forced into the enclosed space through an orifice in the side of the tube. The 
films assumed the form of spherical segments, and it was found that that produced 
by the thinner film was the less curved. The inference that the superficial tension 
increased as the thickness of the film diminished, was supported by other experiments, 
for an account of which we must refer to the original paper. We may, however, 
remark that the large magnitude assigned to the radius of molecular attraction, and 
the statement that the superficial tension is increased instead of diminished when the 
thickness of a film is less than twice that magnitude, are opposed to the views of most 
physicists.J The observations both of Plateau and Ludtge are open to the objection 
that there is no proof that the soap solution used remained unaltered during the 
experiments. 
A mixture of glycerine and water will gain or lose water according to the magnitude 
of the tension of the aqueous vapour in its neighbourhood. The superficial tension of 
a soap solution would thus tend to increase in damp, to decrease in dry, air. The direct 
effects of absorption or evaporation might, however, be diminished or even reversed by 
the changes of temperature with which they were accompanied. Plateau, to prevent 
the absorption of moisture, enclosed the bubble in a small glass vessel, at the bottom 
of which were placed some sticks of caustic potash. This arrangement would tend, and 
* ‘ Beibliitter,’ Bd. i., p. 19. 
t ‘ Die Kinetische Tlieorie der Gase,’ 1877, p. 236. 
t See ‘ Statique des Liquides,’ Plateau, tom. i., p. 205, 1873. Also ‘ Tlieorie Mecanique de la Chaleur,’ 
Dupre, p. 358, 1867. 
