G 28 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
thickness of the bubble. His method, # as is well known, consisted in forming a 
bubble at the end of a tube which was bent so as to serve as a water-manometer for 
the measurement of the internal pressure. He concluded that the radius of molecular 
action was less than 59xlO -G mm., because no regular change was observed in 
the case of a bubble which thinned till it displayed a colour which, according to 
Newton’s scale, indicated twice that thickness, viz., the pale yellow of the first order. 
Ludtge t formed a soap film at the end of a tube, and, when it had become thin, 
closed the other end by a new film. Some air was then forced in, the films assumed 
the form of spherical segments, and their curvatures were compared. He concluded 
that the thicker film had the less surface tension, and that almost immediately after 
its formation the thickness of a film was less than twice the radius of molecular 
action. 
These experiments were carefully repeated by Van der Mensbrugghe, J who kept 
one of the films moderately thick by dropping liquid upon it, so that it displayed the 
red and green of the higher orders. He describes the other film as showing bright 
colours, especially towards the top. He could not detect any change in the curvature 
of the films, and concluded that the tensions were the same, though the thicknesses 
were different. Later, Van der Mensbrugghe’s views appear to have undergone 
some modification. He based an explanation of a number of curious phenomena on 
the principle, enunciated by Sir William Thomson, that if a soap film is extended it 
is cooled. He thought, therefore, that “ les experiences de Ludtge s’expliquent de 
la maniere la plus nette pourvu que les augmentations de tension admises par le 
physicien allemand dans certaines parties d’une lame soient attributes au developpe- 
ment de surfaces fraiches sur les deux faces de cette lame, et a la diminution de 
temperature qui en resulted’§ 
If Van der Mensbrugghe’s later views are correct it is evident that his earlier 
test of Ludtge’s observations is open to objection, as, if a film be kept thick by 
dropping liquid upon it, new surfaces must necessarily be produced. Indeed, he seems 
to think that much less potent causes might produce results which can be easily 
detected. 
We describe one of his experiments, to give an example of his method of applying 
Sir William Thomson’s law||:— 
A circular hole about 4 cm. in diameter was bored in an iron plate furnished with 
a handle. The edge of the hole was bevelled, and it was made the common basis of 
two bubbles forming spherical segments which enclosed a lens-shaped space. If the 
plate were placed horizontally, if the weights of the films were negligible, and if 
* Plateau, ‘ Statique des Liquides,’ 1873, vol. 1, p. 210. 
+ 1 Poggendorff, Annalen,’ vol. 139, 1870, p. 620. 
+ ‘ Bruxelles, Acad. Sci. Bull.,’ vol. 30, 1870, p. 322. 
§ ‘ Bruxelles, Acad. Sci. Mem.,’ vol. 43, 1882 (No. 4, p. 18). 
|| Loc. cit., p. 19. 
