Joseph Plateau does many experiments about oil-equilibrium shapes in the water-alcohol mixture. They are rather involved and later he reproduces them with thin soap films. The same experiments are then a lot easier and lead to the same results. Thin soap films are so light that in fact they "are not subjected to gravity" and take equilibrium shapes, analogous to those of oil in a water-alcohol mixture. A typical example is the soap bubble, which has a spherical shape just like the oil sphere in the Plateau experiment.

To do his experiments, Plateau uses a solution he calls "liquide glycérique". It is François Donny (1822-1896) who suggests the composition. Some experimenting leads to the following composition: 3 parts of a watery filtered solution of Marseille soap and 2 parts of pure glycerine. Plateau mentions that it is not always simple to obtain the products in the necessary purity and concentration. When a figure made of thin wire is briefly submerged in the liquid, thin soap films form between the wires.

Wire figures for the study of soap films
Photo MD.
Museum for the History of Sciences, Ghent

For his research concerning the laws governing the formation of laminar soap films Plateau has some 80 wire figures made. Among them are several that give lovely shapes. After submersion in "liquide glycérique", the rightmost figure shows a shape reminiscent of the calyx of a tulip


Stereo pictures of laminar soap films formed on the wire shapes by Plateau

Photo A.L. Neyt, Ghent, about 1880. Collection J.Plateau, Ghent

Plateau finds the following laws:
1. each wire edge supports one film
2. at a liquid edge no more than 3 films can come together; they then form angles of 120°
3. the liquid edges that come together in one point are always in the number of 4 and form angles of 109°28'

The laminar surfaces that are formed in these experiments hardly differ from what Plateau obtained with oil masses in a water-alcohol mixture. Moreover, due to interference in the incident light, the soap films show all the colours. From this Plateau succeeds in calculating the thickness of the films, and from that the action radius of the molecular forces that hold together the films.