Professor Reinold and Sir Arthur Ruecker have shown that this zone is no longer exactly spherical; and from this we must conclude that the superficial tension, constant for all thicknesses above a certain limit, commences to vary when the thickness falls below a critical value, which these authors estimate, on optical grounds, at about fifty millionths of a millimetre.
From experiments on capillarity, Prof.Quincke has obtained similar results with regard to layers of solids.

But it is not only capillary properties which allow this characteristic to be revealed.

All the properties of a body are modified when taken in small mass; M.Meslin proves this in a very ingenious way as regards optical properties, and Mr Vincent in respect of electric conductivity.

M.Houllevigue, who, in a chapter of his excellent work, _Du Laboratoire a l'Usine_, has very clearly set forth the most interesting considerations on atomic hypotheses, has recently demonstrated that copper and silver cease to combine with iodine as soon as they are present in a thickness of less than thirty millionths of a millimetre.

It is this same dimension likewise that is possessed, according to M.Wiener, by the smallest thicknesses it is possible to deposit on glass.