Note on the Experiments of Moser. 39 



time, it will also be seen, that the portions which had been 

 covered by the diaphragm condense a more opake layer of 

 vapour and apparently thicker than that which covers the 

 other portions on which the vapour was condensed in the first 

 experiment. It does not appear difficult to explain this fact 

 by the general principles of adhesion. A drop of water placed 

 on a surface of glass or upon any other substance, however 

 polished it may be, always preserves a more or less globular 

 form, according to the mass of the drop and the cleanness of 

 the surface itself. Dutrochet has lately proved, that in order 

 for a drop of water to spread itself in a thin layer on a glass 

 plate, it is necessary to give this plate a fresh surface which 

 has not yet been subjected to the contact of atmospheric air. 

 Upon every other surface the drop remains globular, and if 

 the layer of condensed vapour which is obtained by breathing 

 upon a polished surface be examined with a lens, we see that 

 this layer is composed of a quantity of infinitely small globules 

 of water, which differ only by their size from the globules of 

 water that are obtained in letting fall this liquid on a surface 

 covered with dust or oil. It is natural to admit that when 

 some portions of a surface have been covered with these glo- 

 bules, which by their union form the condensed vapour, they 

 have preserved, and continue for a long time to preserve, a 

 thin layer of moisture, a layer of globules smaller than those 

 which had been formed at the beginning of the condensation. 

 No physicist is ignorant of the tenacity with which these layers 

 of water, or rather these veils of moisture, adhere to the sur- 

 face of solid bodies, and how difficult it is, even with the aid 

 of heat, to get rid of them entirely. 



If, when in the state we have just described, we present to 

 this surface a fresh vapour to condense, the points which have 

 preserved a more evident humid veil exert upon the globules 

 of the fresh vapour which is condensed a force of adhesion 

 which will be manifested by effects different from those which 

 will take place on the other points. At the points where the 

 humid layer will be most conspicuous the adhesion will take 

 place between two aqueous layers, or at least between an 

 aqueous layer and some points more covered with water than 

 the others are. The globular state will then be less developed, 

 and the liquid will have a tendency to form a smooth layer 

 rather than globules ; a greater transparency on these parts 

 will be the result of this, and, by analogy with the property 

 which water in a globular form possesses when in contact with 

 hot metals, a quicker evaporation on the points where the 

 globules are less well formed. 



Let us mention another experiment made by M. Moser, 



