380 Miscellaneous Intelligence, 



other substances. It is equally known, from experiment, that the 

 surface of the same solid body susceptible of being moistened in 

 succession by different liquids is, in similar circumstances, vari- 

 ously wetted, according to the nature of the liquid : thus, at the 

 same temperature, a plate of glass wetted with alcohol will retain 

 a thicker layer of the fluid on its surface than it would have done 

 if wetted with water. 



This property, possessed by solid bodies of retaining on their sur- 

 faces a stratum more or less thick, according to the fluid by which 

 they are wetted, is particularly evident when the solid is reduced 

 to the state of fine powder, and is diffused through the liquid in such 

 quantity that, by their vicinity, the atmospheres adhering to them 

 mutually penetrate each other. Experiment then shews that 

 through the intervention of these atmospheres, the particles tend 

 to approach each other with a force which is greater, as the parti- 

 cles are nearer to each other. Hence mutual actions and re-actions, 

 which transmitted to the interposed fluid, subject it to new forces 

 and pressures, of which the intensity may be appreciated by means 

 of the hydrometer. 



I have shewn, in a preceding memoir*, that the degrees of this 

 instrument then differed, and should differ, from those indicating 

 merely the specific gravity of the mixture of fluid and solid matter. 

 In the latter case, the specific gravity, indicated by the instrument, 

 expresses merely the force with which eacd molecule of the mix- 

 ture considered as homogeneous gravitates towards the centre of 

 the earth ; whilst, in the first case, the instrument not only indi- 

 cates the specific gravity of the liquid interposed between the 

 solid particles, but also the force with which the liquid gravitates 

 on these molecules. 



Although the experiments by which these facts have been de- 

 termined leave no doubt, yet their importance, and the importance 

 of their consequences is such as to require, if possible, other proof ; 

 I have, therefore, endeavoured to render the attraction exerted 

 through the intervention of a liquid sensible upon large surfaces, 

 and, if possible, to measure rigorously the intensity of this action 

 with reference to the distance at which it is exerted. 



If two plane surfaces be imagined, suspended vertically in a 

 liquid by which they are wetted, the stratum of liquid adhering to 

 them will form on each a kind of fixed envelope. If, then, the 

 surfaces be approximated to each other until the distance between 

 them is so small as to allow of their liquid envelopes penetrating 

 each other, these surfaces should mutually attract each other with 

 a force increasing as the distance is diminished. To render sen- 

 sible, and appreciate the effect of this force, conceive that moving 

 the moistened surfaces from the perpendicular position in which 



* On Liquid Atmospheres, and their influence on the solid particles they 

 envelope. — Mem. tie V Acadtmie Royale, iv. 1819 and 1820. 



