ITS PHYSICAL THEORY. 37 



of the surface of the plane heing always opposite to that of the liquid. They must, 

 therefore, attract each other. I express here only a fact, not involving any disputed hy- 

 pothesis whatever, as to whether that development of electricity originates in the mere 

 contact of the bodies, their chemical action, or any other cause; but it is a fact, that 

 when any solid reposes upon any fluid, provided its surface does not become wetted, a 

 development of electricity uniformly takes place, and a powerful degree of attraction 

 must necessarily ensue. 



118. The postulate here introduced requires explanation, for electric excitement is not 

 observed if the solid surface is wetted. Solids bear a peculiar relation to liquids, being 

 wetted or not wetted by them. Most solids, for instance, are wetted by water, and but 

 few by mercury ; the surface of the glass is readily moistened by alcohol or oil, but not 

 by melted sulphur or mercury ; hence the latter, from its not adhering to the skin, was 

 called by the older chemists aqua non madefaciens manus. The circumstance that no 

 electrical excitement is observed when a solid surface is wet, might appear, at first sight, 

 contradictory to the hypothesis here assumed. A more accurate examination, however, 

 places it in a very different light, and shows that the phenomena observed are exactly 

 such as they ought to be hypotheticallv. If a disk of glass is placed on the surface of the 

 water and then removed, the gold leaves of the annexed electroscope are not affected, for, 

 strictly speaking, no rupture has taken place between the solid and the fluid ; the thin 

 film of the latter in contact with the former still remains so : it is only the cohesion of 

 the watery particles that is overcome, not the adhesion of the solid to the fluid, and 

 hence no electrical development appears. 



119. Geometers have shown the exact relation a solid must bear to a fluid to be wet- 

 ted by it. It results from the mathematical investigations of CLAIRAUT, that if the at- 

 traction of the particles of the solid for those of the fluid is more than half the attrac- 

 tion of these last for each other, the solid will be wetted ; but if it be less than half, the 

 solid will not be wetted. An experimental proof of this may be obtained by counter- 

 poising a disk of glass, at the end of one of the arms of a balance, by weights in the 

 scale, and then lowering it on the surface of some mercury in a cup ; it will be found 

 that a certain weight must be added in the scale to detach it. Next, in place of the 

 disk of glass, substitute a plate of amalgamated copper, of the same size and weight ) 

 and ascertain the force required to detach it ; this will uniformly be found more than 

 double the former weight. The first weight expressed the attractive force existing be- 

 tween a surface of glass and mercury ; the second, the cohesion of a cylinder of mer- 

 cury of the same diameter, and the numbers obtained experimentally corroborate the 

 investigations of Clairaut. 



120. I dwell on this part of the phenomenon because it is of no small importance ; 

 the same conditions that determine whether or not the surface of a solid is to be 

 wetted, determine also whether a liquid shall pass through a pore, and move forward 

 in a capillary vessel. , 



121. The difficulty arising from the non-development of electricity, where the solid 

 surface is wetted, being thus dismissed, we next inquire whether the hypothesis here 

 assumed will give numerical results analogous to those procured by experiment ; in 



