484 Sir William Thomson [Jan. 29, 



a force of attraction between the finger and the drop hanging from 

 it, and between the matter on the two sides of any horizontal plane 

 you like to imagine through the hanging water. These forces are 

 millions of times greater than what you would calculate from the 

 Newtonian law, on the supposition that water is perfectly homo- 

 geneous. Hence either these forces of attraction must, at very small 

 distances, increase enormously more rapidly than according to the 

 Newtonian law, or the substance of water is not homogeneous. We 

 now all know that it is not homogeneous. The Newtonian theory of 

 gravitation is not surer to us now than is the atomic or molecular 

 theory in chemistry and physics ; so far, at all events, as its assertion 

 of heterogeneousness in the minute structure of matter apparently 

 homogeneous to our senses and to our most delicate direct instru- 

 mental tests. Hence, unless we find heterogeneousness and the 

 Newtonian law of attraction incapable of explaining cohesion and 

 capillary attraction, we are not forced to seek the explanation in a 

 deviation from Newton's law of gravitational force. In a little 

 communication to the Eoyal Society of Edinburgh twenty-four years 

 ago,* I showed that heterogeneousness does suffi.ce to account for any 

 force of cohesion, how^ever great, provided only we give sufficiently 

 great density to the molecules in the heterogeneous structure. 



Nothing satisfactory, however, or very interesting mechanically, 

 seems attainable by any attempt to work out this theory without 

 taking into account the molecular motions which we know to be 

 inherent in matter, and to constitute its heat. But so far as the main 

 phenomena of capillary attraction are concerned, it is satisfactory to 

 know that the complete molecular theory could not but lead to the 

 same resultant action in the aggregate as if water and the solids 

 touching it were each utterly homogeneous to infinite minuteness, and 

 were acted on by mutual forces of attraction sufficiently strong 

 between portions of matter which are exceedingly near one another, 

 but utterly insensible between portions of matter at sensible 

 distances. This idea of attraction insensible at sensible distances 

 (whatever molecular view we may learn, or peo23le not now born may 

 learn after us, to account for the innate nature of the action), is indeed 

 the key to the theory of capillary attraction, and it is to Hawksbee f 

 that we owe it. Laj)lace took it up and thoroughly worked it out 

 mathematically in a very admirable manner. One part of the theory 

 which he left defective — the action of a solid upon a liquid, and the 

 mutual action between two liquids — was made dynamically perfect 

 by Gauss, and the finishing touch to the mathematical theory was 

 given by Neumann in stating for liquids the rule corresponding to 

 Gauss's rule for angles of contact between liquids and solids. 



Gauss, expressing enthusiastic appreciation of Laplace's work, 

 adopts the same fundamental assumption of attraction sensible only 



* Proceedings of the Royal Society of Edinburgh, April 21, 1862 (vol, iv.). 

 t Royal Society Transactions, 1709-13. 



