4 POPULAR LECTURES AND ADDRESSES. 



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 instrumental tests. 

 Hence, unless we find heterogeneousness and the 

 Newtonian law of attraction incapable of explain- 

 ing cohesion and capillary attraction, we are not 

 forced to seek the explanation in a deviation from 

 Newton's law of gravitational force. In a com- 

 munication to the Royal Society of Edinburgh 

 twenty-four years ago, 1 I showed that heterogene- 

 ousness does suffice to account for' any force of 

 cohesion, however 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 



1 "Note on Gravity and Cohesion," Proceedings of the Royal 

 Society of Edinburgh, April '21, 1862 (vol. iv.). This paper is 

 reprinted in full as Appendix B to the present article. 



