Relationship between Molecular Cohesion 491 



uncontrolled; but in liquids it is overcome by a cohesive force, 

 while the particles still retain a power of moving freely in all 

 directions; and in solids the same cohesion is accompanied 

 by a stronger or weaker resistance to all lateral motion, which 

 is perfectly independent of the cohesive force and which must 

 be cautiously distinguished from it." "It. is sufficient to 

 suppose the force of cohesion nearly or perfectly constant in its 

 magnitude throughout the minute distance to which it extends, 

 and owing its apparent diversity to the contrary action of 

 the repulsive force, which varies with the distance. Now, 

 in the internal parts of a liquid, these forces hold each other 

 in a perfect equilibrium, the particles being brought so near 

 that the repulsion becomes precisely equal to the cohesive 

 force that urges them together," etc. 



Young thus assumed that the cohesion extended but 

 a short distance, with slight variation in intensity and that 

 it then ended abruptly. So far as I can find, he made no 

 suggestion how it came to end abruptly; but if it be assumed 

 that it does not penetrate matter, it is seen that it must end 

 abruptly at the next layer of molecules. Young tried to esti- 

 mate how far the cohesive force really extended, and found a 

 value surprisingly near the order of magnitude of that now 

 known to be the distance apart of the centers of two molecules. 

 His reasoning on this point is extremely ingenious, and is 

 of interest as the first estimate of molecular dimensions. 



Lord Rayleigh 1 says anent this computation of Young's: 

 "One of the most remarkable features of Young's treatise is 

 his estimate of the range "a" of the attractive force on the 

 basis of the relation T = Y 3 aK. Never once have I seen it 

 alluded to, and it is, I believe, generally supposed that the 

 first attempt of this kind is not more than twenty years old. 

 It detracts nothing from the merit of this wonderful specu- 

 lation that a more precise calculation does not verify the 

 numerical coefficient in Young's equation. The point is 



1 Rayleigh: "On the Theory of Surface Forces," Phil. Mag., [5] 30, 285-298, 

 456-475 (1890). Collected papers, 3i 396. 



