122 DIRECT PROPERTIES OF MOLECULES 339 



metre. He starts with the assumption that we may consider 

 the molecules as small drops which result from larger ones 

 that are broken up by collisions with each other and with the 

 sides of the containing vessel. Such a division of a drop into 

 two smaller ones is possible only when the kinetic energy of 

 the collision is greater than the work which must be spent 

 in overcoming the forces of cohesion and producing the in- 

 crease of the capillary surface that results from the division. 

 Since both the kinetic energy of the molecules and the 

 intensity of the capillary pressure are known, the limit at 

 which divisibility ceases can be determined. Houlle- 

 vigue 1 and H. A. Wilson 2 tried to find the same limit by 

 determining the conditions under which the latent heat of 

 vaporisation and the capillary pressure of a drop of water 

 equilibrate each other ; and the results of both give 

 O13 millionth of a millimetre as the smallest diameter 

 which a drop of water can have. 



Phenomena can be adduced from other branches of 

 physics, such as optics, for example, which may serve to 

 give a determination of the size of the smallest particles. 

 Our first idea would be to get an estimate of it from the 

 size of the smallest objects that are visible microscopically ; 

 but the limit of visibility is, unfortunately, reached far too 

 soon in consequence of the diffraction of light in the micro- 

 scope, so that only a 4,000th part of a millimetre is recog- 

 nisable. 3 The dispersion of light can, on the contrary, be 

 utilised for an estimation in this respect, as it is caused by 

 the action of the material particles on light ; and Lord 

 Kelvin 4 has so employed it. It is sufficient to mention 

 here that the simple laws of dispersion in transparent media 

 cannot be correct if only a few particles are to be found in 

 the length of a wave of light. If there are many, the 

 distance separating two neighbouring particles must be 

 much smaller than the length of a wave of light, and 



1 Journ. de Phys. 1896 [3] v. p. 159. 



2 Chem. News, 1896, Ixxiii. p. 63. 



3 Helmholtz, Pogg. Ann. 1884, Jubelband, p. 557. 



4 Nature, 1870, i. p. 551. 



z 2 



