178 Brunonian or Particle Movements. 



among the heaviest bodies with which the chemist is brought in 

 contact. 



Having thus briefly regarded the development of our know- 

 ledge concerning these microscopic movements, let us consider 

 the explanation of them afforded by science. They are due to 

 what is known as surface tension or capillarity. This subject is 

 one of the most abstruse of the departments of physical science. 

 It has ocupied the mind of Newton, of Laplace, and of Clairaut 

 in earlier days, and of Helmholtz, Lord Kelvin, and Professor 

 Clerk Maxwell of more recent days. It is far too vast a subject 

 to be taken up here, and we must refer the hearer to some authori- 

 tative work like the article on the subject in the " Encyclopaedia 

 Britannica " from the pen of your late eminent neighbour, Pro- 

 fessor Clerk Maxwell, one of the clearest intellects produced by 

 the Stewartry, which has been famed for so many illustrious 

 names. 



The branch of capillary science most closely connected with 

 our present topic is that which treats of the tension of films of 

 liquid in contact with liquid, and of liquid in contact with solid. ■ 

 The chief investigator along these lines is the German savant, 

 Quincke, who has been investigating these subjects for over fifty 

 years, and I hope is still pursuing these researches. He forms, 

 like so many of his countrymen, an example of the real meaning 

 of the word thoroughness. His work is largely quoted by Clerk 

 Maxwell in the article to which reference has been made. As an 

 instance of the action of liquid on liquid take the case of the 

 " tears " of wine. Along the glass is spread a film of wine. The 

 alcohol evaporates in places and leaves the film weaker in alcohol 

 and therefore denser. The denser part draws the lighter part 

 into a ring some distance above the surface of the liquid and 

 presently gathers that ring into drops, which roll down the gk.ss 

 and are known to connoisseurs as " tears." 



A somewhat similar action takes place at the meeting place 

 of solid and liquid. If the solid be a gum like gamboge it is 

 slightly soluble in water. A thin film is formed of different 

 density from the water. Movement takes place as in the case 

 just mentioned. And the movement alters the equilibrium of the 

 surrounding layers. And there is further movement. Fresh 



