XII. 



ON CRYSTALLINE AND SLATY CLEAVAGE* 



"TTTTEEN the student of physical science has to in- 

 VV vestigate the character of any natural force, 

 his first care must be to purify it from the mixture of 

 other forces, and thus study its simple action. If, for 

 example, he wishes to know how a mass of liquid would 

 shape itself if at liberty to follow the bent of its own 

 molecular forces, he must see that these forces have 

 free and undisturbed exercise. We might perhaps 

 refer him to the dew-drop for a solution of the ques- 

 tion; but here we have to do, not only with the action 

 of the molecules of the liquid upon each other, but 

 also with the action of gravity upon the mass, which 

 pulls the drop downwards and elongates it. If he 

 would examine the problem in its purity, he must do as 

 Plateau has done, detach the liquid mass from the 

 action of gravity; he would then find the shape to be a 

 perfect sphere. Natural processes come to us in a 

 mixed manner, and to the uninstructed mind are a 

 mass of unintelligible confusion. Suppose half-a- 

 dozen of the best musical performers to be placed in 

 the same room, each playing his own instrument to 

 perfection, but no two playing the same tune; though 

 each individual instrument might be a source of per- 

 fect music, still the mixture of all would produce mere 

 noise. Thus it is with the processes of nature, where 

 mechanical and molecular laws intermingle and create 



* From a discourse delivered in the Royal Institution of 

 Great Britain, June 6, 1856. 



