304: FRAGMENTS OF SCIENCE. 



XII. 



ON CRYSTALLINE AND SLATY CLEAVAGE* 



WHEN the student of physical science has to investi- 

 gate 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 question ; 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 

 miud 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 perfect 

 music, still the mixture of all would produce mere noise. 



1 From a discourse delivered in the Royal Institution of Great 

 Britain, June 6, 1856. 



