XII 



ON CRYSTALLINE AND SLATY CLEAVAGE 1 



WHEN the student of physical science has to in- 

 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 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 downward 

 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 perfect 



1 From a discourse delivered in the Royal Institution of Great Britain, 

 June 6, 1856. 



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