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IV. Compa7'ative View of the Cleavage of Crystals and Slate 

 Rocks. By John Tyndall, Esq., F.R.S. ^c* 



WHEN the student of physical science has to investigate 

 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 water would shape itself, supposing it to be at liberty to follow 

 the bent of its own molecular forces, he must see that these 

 forces have free aad 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, withdraw the liquid mass from the action 

 of gravity, and he would then find the shape of the mass to be 

 perfectly spherical. 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 : though each individual instrument might be a 

 wellspring of melody, still the mixture of all would produce mere 

 noise. Thus it is with the processes of nature. In nature 

 mechanical and molecular laws mingle and create apparent con- 

 fusion. Their mixture constitutes what may be called the noise 

 of natural laws, and it is the vocation of the man of science to 

 resolve this noise into its components, and thus to detect the 

 " music '' in which the foundations of nature are laid. 



The necessity of this detachment of one force from all other 

 forces is nowhere more strikingly exhibited than in the phseno- 

 mena of crystallization. I have here a solution of sulphate of 

 soda. Prolonging the mental vision beyond the boundaries of 

 sense, we see the atoms of that liquid, like squadrons under the 

 eye of an experienced general, arranging themselves into batta- 

 lions, gathering round a central standard, and forming themselves 

 into solid masses, which after a time assume the visible shape of 

 the crystal which I here hold in my hand. I may, like an ignorant 

 meddler wishing to hasten matters, introduce confusion into this 

 order. I do so by plunging this glass rod into the vessel ; the 

 consequent action is not the pure expression of the crystalline 

 forces ; the atoms rush together with the confusion of an unor- 



* A Discourse delivered at the Royal Institution on the evening of 

 Friday the 6th of June, 1856, and printed in the * Proceedings ' of the 

 Institution. 



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