Mixture^ and its Effects. 99 



attraction for the fixed salt than the alkali ; but 

 though I take great pains to mix them, they will 

 not act; but when dissolved they immediately 

 act on one another. 



Second. Salt of amber mixed with potass 

 while kept dry will continue separate, and the 

 brown and white particles may be distinguished 

 by a microscope ; but if water is thrown on them, 

 they immediately act upon each other. 



The reason why fluids promote their action 

 upon one another seems to be, that they 

 bring their particles into closer contact than it 

 was possible to bring them by any other means. 

 The surfaces of solids are rough, and there 

 seems to be an atmosphere over them which 

 prevents them from uniting. Sir Isaac Newton 

 calls it a tenacious atmosphere, and extremely 

 subtile, upon which depends a variety of elec- 

 trical facts. When a solid is applied to another 

 solid, the particles of each are united by the at- 

 traction of cohesion ; therefore they will not act 

 upon each other, but they will act readily when 

 this attraction of cohesion is removed by solution. 

 But whatever the cause, the law is founded in 

 experience. The necessary fluidity is obtained 

 by dissolving the body in a fluid that produces 

 no change in its qualities : thus, salt is dissolved 

 in water ; or by melting it, as metals ; or by 

 converting it into vapour, as in bodies which 

 assume the form of vapour before they become 



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