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in cases of alloys. Silver coins, after having lain long in the earth, 

 have been found covered with a salt of copper. This may be ex- 

 plained by supposing that the alloy of copper, at the surface of the 

 coin, enters into combination with the carbonic acid of the soil, and 

 being thus removed, its place is supplied by a diffusion from within; 

 and in this way it is not improbable that a considerable portion of the 

 alloy may be exhausted in the process of time, and the purity of the 

 coin be considerably increased. 



Perhaps, also, the phenomenon of what is called segregation, or 

 the formation of nodules of flint in masses of carbonated lime, and of 

 indurated marl in beds of clay, may be explained on the same prin- 

 ciple. In breaking up these masses, it is almost always observed, 

 that a piece of shell or some extraneous matter occupies the middle, 

 and probably formed the nucleus, around which the matter was accu- 

 mulated by attraction. The difficulty consists in explaining how the 

 attraction of cohesion, which becomes insensible at sensible distances, 

 should produce this effect. To explain this, let us suppose two sub- 

 stances uniformly diffused through each other by a slight mutual at- 

 traction, as in the case of a lump of sugar dissolved in a large quan- 

 tity of water, every particle of the water will attract to itself its pro- 

 portion of the sugar, and the whole will be in a state of equilibrium. 

 If the diffusion at its commencement had been assisted by heat, and 

 this cause of the separation of the homogeneous particles no longer 

 existed, the diffusion might be one of unstable equilibrium ; and the 

 slightest extraneous force, such as the attraction of a minute piece of 

 shell, might serve to disturb the quiescence, and draw to itself the 

 diffused particles which were immediately contiguous to it. This 

 would leave a vacuum of the atoms around the attracting mass: for 

 example, as in the case of the sugar, there would be a portion of the 

 water around the nucleus deprived of the sugar; this portion of the 

 water would attract its portion of sugar from the layer without, and 

 into this layer the sugar from the layer next without would be dif- 

 fused, and so on until, through all the water, the remaining sugar 

 would be uniformly diffused. The process would continue to be re- 

 peated, by the nucleus again attracting a portion of the sugar from 

 the water immediately around it, and so on until a considerable accu- 

 mulation would be formed around the foreign substance. 



We can in this way conceive of the manner by which the molecu- 

 lar action, which is insensible at perceptible distances, may produce 

 results which would appear to be the effect of attraction acting at a 

 distance. 



