392 Scientific Intelligence — Geology. 



earth, have been found covered with a salt of copper. This may be 

 explained 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 

 principle. In breaking up these masses, it is almost always ob- 

 served that a piece of shell or some extraneous matter occupies the 

 middle, and probably formed the rmcleus, around which the matter 

 was accumulated by attraction. The difficulty consists in explain- 

 ing how the attraction of cohesion, which becomes insensible at sen- 

 sible distances, should produce this effect. To explain this, let us 

 suppose two substances uniformly diffused through each other by a 

 slight mutual attraction, as in the case of a lump of sugar dissolved 

 in a large quantity of water, every particle of the water will attract 

 to itself its proportion of the sugar, and the whole will be in a state 

 of equilibrium. If the diffusion at its commencement had been as- 

 sisted 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 attrac- 

 tion of a minute piece of shell, might serve to disturb the quiescence, 

 and to 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 diffused, and so on, until, through all the 

 water, the remaining sugar would be uniformly diffused. The pro- 

 cess would continue to be repeated, by the nucleus, again, attracting 

 a portion of the sugar from the water immediately around it, and so 

 on, until a considerable accumulation would be formed around the 

 foreign substance. We can in this way conceive of the manner by 

 which the molecular action, which is insensible at perceptible dis- 

 tances, may produce results which would appear to be the effect of 

 attraction acting at a distance. — Proceedings of the American Phil. 

 Society, vol. iv., No. 33, p. 176. 



35. Nevj Map of the Island of Sardinia. — General della Marmora, 

 Director of the Naval School of Genoa, and Member of the Royal 

 Academy of Turin, has just published his new map of the Island 

 of Sardinia, which has been engraved by Desbuissons. It is the 

 result of 20 years' labour, and has been executed at the expense of 

 the author. 



