Crystalline Structures of Rocks. 79 



ticed. It is neither concerned in the disposition of original and 

 similar molecules, nor in arranging them into geometric forms. Yet 

 its phenomena bespeak a tendency in the particles, or finer frag- 

 ments constituting stones, to arrange themselves, by a predomi- 

 nant attraction, into certain forms rather than others; however 

 irregular or uninfluenced by geometrical rules these may be. A 

 simple and obvious instance of this tendency may be seen in the 

 disposition assumed by fine powders or sand under water, where 

 these are therefore free to move. 



That it exists in bodies fluid from fusion, is proved by the ap- 

 pearances that occur in the slow-cooling of liquid basalts artificially 

 fused. Lastly, that it may happen in solid bodies, is proved by 

 the phenomena which take place in heated sandstones. If the 

 improbability of this latter case should be objected, it must be 

 remembered that, in Mr. Watt's experiments, the crystalline change 

 took place in the trap after it had ceased to be fluid ; that the 

 experimentsof Dr. Brewster point out the changes which take place 

 in the crystallization of solid glass from changes of temperature ; 

 and that those of Reaumur prove analogous changes in the same 

 substances at higher temperatures, still short of fusion. In a series 

 of experiments instituted for the same purpose, I have also proved 

 that every metal can change its crystalline arrangement while 

 solid, and many of them at very low temperatures. In fact the 

 power of motion in the particles of solid bodies, is proved by their 

 changes of dimension on alternations of temperature ; and it is not 

 therefore extraordinary that in those which have the properties of 

 crystallizing, a tendency to their peculiar crystalline forms should 

 occur. It is also not surprising if, being thus in motion, they 

 should assume other or less regular forms, as they do from the 

 fluid state. 



We have uo right to assume that the parts of such matter may 

 not have the power, by mutual attraction, of assuming forms that 

 are not geometrical, even though they should be heterogeneous 

 and shapeless ; knowing nothing of the nature and laws of that 

 force by which similar and definite molecules affect geometrical 

 ones. The limit between crystalline and mechanical attraction 



