190 ON THE CONCRETIONARY AND 



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 

 free to move. That it exists in bodies fluid from 

 fusion, is proved by the appearances 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 p^ace in heated sandstones, in 

 trap after it has ceased to be fluid, and in solid glass ; 

 which undergoes a change of internal crystallization 

 from changes of temperature, and even effloresces, as 

 in the achromatic object-glasses of telescopes. In a 

 series of experiments instituted for the same purpose, 

 I have also proved that every metal can completely 

 change its crystalline arrangements 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 alterations 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 and less regular forms, as they 

 do from the fluid state. 



We have no right to assume that the parts of such 

 matter may not have the power, by mutual attraction, 

 of assuming forms which 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 geo- 

 metrical forms. The limit between crystalline and 

 mechanical attraction may be undefined, and so may 

 the resulting forms. Thus the concretionary structure 

 may bear a real analogy to crystallization, or it may 

 even be supposed a modification of that process. We 



