414 TEE INSTABILITY OF THE HOMOGENEOUS. 



parts. Usually the heterogeneity produced by the action 

 of chemical forces on the surfaces of masses, is not strik- 

 ing; because the changed portions are soon washed away, 

 or otherwise removed. But if this is prevented, compara- 

 tively complex structures result. Quarries of trap-rock 

 contain some striking examples. Xot unfrequently a piece 

 of trap may be found reduced, by the action of the weather, 

 to a number of loosely-adherent coats, like those of an 

 onion. Where the block has been quite undisturbed, we 

 may trace the whole series of these, from the angular, 

 irregular outer one, through successively included ones in 

 which the shape becomes gradually rounded, ending 

 finally in a spherical nucleus. On comparing the original 

 mass of stone with its group of concentric coats, each of 

 which differs from the rest in form, and probably in the state 

 of decomposition at which it has arrived, we get a marked 

 illustration of the multiformity to which, in lapse of time, 

 a uniform body may be brought by external chemical 

 action. The instability of the homogeneous is equal- 



ly seen in the changes set up throughout the interior of a 

 mass, when it consists of units that are not rigidly bound to- 

 gether. The atoms of a precipitate never remains separate, 

 and equably distributed through the fluid in which they 

 make their appearance. They aggregate either into crystal- 

 line grains, each containing an immense number of atoms, 

 or they aggregate into flocculi, each containing a yet larger 

 number; and where the mass of fluid is great, and the process 

 prolonged, these flocculi do not continue equi-distant, but 

 break up into groups. That is to say, there is a destruc- 

 tion of the balance at first subsisting among the diffused par- 

 ticles, and also of the balance at first subsisting among the 

 groups into which these particles unite. Certain solutions 



of non-crystalline substances in highly volatile liquids, exhib- 

 it in the course of half an hour a whole series of changes that 

 are set up in the alleged way. If for example a little shell- 

 lac-varnish (made by dissolving shell-lac in coal-naphtha 



