^ 



238 



MINEllALOG Y. 



[Sect. VIII. 



Althou 



o 



under wliich they may be formed, it is found that many 

 substances will bear an admixture, sometimes consider- 

 able, of other substances, without having their power to 

 crystallize in certain fundamental forms prevented, the 

 particles of the one substance, as it were, compelling 

 those of the other to adjust themselves in a subordinate 

 manner to them. This is more especially the case where 

 the admixture is clearly mechanical, as regards the sub- 

 stance so compelled to adjust its particles that they do 

 not interfere with the fundamental or primary form of 

 crystal of the more powerful body. Of this one of the 

 most marked is the well-known crystallized sandstone, as 

 it is often termed, of Fontaincbleau, where grains of sili- 



ceous sand are gathered up by crystals of carbonate of 



lime, and do not injure the form of the latter. It is not 

 im.probable that the matter found in many minerals, and 

 considered extraneous, may often thus be mechanically 

 mixed. However this may be, the proportions in which 

 one substance may be sometimes mingled with another, 

 the crystallization obtained being that proper to the minor 

 quantity (as if though less in that respect it was greater 



in crystalline power) 



are very remarkable. 



M 



Beudaut was enabled to produce crystals of the form of 

 sulphate of iron which contained 85 per cent, of sulphate 

 of zinc, the remainder only (15 per cent.) being composed 

 of the substance which gave the form to the crystals.* 



With regard to dimorphism, or the crystallization of 

 the same chemically composed substance in two distinct 

 forms, considered fundamentally different, we have 



* Beudaut, ' Annales des Mines ' (1817), tome ii . p. 10. 



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