44 STRUCTURE OF MINERALS. 



DIMORPHISM. POLYMORPHISM. 



It was formerly supposed that the same chemical com* 

 pound could have but a single mode of crystallization. But 

 later researches have discovered that there are many in- 

 stances of substances crystallizing according to two distinct 

 systems. Thus sulphur at different times crystallizes in ob- 

 lique prisms and right rhombic octahedrons, or according to 

 the two systems monoclinic and trimetric. Carbonate of 

 lime at one time takes on the rhombohedral form, and is 

 then called calc spar ; at another, that of a rhombic prism, 

 and it is then termed aragonite. Again, sulphuret of iron 

 presents us both with cubical (monometric) crystals and 

 rhombic prisms (trimetric.) As far as investigation has gone, 

 it has appeared that one of these forms is assumed at a lower 

 temperature than the other ; and this takes place uniformly, 

 so that the temperature attending solidification, in certain 

 cases at least, determines the forms and system of crystal- 

 lization. How far other causes operate is unknown. 



This property is termed dimorphism, (from the Greek dis, 

 two or twice, and morphe, form,) and a substance presenting 

 two systems of crystallization is said to be dimorphous. In 

 addition to the above, garnet and idocrase, the one dodeca- 

 hedral, and the other square-prismatic, are different forms 

 of the same substance. Rutile, which is dimetric, anatase, 

 dimetric also, but of different dimensions, and Brookite, which 

 is trimetric, are three distinct forms of the same substance, 

 oxyd of titanium. In this last case, the property has been 

 called trimorphism, (from the Greek tris, three times, and 

 morphe, form.) As the number of forms may be still greater, 

 the more general term polymorphism (polus, many, and 

 morphe) has been introduced to include all cases, whatever 

 the number of forms assumed. 



A polymorphous substance in its different states presents 

 not merely difference of form. There is also a difference in 

 hardness, specific gravity and luster, in fact, in nearly all 

 physical qualities. Aragonite has the specific gravity 2*93, 

 and calc spar only 2*7 ; the hardness of aragonite is 3f , 

 and that of calc spar but 3. 



May the same substance crystallize under more than one fundamen 

 tal form 1 Mention examples. What is this property called ? Wha* 

 is said of oxyd of Titanium ? What is trimorphism? polymorphism 

 What other differences beside that of form are connected with poly 

 morphism 1 



