CAUSE OF DIMORPHISM. 95 



It has been also discovered by Rose, that at 

 the temperature of 50, carbonate of lime assumes 

 the form of rhombohedric crystals, as in calc 

 spar; whereas, at the temperature of 150, it 

 takes the cubic form, as in Aragonite, the com- 

 position of which is the same. And when rhombic 

 crystals of calc spar are heated to 212, they are 

 changed to the cubic form, without any change 

 in its chemical composition, if we except the 

 addition of caloric, which is the cause of all 

 molecular aggregations, and therefore the creator 

 of forms. Corresponding with the foregoing facts, 



vapours, liquids, and glass. But in the square prismatic and rhom- 

 bohedric classes, the expansion by caloric is dissimilar in two direc- 

 tions ; while in the right prismatic, the oblique prismatic, and the 

 doubly oblique prismatic, it is dissimilar in three directions. And 

 when a ray of light is made to pass through any of these bodies, or 

 through glass unequally cooled, and solids of unequal density, 

 whether from unequal temperature or pressure, it is refracted in 

 two different pencils, more or less inclined to one another, accord- 

 ing to the molecular arrangement of the body, and the direction in 

 which the pencil is incident ; thus producing all the phenomena 

 of double refraction, which also occurs when it passes through 

 resins, gums, jellies, horn, shells, bones, elastic integuments, 

 and animal lenses. Whenever a ray of light is thus divided, 

 both the transmitted rays are said to be polarized ; that is, 

 its properties are so far changed that it is incapable of under- 

 going reflexion or refraction, except at certain angles ; or of 

 transmission through transparent bodies, except in particular 

 positions. For example, a plate of tourmalin permits the rays to 

 be transmitted in one position, but in a position perpendicular to 

 this it arrests and stifles them, because in these two rectangular 

 directions its particles are differently arranged. It is there- 

 fore manifest that all the phenomena of crystallization, the re- 

 flexion, refraction, double refraction, polarization, and doubtless 

 the diffraction of light, are immediately connected with the theory 

 of caloric. 



