96 CAUSE OF DIMORPHISM. 



Mitscherlich has shewn that the mutual inclina- 

 tion of the crystalline planes in calc spar is 

 altered eight and a half minutes of a degree be- 

 tween the temperature of 32 and 212; while in 

 other cases it varies nearly 1 in different speci- 

 mens of the same salt. For a full account of his 

 discoveries, see Ann. de Chym. et de Phys. torn, 

 xiv. 172; xix. 350; xxiv. 264, 355. 



In accordance with the above facts, it has been 

 discovered that on crystallizing from solution in 

 the bisulphuret of carbon, or in oil of turpentine, 

 at temperatures below 100, sulphur assumes the 

 octohedric form, with rhombic bases ; but that 

 when melted by itself and allowed to cool slowly, 

 it takes the form of an oblique rhombic prism on 

 solidifying at 230. (Graham's Elements of Che- 

 mistry, p. 189.) It has also been ascertained 

 that pure carbon occurs in the form of regular 

 octohedrons, as in the diamond, but in six sided 

 plates in Graphite. And Professor Johnston has 

 recently shown that there is a large class of 

 bodies composed of the same elements in the same 

 proportions, which crystallize in two, if not more 

 primitive forms. But since the time of Dr. Black,- 

 chemists have attended so little to the agency of 

 caloric in the various transformations of matter, 

 that they have scarcely ranked it among their 

 elements, although it constitutes by far the largest 

 proportion by volume of solid bodies. 



The phenomena of isomerism are no less re- 

 markable, as illustrating the influence of caloric 



