﻿ATOMIC HEAT TO CRYSTALLINE FORM. 



121 



of a crystal, is beautifully and conclusively shown in the following example : Calca- 

 reous spar, heated to 180° F., has its specific gravity decreased in the proportion of 

 1 : ~„ Tl ; its principal axis expanded by 0.001961 ; and each horizontal axis contracted 

 by 0.00056 ; in consequence of which the obtuse angles of the primitive rhombohedron 

 diminish, and the acute angles increase by 8' 34". Now, according to Kopp, when 

 its principal axis = 0.85440, and its obtuse angle = 105°.5', the specific gravity 

 of calc spar is 2.7220. When heated, its specific gravity becomes 2.71675, and its 

 atomic volume passes from 232.36 to 232.80; or, in other words, a modification of 

 form ensues. 



The phenomenon of dimorphism presents us with many examples strongly confirma- 

 tory of the significant connection, here indicated, between form and capacity for heat. 

 Still more do they conduct us to some positive notions concerning the active agency 

 of caloric in producing crystalline form. " Crystals formed at one particular tempe- 

 rature," says Gmelin, " and then exposed to that temperature at which crystals of a 

 different kind are produced, often lose their transparency, and, without alteration of 

 external form, become changed into an aggregate of small crystals of the latter kind. 

 We may therefore imagine that the atoms of the solid crystal displace one another in 

 such a manner as to bring about the particular arrangement which they are disposed 

 to assume at the altered temperature, the new arrangement belonging to a different 

 crystalline system."* Carbon, as found in the diamond, which belongs to the regular 

 system, has a different atomic heat from carbon as found in rhombohedral graphite. 

 In fact, all the varieties of carbon differ in the same respect, as shown in the following 

 table : 



Varieties of 



Specific 



Atomic 



Atomic 



Carbon. 



Heat. 



Heat. 



Volume. 



Diamond. 



0.1192 D,M. 









0.1469 Rg. 



0.8814 



1.690 



Graphite : 









Natural, 



0.2019 Rg. 



1.2114 





Artificial, 



0.1970 Rg. 



1.1820 



2.857 



Coke: 









3 varieties, 



0.2036 Rg. 



1.2216 







0.2017 Rg. 



1.2102 







0.2031 Rg. 



1.2186 



3.333 



Charcoal: 









Strongly ignited, 



0.2415 Rg. 







(t a 



0.2009 D,M. 



1.3272 





Moderately heated, 



0.2964 D,M. 



1.7784 





From Sugar, 



0.1592 Rg. 



0.8552 





" 01. Terebinth, 



0.1801 Eg. 



1.0806 





Lamp Black, 



0.2570 Av. 



1.5420 





Animal, 



0.2608 Rg. 



1.5648 





Carbon thus furnishes us with an excellent example of the connection between the 

 calorific capacity of a body and its degree of aggregation. 



* Gmelin. Op. cit., vol. i. p. 98. 



