﻿120 MEIGS ON THE RELATION OF 



arsenic is about twice that of sulphur. The atomic heat of Fe 2 S 4 is 15.60384, of 

 Fe 2 AsS 2 , 16.35392. The cyanides replace the chlorides, three atoms occupying the 

 space and performing the function of one. The atomic heat of cyanogen is 14.4222, 

 that of chlorine, 14.4627. 



We come now to inquire why the atomic heat and volume in many of the isomor- 

 phous groups are not absolutely identical. Many bodies called isomorphous are 

 not strictly so. Their corresponding angles sometimes vary several degrees. A 

 variation of 59' has been observed by Mitscherlich in the analogous angles of 

 different specimens of the same salt. In the sulphates of barytes and strontia the 

 angles of the rhombs differ by 2° 48'; in the carbonates of lime and magnesia by 

 2° 36'; and in the sulphates of zinc and magnesia by 38'. Inequalities also exist 

 between the axes of such plesimorphic bodies. Now the interdependence of crystalline 

 condition and atomic volume has been well and beautifully shown by Kopp, as the 

 result of an attentive study of the various classes of plesimorphic and strictly isomor- 

 phic compounds. That careful physicist found that variations in the individual 

 angles of a crystal were accompanied by corresponding variations in its atomic volume. 

 But the angular value of a crystal depends upon the proportions between its axes, and 

 these cannot suffer alteration without changing the density, and, consequently, its 

 specific gravity — alterations necessarily preceding variation in atomic volume. 

 Elongation of the principal axis increases the atomic volume ; thus, step by step, we 

 are led to the conviction that crystalline form is inseparably connected with, and 

 dependent upon, peculiar axial proportions. Whatever modifies these proportions in 

 a definite manner, must be considered as the ultimate cause of the plesimorphic 

 differences referred to above. In this connection, the observations of Mitscherlich, 

 Fresnel and others, upon the application of heat to crystals, may be studied with much 

 advantage, as they are well calculated to lead to results as positive as they are impor- 

 tant. According to these gentlemen, when crystals not belonging to the regular 

 system are heated, their axes expand unequally and change the magnitude of their 

 angles. Crystals of the right prismatic system have their three axes expanded differ- 

 ently. In arragonite, on raising the temperature from 32° to 212°, the inclination 

 of the lateral faces increases by 2' 46'', and that of the terminal faces diminishes by 

 5' 29" ; gypsum is more expanded by heat in the direction of the principal axis than 

 in that of the lateral axes. In bitter spar, the obtuse angle of the primitive rhombo- 

 hedron diminishes when the temperature is raised from 32° to 212° by 4' 6"; in 

 ferruginous bitter spar, by 3' 29"; in iron spar, containing a considerable quantity of 

 manganese, by 3' 31''; and in pure iron spar, by 2' 22". In crystals of the rhombo- 

 hedral system, the expansion is the same in the directions of the three secondary 

 axes, but different in that of the principal axis* But the positive and methodical 

 influence of heat in changing the density, axes and angles, and, therefore, the form 



*Gmelin. Hand-Book of Chemistry, Cavendish Society edition, vol. i. p. 233. 



