362 THE PRINCIPLES OF SCIENCE. 



cipal axis, but more or less rapidly in the direction of this 

 axis, so that the isothermal surface is an ellipsoid of revo- 

 lution round that axis. 



Nearly the same statement may be made concerning 

 the third or hexagonal or rhombohedral system of 

 crystals, in which there are three axes lying in one plane 

 and meeting at angles of 60, while the fourth axis is 

 perpendicular to the other three. The hexagonal prism 

 and the rhombohedron are the two commonest forms 

 assumed by crystals of this system, and in ice, quartz, 

 and calc-spar, we have abundance of beautiful specimens 

 of the various forms produced by the modification of the 

 primitive form. Calc-spar alone is said to crystallize in 

 at least 700 varieties of forms. Now of all the crystals 

 belonging both to this and the dimetric class, we know 

 that a ray of light passing in the direction of the prin- 

 cipal axis will be refracted singly as in a crystal of the 

 regular system ; but in every other direction the light 

 will suffer double refraction being separated into two rays, 

 one of which obeys the ordinary law of refraction, but the 

 other a much more complicated law. The other physical 

 properties vary in an analogous manner. Thus calc-spar 

 expands by heat in the direction of the principal axis, but 

 contracts by a small quantity in directions perpendicular 

 to it. So closely indeed are these various physical pro- 

 perties correlated that Mitscherlich, having observed the 

 law of expansion in calc-spar, was enabled to predict that 

 the double refracting power of the substance would be de- 

 creased by a rise of temperature, as was proved by expe- 

 riment to be the case. 



In the fourth system, called the trimetric, rhombic, or 

 right prismatic system, there are three axes, at right 

 angles, but all unequal in length. It may be asserted 

 in general terms that the mechanical properties vaiy in 

 such crystals in every direction, and heat spreads so that 



