40 THE PROGRESS AND SPIRIT OF 



the atomic theory is far from being limited in application to 

 this single science. We have seen that the other great forces 

 are known to us by their actions on and through matter, 

 such actions and changes, whether from light, heat, electricity, 

 or dynamic force, giving foundation to the several sciences 

 which bear these names. Co -related as they all are with 

 chemical phenomena, we might expect some corresponding 

 relation to that atomic constitution of bodies, from which 

 modern chemistry has drawn its greatest discoveries. And 

 accordingly we find numerous and striking proofs to this 

 effect, furnished by those who are seeking to solve experi- 

 mentally these high problems, and thereby to establish new 

 connections in the sciences, and laws common to all. We may 

 take as a most instructive example, the various and beautiful 

 phenomena of crystalline bodies, in their relation to heat, light, 

 and electricity. The crystal itself, whatever the matter com- 

 posing it, must be regarded as a substance, the component 

 molecules of which are compelled by a force or affinity 

 (what we may provisionally call polarity) to assume certain 

 definite positions, determining both the inner structure and 

 outer form. The three forces just named all affect most cu- 

 riously this molecular arrangement. Mitscherlich has shown 

 that while octaedral crystals expand equally in all directions 

 from heat, other crystals, not in this group, change the 

 measure of their angles with every change of temperature. 

 He has further shown that great alterations may be effected 

 by heat in the internal structure of crystals (as in the case of 

 certain prismatic crystals evolving octaedrons under exposure 

 to the sun's heat), without affecting their solidity or altering 

 their external form. The geometry of crystals, indeed, we 

 may fairly say, is but the outer side of the science; the 

 atomic relations and changes within put other and deeper 

 considerations before us. 



The fact, now attested in so many ways, that molecular 



