OF NATURAL PHILOSOPHY. 243 



it were on the immediate mechanism of solid sub- 

 stances, are remarkably modified by their crystalline 

 structure. Thus, in transparent solids, the course 

 taken by the rays of light, in traversing them, as 

 well as the properties impressed upon them in so 

 doing, are intimately connected with this structure. 

 The recent experiments of M. Savart, too, have 

 proved that this is also the case with their power 

 of resistance to external force, on which depends 

 their elasticity. Crystallized substances, according 

 to the results of these experiments, resist compres- 

 sion with different degrees of elastic force, according 

 to the direction in which it is attempted to compress 

 them ; and all the phenomena dependent on their 

 elasticity are affected by this cause, especially those 

 which relate to their vibratory movements and their 

 conveyance of sound. 



(266.) There can be little doubt that modifica- 

 tions, similarly depending on the internal structure 

 of crystals, will be traced through every depart- 

 ment of physics. In that interesting one which 

 relates to the action of heat in expanding the 

 dimensions of substances, a beginning has already 

 been made by Professor Mitscherlich. It had long 

 been known that all substances are dilated by heat, 

 and no exception to this law has been found, so 

 long as we regard the bulk of the heated body. 

 Thus, an iron rod when hot is both longer and 

 thicker than when cold ; and the difference of di- 

 mension, though but trifling in itself, is yet capable 

 of being made sensible, and is of considerable con- 

 sequence in engineering. Thus, too, the quicksilver 

 in a common thermometer occupies a larger space 

 R 2 



