244 DISCOURSE ON THE STUDY 



when hot than when cold ; and being confined by 

 the glass ball, (which also expands, but not so much 

 in proportion^) it is forced to rise in the tube. These 

 and similar facts had long been known ; and accu- 

 rate measures of the total amount of dilatation of 

 a variety of different bodies, under similar accessions 

 of heat, had been obtained and registered in tables. 

 But no one had suspected the important fact, that 

 this expansion in crystallized bodies takes place 

 under totally different circumstances from what 

 obtains in uncrystallized ones. M. Mitscherlich has 

 lately shown that such substances expand differently 

 in different directions, and has even produced a 

 case in which expansion in one direction is actually 

 accompanied with contraction in another. This 

 step, the most important beyond a doubt which 

 has yet been made in pyrometry, can however only 

 be regarded as the first in a series of researches 

 which will occupy the next generation, and which 

 promises to afford an abundant harvest of new 

 facts, as well as the elucidation of some of the 

 most obscure and interesting points in the doctrine 

 of heat. 



(267.) From what has been said, it is clear that it 

 we look upon solid bodies as collections of particles 

 or atoms, held together and kept in their places by 

 the perpetual action of attractive and repulsive 

 forces, we cannot suppose these forces, at least in 

 crystallized substances, to act alike in all directions. 

 Hence arises the conception of polarity, of which we 

 see an instance, on a great scale, in the magnetic 

 needle, but which, under modified forms, there is 

 nothing to prevent us from conceiving to act among 



