272 CHEMICAL TRANSFORMATIONS. 



does not crystallize; but when a grain of sand is thrown into 

 the solution, or when it receives the slightest movement, the whole 

 liquid becomes suddenly solid with the evolution of heat. The 

 same phenomenon happens with water, for this liquid may be 

 cooled much under 32° F. (0° C), if kept completely undisturbed, 

 but solidifies in a moment when put in motion. 



The atoms of a body must in fact be set in motion before they 

 can overcome the vis inertia so as to arrange themselves into cer- 

 tain forms. A dilute solution of a salt of potash, mixed with tar. 

 taric acid, yields no precipitate whilst at rest ; but if the motion 

 is communicated to the solution by agitating it briskly, crystals 

 of cream of tartar are instantly deposited. A solution of a salt 

 of magnesia also, though not rendered turbid by the addition of 

 phosphate of ammonia, deposits the phosphate of magnesia and 

 ammonia on those parts of the vessel touched with the rod em- 

 ployed in stirring. 



In the processes of combination and decomposition under 

 consideration, motion, by overcoming the via inertia, gives rise 

 immediately to another arrangement of the atoms of a body, that 

 is, to the production of a compound which did not before exist in 

 it. Of course' these atoms must previously possess the power of 

 arranging themselves in a certain order, otherwise both friction 

 and motion would be without the smallest influence. 



The simple permanence in position of the atoms of a body, 

 is the reason that so many compounds appear to present themselves, 

 in conditions, and with properties, different from those which 

 they possess when they obey the natural attractions of their atoms. 

 Thus sugar and glass, when m lted and cooled rapidly, are 

 transparent, of a conchoidal fracture, and elastic and flexible to 

 a certain degree. Put the ormer becomes dull and opaque on 

 keeping, and exhibits, by cleavage, crystalline faces which belong 

 to crystallized sugar. Glass assumes also the same condition, 

 when kept soft by heat fur a long period ; it becomes white, 

 opaque, and so hard as to strike fire with steel. Now, in both 

 these bodies, the atoms evidently have different positions in the 

 two forms. In the first form their attraction did not act in the di- 

 rection in which their power of cohesion was strongest. It is 

 known also, that when sulphur is melted and cooled rapidly by 



