296 CHEMICAL TRANSFORMATION& 



go that change; and combinations and decompo- 

 sitions are the consequence. But when the atoms 

 of the second body are not capable of such an action, 

 any further disposition to change ceases from the 

 moment at which the atoms of the first body assume 

 the state of rest, that is, when the changes or trans- 

 formations of this body are quite completed. 



This influence exerted by one compound upon the 

 other, is exactly similar to that which a body in the 

 act of combustion exercises upon a combustible body 

 in its vicinity ; with this difference only, that the 

 causes which determine the participation and dura- 

 tion of these conditions are different. For the cause, 

 in the case of the combustible body, is heat, which 

 is generated every moment anew ; whilst in the phe- 

 nomena of decomposition and combination which we 

 are considering at present, the cause is a body in 

 the state of chemical action, which exerts the de- 

 composing influence only so long as this action 

 continues. 



Numerous facts show, that motion alone exercises 

 a considerable influence on chemical forces. Thus, 

 the power of cohesion does not act in many saline 

 solutions, even when they are fully saturated with 

 salts, if they are permitted to cool whilst at rest. 

 In such a case, the salt dissolved in a liquid does not 

 crystallize; but when a grain of sand is thrown into 

 the solution, or when it receives the slightest move- 

 ment, the whole liquid becomes suddenly solid while 

 heat is evolved. The same phenomenon happens 

 with water, for this liquid may be cooled much under 

 32^ F. (Qo 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 inerticB so as to ar- 

 range themselves into certain forms. A dilute solution 

 of a salt of potash mixed with tartaric acid yields no 

 precipitate whilst at rest ; but if motion is communi- 

 cated to the solution by agitating it briskly, solid 

 crystals of cream of tartar are deposited. A solu- 



