226 CHEMICAL TRANSFORMATIONS. 



tible body in its vicinity ; with this difference only, 

 that the causes which determine the participation 

 and duration 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 phenomena of decomposition and combination, 

 which we are considering at present, the cause is a 

 body in the state of chemical action, which exerts 

 the decomposing 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 crystallise, but when a grain of sand is thrown 

 into the solution, or when it receives the slightest 

 movement, 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 (0 C.), if kept completely undis- 

 turbed, but solidifies in a moment when put in 

 motion. 



The atoms of a body must in fact be set in mo- 

 tion before they can overcome the vi$ inertice so 

 as to arrange themselves into certain forms. A 

 dilute solution of a salt of potash mixed with tar- 

 taric acid yields no precipitate whilst at rest ; but 

 if motion is communicated to the solution by agi- 



