INTRODUCTION. XY 



natural state would depend entirely upon the degrees of 

 affinity existing between their component particles. But 

 its action is balanced and modified by that of the fluid of 

 heat, which enters, in various proportions, into all sub- 

 stances, and which tends to separate, one from another, the 

 elements which affinity draws together. Affinity, alone, 

 would form only solid masses, inactive, and more or less 

 compact. The action of heat, alone, would produce only 

 gases, or aerial substances. But the combined action of 

 these agents presents to us bodies either in a solid, liquid, 

 or fluid state, according to the degree of intensity with 

 which one or the other force acts upon the component 

 parts. The natural state of bodies, then, is owing to the 

 combined action of the law of affinity, which brings their 

 particles into union, and the interposing fluid of heat, 

 which separates them from each other. 



The variations which the atmosphere undergoes during 

 the different seasons of the year are sufficient to produce 

 changes of consistency in some bodies. Water, for in- 

 stance, is either solid, liquid, or aeriform, according to the 

 temperature of the air. 



Man, who governs the power of heat at his pleasure, 

 can produce all these remarkable changes in the natural 

 state of bodies. He can augment or diminish their con- 

 sistency at his will, and cause them to assume either the 

 sohd, liquid, or gaseous form, according as he adds or 

 takes away that fluid. 



The changes produced by the addition or subtraction 

 of heat are not permanent. The body returns to its 

 natural state, the moment the cause has ceased to 

 operate, — ^imparting to the surrounding substances the 

 excess of fluid it has imbibed, or receiving from them 



