HEAT ACTING ON COMPOUNDS. 255 



m all their various forms ; and, chymically, into their 

 constituent parts or elements, in all kinds of matter 

 except those which we consider them, and we con- 

 sider them simple, just because we have not been 

 able to divide them chymically. 



Our means and methods of decomposition have 

 been much improved since the time when fire, and 

 air, and water, and solid matter under the general 

 name of earth, were considered as the four elements 

 of all created things. And as we find in every 

 case of decomposition that the constituent or ele- 

 mentary parts have all different qualities, and that 

 the compound has qualities of which we could have 

 had no knowledge or even suspicion, if we had 

 known the elements only in their separate states, 

 we are enabled to say to what the properties which 

 we observe in different kinds of matter are owing. 

 But, as every new combination is attended with new 

 properties, we have strong grounds for believing that 

 every property of matter, and every change in the 

 appearance of any portion of matter, is the result 

 of combination : that the property which we find 

 originally in any substance is the result, or effect, 

 of a combination which took place before we ex- 

 amined that substance ; and that every change which 

 we find to take place in any substance is the result 

 of a combination immediately preceding that change. 

 The combination may take place in two ways, be- 

 cause it may, in the case of the individual substance, 

 be either an adding to it or a taking away from it ; 

 and the addition or the subtraction may either be 

 that which we can obtain and examine in a separate 

 state, or it may not. It may happen, also, that those 

 two modes of change are combined, and the com- 

 binations of them may be varied, it may consist 

 of any two of them, or of any three, or of all the 

 four. 



Removing a spot of tar is a familiar instance of 

 that. Common soap will not dissolve the tar, and 

 neither tar nor grease will dissolve in water; but 



