160 DECOMPOSITION. 



sufficient cooling, both might be condensed into 

 liquids, and crystallized into solids. We do not 

 know that these elements of water are abso- 

 lutely simple ; but we call them so, just because 

 we are not able to resolve any of them into two 

 substances bearing different properties ; and the 

 ancients thought water simple, and called it an 

 " element," for the same reason. But we can 

 work any of those (to us) simple substances 

 through a very great range of temperature, and 

 still get them back again in the very state with 

 which we set out. But bring them together 

 in the proportions in which they form water, 

 and apply a lighted match, and the combus- 

 tion is terrible, probably the most brilliant dis- 

 play of the action of heat with which we are 

 acquainted, and perfectly irresistible in its effects. 

 When those elements are in sufficient quantity, 

 and free to mix with their natural rapidity, as 

 much heat would come out of the materials of 

 a pitcher of water, when passing from the state 

 of separate airs or gases, to that of the com- 

 pound liquid, as would suffice to kindle the globe, 

 or loosen from their cohesion the particles of 

 any substance in nature, whether compound or 

 simple. 



The progress of decomposition is always the 

 same as that which is produced by the action of 

 heat ; the solid is first changed to a liquid, and 

 then the liquid into an air or vapour ; but there 

 are many cases in which the process is altogether 

 invisible ; and there are others in which the two 

 parts of it follow each other so closely that we 

 cannot distinguish them. There is no doubt, 

 however, of the perfect uniformity of the process; 



