276 DECOMPOSITION BY HEAT. 



formed. These opposite effects are not very easily ex- 

 plained. The action of heat is to set free the carbonic 

 acid ; and it can only be by supposing that considerable 

 differences of temperature reverse the laws of affinity, 

 that we can at all understand this phenomenon. That 

 different effects result at high temperatures from those 

 which prevail at low ones,, recent experiments prove to 

 us, particularly those of Boutigny, already quoted when 

 considering decomposition by calorific action. 



Under the term chemical affinity, which we regard as 

 a power acting at insensible distances, and producing a 

 change in bodies, we are content to allow ourselves to 

 believe that we have explained the great operations of 

 nature. We find that the vegetable and animal king- 

 doms are composed of carbon, hydrogen, oxygen, and 

 nitrogen. The granite mountains of the earth, and its 

 limestone hills, and all its other geological formations, 

 are found to be metals and oxygen, and carbon and 

 sulphur, disposed to settle in harmonious union in their 

 proper places by chemical affinity. But what really is 

 the power which combines atom to atom, and unites 

 molecule to molecule? Can we refer the process to 

 heat ? The influence of caloric, although by changing 

 the form of bodies it sometimes assists combination, is 

 to be regarded rather as in antagonism to the power of 

 cohesion. Can it be thought that electricity is active in 

 producing the result? During every change of state, 

 those phenomena which we term electrical are mani- 

 fested ; but we thereby only prove the general diffusion 

 of the electric principle, and by no means show that 

 electricity is the cause of the chemical change. Can 

 light determine these change ? It is evident, although 

 light may be a disturbing power, that it cannot be the 

 effective one ; for many of these decompositions and re- 

 compositions are constantly going on within the dark 

 and silent depths of the earth, to which a sunbeam 

 cannot reach. That the excitation on the surface of the 



