Matter and Energy. 23 



molecules of water, some complex substances are reduced to 

 simpler forms. 



(c) Similarly, the addition of one or more molecules of 

 water hydration may cause a complex molecule to re- 

 solve itself into several molecules, each having a simpler 

 composition than that of the original molecule. 



(d) Undoubtedly the commonest mode of decomposition 

 is oxidation, that is to say, the constant tendency of the con- 

 stituent atoms of complex molecules to give up their weaker 

 affinities, and to unite with free or loosely combined oxygen. 

 All the changes taking place in decaying vegetal or animal 

 matter are dependent, more or less, on the oxidation and 

 removal of the several constituents of the decaying substance 

 in the form of carbonic acid, water, and other simple com- 

 pounds. The principle at the bottom of disinfection is the 

 supply of a large quantity of free or loosely combined oxygen 

 which is available for the oxidation and rendering harm- 

 less of putrescent and injurious matter. 



(e) Deoxidation, or the withdrawal of oxygen, is of much 

 rarer occurrence, but does take place in certain cases. 



Instances of these actions in plenty will present them- 

 selves to us as we proceed. They are omitted here, as they 

 would otherwise be the means of introducing a larger number 

 of new terms than it would be possible to pause to explain 

 at this stage. 



By way of recapitulation, we may classify chemical com- 

 pounds in the following manner : 



Simple Complex 



Stable Unstable 



Inorganic Organic 



Found in the environment Found in the organism 



It must be borne in mind, however, that these different 

 groups flow into one another, and are by no means perfectly 

 distinct. A few illustrations will make this sufficiently 

 clear. 



