INTRODUCTION 9 



other hand, is one in whose formation heat or energy was 

 absorbed. Consequently exothermic compounds are stable 

 and require some power to decompose them, while endothermic 

 compounds are unstable and give out heat or energy when 

 they decompose ; they are often explosive. A chemical change 

 evolving heat is described as an exothermic reaction, while 

 one in which heat or other form of energy is absorbed is called 

 an endothermic reaction. 



Orgfanic Matter is, strictly speaking, matter which has 

 been produced by organisms — i.e., animals or plants — but the 

 term is used in a wider sense in chemistry for any compound 

 of carbon, whether produced by life-processes or artificially. 

 Almost all forms of organic matter, when strongly heated out 

 of contact with air, blacken, owing to the liberation of carbon; 

 with free access of air combustion occurs, and carbon dioxide 

 and other products are formed. 



Oxidation and Reduction.— By oxidation, literally 



speaking, is meant union with oxygen, but in a chemical 

 sense the term is given a wider significance, viz., combination 

 with more oxygen or with some substance playing the part of 

 oxygen. Thus the conversion of a ferrous compound, e.g., 

 ferrous chloride, FeCl,, into a ferric compound, ferric chloride, 

 FeClg, is often called by chemists a process of oxidation, though 

 no oxygen may be concerned in it. 



Reduction is used in exactly the opposite sense. A sub- 

 stance which brings about oxidation of others is called an 

 '* oxidising agent," while one which removes oxygen or any- 

 thing which plays the part of oxygen is called a *' reducing 

 agent." Common oxidising agents are air, nitric acid and 

 nitrates, chlorates, chlorine, <fec. ; common reducing agents 

 are easily oxidisable metals, e.g., zinc, partially oxidised sub- 

 stances such as sulphurous acid, H,S03, many forms of decaying 

 organic matter (especially when under water), &c. 



Putrefaction: — a process of decomposition, sometimes 

 accompanied by oxidation, of carbonaceous matter, produced 

 by the life-processes of bacteria, yeasts, moulds, (fee., and result- 



