DECOMPOSITION OF ORGANIC COMPOUNDS. 291 



The nature of the products formed during this process varies not 

 only with the nature of the substance heated, but also with the tem- 

 perature applied during the operation. The products formed by 

 destructive distillation are invariably less complex in composition, 

 that is, have a smaller number of atoms in the molecule, than the 

 substance which suffered decomposition ; in other words, a complex 

 molecule is split up into two or more molecules less complex in 

 composition. 



Otherwise, the products formed show a great variety of properties ; 

 some are gases, others volatile liquids or solids, some are neutral, 

 others basic or acid substances. In most cases of destructive distilla- 

 tion a non-volatile residue is left, which is nearly pure carbon. 



Action of oxygen upon organic substances. Combustion. 

 Decay. All organic substances are capable of oxidation, which 

 takes place either rapidly with the evolution of heat and light and is 

 called combustion, or it takes place slowly without the emission of 

 light, and is called slow combustion or decay. The heat generated 

 during the decay of a substance is the same as that generated by 

 burning the substance ; but as this heat is liberated in the first 

 instance during weeks, months, or perhaps years, its generation is so 

 slow that it can scarcely be noticed. 



No organic substance found or formed in nature contains a suffi- 

 cient quantity of oxygen to cause the complete combustion of the 

 combustible elements (carbon and hydrogen) present; by artificial 

 processes such substances may, however, be produced, and are then 

 either highly combustible or even explosive. 



During common combustion, provided an excess of atmospheric oxygen be 

 present, the total quantity of carbon is converted into carbon dioxide, hydrogen 

 into water, sulphur and phosphorus into sulphuric and phosphoric acids, while 

 nitrogen is generally liberated in the elementary state. 



During the process of decay the compounds mentioned above are produced 

 finally, although many intermediate products are generated. For instance : If 

 a piece of wood be burnt, complete oxidation takes place ; intermediate pro- 

 ducts also are formed chiefly in consequence of the destructive distillation of 

 a portion of the wood, but they are consumed almost as fast as they are pro- 

 duced, as was mentioned in connection with the consideration of flame. Again, 

 when a piece of wood is exposed to the action of the atmosphere, it slowly 

 burns or decays. The intermediate products formed in this case are entirely 

 different from those produced during common combustion. 



Common alcohol has the composition C 2 H 6 O ; in burning, it requires 

 six atoms of oxygen, when it is converted into carbon dioxide and water : 

 C 2 H 6 + 6O = 2CO 2 + 3II 2 O. 



