I. INTRODUCTION 3 



measured by the respective numbers of atoms present, not by their 

 respective weights. Eegarded in this way, hydrogen is of relatively 

 far greater importance and abundance than is usually estimated 

 (vide p. 16). 



Oxygen is perhaps the most important element known. It is by 

 far the most abundant, and takes part in a greater number of the 

 chemical changes occurring in nature than any other element. It is, 

 indeed, chiefly remarkable for its activity and its power of uniting with 

 almost all other elements. 



Its method of preparation and chief properties are well known to 

 all students of chemistry, being appropriately chosen for consideration 

 early in their course of study. 



Its union with- other bodies is usually attended with the evolution 

 of much heat and often light. Present in the free state in air, it plays 

 an important part in the chemical actions attendant upon the pro- 

 cesses of respiration, combustion, decay and almost all the forms of 

 " weathering " which occur around us. 



Oxidation, i.e., union with oxygen, is a process of great importance. 

 The life of animals especially, may be said almost to consist of oxida- 

 tion. So, too, the changes occurring in the soil, the " fermenting " of 

 hay, ensilage, etc., the putrefaction and decay of animal matter, and 

 many other processes are largely dependent upon combination with 

 oxygen. Union with oxygen is almost invariably accompanied by the 

 evolution of heat ; in fact, to union with oxygen most artificial and 

 many natural sources of heat (and hence of energy) owe their efficiency. 

 The rapid combination of substances with oxygen is generally accom- 

 panied by the attainment of a high temperature and is instanced by 

 most processes of combustion or burning. In such cases, the heat 

 evolved is rendered evident, but in others, the slow combination of 

 substances with oxygen evolves the heat so gradually that conduction 

 and radiation are able to carry it away almost as fast as it is produced, 

 consequently no distinct rise of temperature may be perceptible. A 

 very important fact, and one which should always be kept in mind, is 

 that, in all cases, the union of a given weight of a substance with 

 oxygen evolves the same quantity of heat, however slowly or quickly 

 the process of oxidation may take place ; provided, of course, that the 

 final product be the same. 



It is thus possible to determine experimentally the actual quantity 

 of heat (and thus of energy) evolved by the union of any fixed weight 

 of various combustibles with oxygen, and the numbers so obtained will 

 apply to all cases of burning in which these combustibles take part. 



Heat is measured by the quantity of water which it can raise 

 through 1 C. (or in some cases 1 R). The number of units of mass 

 (e.g., pounds or grammes) of water which can be raised through 1 C. 

 by the union of the unit of mass (i.e., 1 Ib. or 1 gramme) of the com- 

 bustible with oxygen, is called the heat of combustion or the calorific 

 power of the substance. 



The following table gives the calorific power of a number of sub- 

 stances : 



