4 INTRODUCTION. I. 



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 effi- 

 ciency. The rapid combination of substances with oxygen is 

 generally accompanied 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 F.). 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. lib or 

 1 gramme) of the combustible 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 

 substances : 



No. of grammes of water raised 



Substance. I" C. in temperature by the combustion 



of 1 gramme of the substance. 



Charcoal ... ... ... 8080 



Hydrogen ... ... 34,460 



Air-dried w^ood ... ... 2800 



Charred wood ... ... 3600 



Average coal ... ... 7500 



Good coke ... ... 7050 



Albumen (serum) ... ... 5918 



Casein 5860 



