INTRODUCTION 



13 



attended with the production of much heat. Burning or com- 

 bustion is nearly always due to the heat produced by the 

 combination of the substance burnt with the oxygen of the 

 air. It is found, therefore, that any substance which will 

 burn in air (containing its 21 per cent, of free oxygen) will 

 do so with increased brilliancy in pure oxygen. The tem- 

 perature attained and the intensity of the light emitted are 

 always greater in the latter case, though the total quantity 

 of heat evolved by the union of a definite weight of a sub- 

 stance with oxygen is constant, and independent of the 

 circumstances under which the union takes place. 



Quantity of heat is measured by the weight of water which 

 it can raise in temperature through 1° 0. It is therefore 

 possible to determine the amount of heat which is evolved by 

 the union of the unit weight (1 pound or 1 gramme) of a 

 substance with oxygen, and this value will be true under 

 whatever circumstances the union may take place. The 

 number which expresses the number of units of mass (pounds 

 or grammes) of water which are raised in temperature through 

 1* C. by the union of the unit mass (one pound or gramme) 

 of a substance with oxygen is called the heat of combustion^ 

 or the caloi'ific power of the substance. The following 

 table gives the calorific power of a number of important 

 substances : 



In ordinary cases of burning, the evolution of heat is 

 readily evident, but in some cases of slow combination with 

 oxygen the heat is evolved so sk)wly that conduction and 

 convection carry it away almost as rapidly as it is produced, 



