LESSONS IN CHEMISTRY. 109 



under the action of chemical attraction, the collision gave rise 

 to the heat and light which attended the action. The heat 

 and light of chemical action being due in reality to atomic per- 

 cussion as the hammer and iron become warm from the blows 

 necessary to shape the iron. As the sulphur and carbon burn in 

 the air, forming the sulphur dioxide in one case and carbon 

 dioxide in the other, we have reason to suppose that oxygen 

 exists abundantly in the air. 



Combustion means any chemical action attended by the evolu- 

 tion of heat, but is usually restricted to the union of oxygen with 

 other substances which results in the evolution of heat and light. 

 Substances which unite with oxygen are said to be combustible, 

 those that do not are incombustible. Most of the elements are 

 combustible, but the compounds resulting from combustion, as 

 the sulphur arid carbon dioxides, are incombustible. Chemical 

 action, or combustion, always produces the same amount of heat 

 whether the action be rapid or slow. When it is slow the heat 

 escapes by conduction or otherwise almost as rapidly as it is 

 formed, so that there is at no time any great intensity of heat and 

 no light, while if the combustion is rapid the heat becomes intense 

 and is accompanied with light as well. 



. The amount of heat generated by chemical action may be 

 measured. This is done by causing the action to take place 

 in such a manner that the heat resulting may, without waste, 

 be applied to raising the temperature of water. The quantity 

 of heat is shown by the number of grams of water it will 

 raise one degree in temperature. From careful experiments in 

 this direction, it has been shown that the chemical combi- 

 nation not only contains definite unvarying proportions of 

 simples, but that in its formation a definite unvarying amount 

 of heat was generated. Chemical combination and the evolution 

 of heat represent work done. When the carbon united with the 

 oxygen forming carbon dioxide, a certain amount of heat was 

 given off, a definite amount of work was done, and the same 

 amount of work will be necessary to separate the oxygen from 

 the carbon as was developed in bringing them together. The law 



