CABBONIC ACID GAS IN AIR. 39 



rectly, and become very hot ; this is the case in the 

 slaking of lime ; but it most usually happens that 

 the mere bringing together of two substances, even 

 though they have an affinity for each other, is not 

 sufficient to cause them to combine. 



35. In these cases combination cannot take place 

 until the substances are heated up to a certain point. 

 Thus, charcoal has a strong affinity for the oxygen of 

 the air, yet it cannot combine with it whilst both are 

 cold ; but as soon as a part of the charcoal is heated 

 redhot, combination commences, and this very act 

 evolves so much heat that the surrounding parts of 

 the charcoal soon begin to burn, and thus the com- 

 bustion, or combination of the charcoal with oxygen, 

 continues and increases, until either the charcoal is 

 all burnt, or the oxygen in the surrounding air is 

 saturated with carbon, and therefore unable to cause 

 the combustion any more. 



36. The common operation of lighting a fire is a 

 daily illustration of this. The fuel contains carbon, 

 or charcoal, ready to combine with the oxygen of the 

 air, but unable to do so, until, by applying a light to 

 it, we heat a portion up to the point required to com- 

 mence combination : after which the heat given out 

 by the chemical action going on, keeps it alight, and 

 causes the combustion to spread to the surrounding 

 fuel. 



37. The atmosphere is composed chiefly of two 

 different gases, called oxygen and nitrogen : but 

 besides this, it also contains a small quantity of a 



