158 AJR. 



moving differently or at rest, by means of which we 

 can know and judge of the motion. We can treat 

 only of what we know ; and thus every attempt to 

 explain the principles or agencies which have been 

 noticed must be made through the medium of those 

 matters in which their effects are displayed. 



Now, however, we come to a real substance, or 

 perhaps, more correctly, to a state in which some 

 substances generally, and all substances at times, 

 exist. That substance is AIR, the lightest, the softest, 

 the fleetest, the most gentle, and the most obedient 

 of all material things, of which the human senses 

 can have any knowledge. The common atmosphere 

 which we breathe, and without which we could not 

 possibly live, is the type, and most familiar instance 

 of air. But it is the state, and not the substance, 

 that is aerial. Besides water, and other foreign 

 substances, of which it always contains some por- 

 tion, however small, the common air, or atmosphere, 

 consists of two ingredients oxygen and nitrogen, 

 The first of these forms part of water, of every 

 animal and every vegetable, and of many mineral 

 or earthy substances ; and the latter forms part of 

 every animal, and of some vegetables of caoutchouc, 

 or Indian rubber, for instance, and of course of the 

 trees whose juice consolidates into that substance. 



But though the atmospheric compound of oxygen 

 and nitrogen be the type, and, to popular observation, 

 the example of air, yet air may mean any thing, or 

 all things ; because all things, or the elements of 

 which all things are composed, may exist in the state 

 of air. 



The most accurate definition of air is, "matter 

 subdued by heat," so overcome by the tendency to 

 motion which heat imparts, that it has no cohesion, 

 and none of the common properties of matter, ex- 

 cepting gravitation the property which matter never 

 loses, or can lose, while it exists. No matter what 

 the substance be which is in the state of air ; be it 

 hydrogen, which when in a state of air is the lightest 



