156 AIR. 



matter to cohese; light never appears but when it 

 illuminates something ; we know nothing of heat, 

 unless there is something that is warmed by resist- 

 ance to it ; and we can know that there is motion, 

 not merely when something moves, but when 

 there is some other thing 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 mat- 

 ters 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 with- 

 out 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. Be- 

 sides water, and other foreign substances, of 

 which it always contains some portion, 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 caotchouc, 

 or Indian rubber, for instance, and of course of 

 the trees whose juice consolidates into that sub- 

 stance. 



But though the atmospheric compound of oxy- 



