AIR ON MOUNTAINS. 



16. The ancients imagined that air was a simple substance which, 

 entered more or less into the composition of bodies in general, and 

 hence they called it one of the elements ; the others being in their 

 theory of physics, water, earth, and fire. Better informed now, 

 we know that neither air, water, nor earth, are simple or ele- 

 mentary substances, and that fire is not a substance at all, but a 

 physical effect due to the sudden and large production of heat 

 which attends the chemical combination of certain substances. 

 Thus the ancient elements are not elements at all. 



But to return to air, the more immediate object of our attention 

 at present. 



17. Air meaning by that term the air of the atmosphere, the 

 air we breathe, the air through which we behold the firmament, 

 the air whose currents carry our commerce over the ocean from 

 land to land is a compound or mixture made up of two extremely 

 different kinds of air. 



18. As there are many sorts of air having extremely different 

 qualities and properties, although they are alike in appearance, 

 being all invisible, transparent, colourless, light, compressible, and 

 elastic, it has been found convenient to call them by the general 

 name gas (derived from the Saxon word gast], and to limit the 

 application of the term "air" to that particular compound or 

 mixture of gases which constitutes the atmosphere. 



19. The erroneous notion that air was a simple and elementary 

 substance prevailed until the close of the last century, when 

 Lavoisier, the celebrated French philosopher, who was one of the 

 most illustrious of the founders of modern chemistry, showed that 

 it was a mixture of two different gases in definite proportions, 

 called oxygen, and azote or nitrogen. 



A hundred cubic inches of air is a mixture consisting of 80 

 cubic inches of azote, and 20 of oxygen. The result of the most 

 exact analyses differs from this proportion by a minute fraction, 

 which, though not unimportant in certain respects, need not here 

 embarrass the reader, who will do well to fix in his memory this 

 proportion of 80 to 20. 



There are many ways in which this constitution of atmospheric 

 air may be made manifest, some of which, however, involve prin- 

 ciples which would not be comprehended without a more extensive 

 knowledge of chemistry than is expected from our readers in 

 general. The following demonstration will, however, it is hoped, 

 be understood without difficulty. 



Let 100 cubic inches of common air, and 40 cubic inches of the 

 gas called hydrogen, be introduced into a closed fiask. If an electric 

 spark be transmitted through this mixture, which may be easily 

 done, an explosion will take place with a considerable development 



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