164 WELLS'S NATURAL PHILOSOPHY. 



The atmosphere is not, as is generally regarded, innsiblo. 



Is the atmos- "^Jien seen through a great extent, as when we look upward 

 p he re visible ? o o i i- 



in the sky on a clear day, the vault appears of an azure, or 



deep blue color. Distant mountains also appear blue. In both these instances 

 the color is due to the great mass of air through which we direct our vision. 



The reason that we do not observe this color in a small quan- 

 a small quanti- tity of air is, that the portion of colored light reflected to the 

 K) •"/ T"^ ?^^' ^^'° ^^ * limited quantity is insufficient to produce the requis- 

 ite sensation upon the eye, and in this way excite in the mind 

 a perception of the color. Almost aU slightly transparent bodies are exam- 

 ples of this fact. 



If a glass tube of small bore be filled with sherry wine, or wine of a simi- 

 lar color, and looked at through the tube, it will be found to have all the 

 appearance of water, and be colorless. If viewed from above, downward, in 

 the direction of its length, it will be found to possess its original color. In 

 the first instance, there can be no doubt that the wine has the same color 

 as the liquid of which it originally formed a part ; but in the case of small 

 quantities, the color is transmitted to the eye so faintly, as to be inadequate, to 

 produce perception. For the same reason, the great mass of the ocean 

 appears green, while a small quantity of the same water contained in a glass 

 is perfectly colorless. 



Poes air pos- ^^^- -^^'''j ^^ commoii witli otliGr material 

 Imtui 'quau- substaiices, possesses all the essential quali- 

 ties of matter? ^jgg of matter, as impenetrability, inertia, and 

 weight. 



367. The impenetrability of air may be sIio-rti by taking a 

 ■UTiat are , „ , ^ , / , , , . . / . ^ 



proofs of the hollow vessel, as a glass tumbler, and immersing it in water 



impcnetrabiU- ^j^^ jjg niouth downward ; it will be found that the water 

 ty of air / ' 



will not fill the tumbler. If a cork is placed upon the water 



under the mouth of the tumbler, it will be seen that as the tumbler is pressed 



down, the air in it will depress the surface of the water on which the cork 



floats. The diving-bell is constructed on the same principle. 



3G8. The inertia of the air is shown by the resistance which 

 "What are 



proofs of the it opposes to the motion of a body passing through it. Thus, 



inertia of air ? jf ^^q open an umbrella, and endeavor to carry it rapidly with 



the concave side forward, a considerable force wUl be required to overcome 



the resistance it encounters. A bird could not fly in a space devoid of air, 



even if it could exist without respiration, since it is the inertia, or resistance 



of the particles of the atmosphere to the beating of the wings, which enables 



it to rise. The wings of birds are larger, in proportion to their bodies, than 



the fins of fishes, because the fluid on which they act is less dense, and has 



proportionally less inertia, than the water upon which the fins of fishes act. 



To what ex- ^69. Air is highly compressible and perfectly 



tent is air plnctiV 

 compressible? '-iclt'LH^. 



By these two qualities air and aU other gaseous substances 



