168 WELLS'S NATURAL PHILOSOPHY. 



of atmosphere pressing upon the surface, E. If this were not the case, or if 

 there were no air in F D, then the weight of the atmosphere pressing upon 

 the surface E would force the mercury, E B C F, up hito the space, F D. The 

 elasticity of air is, therefure, directly proportionate to the force, or compression, 

 exerted upon it. 



It is evident that the pressure exerted upon the surface, E, Fig. 157, what- 

 ever may be its amount, is that of a column of air reacliing friSm E to the top 

 of the atmosphere, or, as we express it, the weight of one atmosphere. The 

 amount of this pressure, aceurat.ly detenniued, is equal to the weight, or 

 pressure, which a column of mercury 30 inches high would exert on iho 

 same surface. If then, we pour into the tube, A E, Fig. 157, as much 

 mercury as will raise the surface in the leg A B 30 inches above the 

 surface of the mercury in the leg D C. we shall have a pressure on the 

 surlace of E equal to two atmospheres; and since liquids transmit pressure 

 equally in all directions, the same pressure will be exerted on the air included 

 in the leg D F. This will reduce it in volume one halfj or compress it into 

 half the space, and the mercury will rise in the log D F from F to F'. This 

 weight of two atmospheres reduces a given quantity of air into one half its 

 volume. In the same manner, if mercury be again poured into the tube^ 

 E until the surface of the column in A E is GO inches above the level of the 

 mercury in D F, then the air in D F will be compressed into one third of its 

 original volume. In the same manner it could be shown, by continuing these 

 experiments, that the diminution of the volume of air will always be in the 

 exact proportion of the increase of the compressing force, and its volume can 

 also be increased in exact proportion to the diminution of the compressing 

 force. In fact this law has been verified by actual experknent, until the air 

 has been condensed 27 times and rarefied 112 times. 



Air has been allowed to expand into more than 2,000 times its bulk, and 

 it would have expanded still more if greater space had been allowed. Air 

 has also been compressed into less than a thousandth of its usual bulk, so as 

 to become denser than water. In this state it still preserved its gaseous forni 

 and condition. 



376. The fact that air possesses weight, and consequently 

 Was the weight , ' ., , '. , ■, , 



of air known exerts pressure, was not known until about two hundred years 



to the an- g^gQ_ jj^^ ancient philosophers recognized the fact, that air 



was a substance, or a material thing, and they also noticed 



that when a solid, or a liquid, was removed, that the air rushed in and filled 



up the space that had been thus deserted. But when called to give a 



reason for this phenomenon, they said " that nature abhorred an empty 



space," or a " vacuum," and therefore filled it up with air, or some liquid, or 



solid body. 



What is a 377. A vacuum is a space devoid of matter; 



vacuum? -^^ general, we mean by a vacuum a space de- 



void of air. 



No perfect vacuum can be produced artificially ; but confined spaces can 

 be deprived of air sufficiently lor all experimental and practical purposes. 



