COMMON THINGS AIR. 



with a force of fifteen pounds. If the tube were filled with water 

 instead of air, the plug would in that case maintain its position, 

 for the water would not yield in any degree to the pressure. 

 But the case is quite otherwise with air. 

 The moment the pressure is applied, the 

 plug will descend in the tube, squeezing or 

 compressing the air into a less space, and it 

 will continue to descend until the air is 

 compressed half its original bulk. There 

 the compression will cease, and the plug 

 will remain at half its original distance 

 from the bottom B of the tube, as in fig. 2. 

 Thus, if the original height of the plug 

 p above the bottom of the tube were twelve 

 inches, the plug being pressed downwards 

 only by the atmospheric pressure, that is, 

 by 15 Ibs., its height, when pressed by 

 15 Ibs. more, that is, by 30 Ibs. in all, will 

 be six inches. The air, which is compressed 

 into twelve inches by 15 Ibs., is therefore 

 compressed into six inches by 30 Ibs., the 

 volume of the air being diminished in the 

 exact proportion in which this compressing 

 c force is augmented. p 



This experiment may be carried farther 

 with a like result. If the piston be forced 

 down with a weight of 30 Ibs., in addition 

 to the atmospheric pressure, which is 15 Ibs., 

 the whole compressing force will be 45 Ibs. 

 In this case, the compressing force being augmented 

 in the proportion of three to one, the space into which 

 the air is compressed will be decreased in the same 

 ratio, and the plug p will descend to four inches from 

 _ the bottom B. 



In general, therefore, the space into which air will be squeezed 

 by any force will be less in exactly the proportion in which the com- 

 pressing force is increased, it being well understood, nevertheless, 

 that the original pressure of the external air, amounting to 15 Ibs. 

 per square inch, is to be included in the compressing force. 



This property of unlimited and uniformly regular compres- 

 sibility is one of the essential and characteristic properties of air, 

 being one in which no other form of matter participates. Liquids 

 are in general, for all practical purposes, absolutely incompressible. 

 Some solids are compressible in a certain slight degree, but not at 

 all in the general and regular way in which air is compressible. 



