MATTER AND ITS PHYSICAL PROPERTIES. 31 



weather, when the body has been exposed to the open air, the flesh appears to 

 contract, the vessels shrink, and the skin shrivels. 



The phenomena attending change of temperature are conclusive proofs of 

 the universal porosity of material substances, but they are not the only proofs. 

 Many substances admit of compression by the mere agency of mechanical force. 

 Let a small piece of cork be placed floating on the surface of water in a basin or 

 other vessel, and an empty glass goblet be inverted over the cork so that its edge 

 just meets the water. A portion of air will then be confined in the goblet and 

 detached from the remainder of the atmosphere. If the goblet be now pressed 

 downward so as to be entirely immersed, it will be observed that the water 

 will not fill it, being excluded by the impenetrability of the air enclosed in it. 

 This experiment, therefore, is decisive of the fact that air, one of the most 

 subtile and attenuated substances we know of, possesses the quality of impene- 

 trability. It absolutely excludes every other body from the space which it 

 occupies at any given moment. 



But although the water does not fill the goblet, yet if the position of the cork 

 which floats upon its surface be noticed, it will be found that the level of the 

 water within has risen above its edge or rim. In fact, the water has partially 

 filled the goblet, and the air has been forced to contract its dimensions. This 

 effect is produced by the pressure of the incumbent water forcing the surface 

 in the goblet against the air, which yields until it is so far compressed that it 

 acquires a force able to withstand this pressure. Thus it appears that air is 

 capable of being reduced in its dimensions by mechanical pressure, indepen- 

 dently of the agency of heat. It is compressible. 



That this effect is the consequence of the pressure of the liquid, will be 

 easily made manifest by showing that, as the pressure is increased, the air is 

 proportionally contracted in its dimensions ; and as it is diminished, the dimen- 

 sions are, on the other hand, enlarged. If the depth of the goblet in the water 

 be increased, the cork will be seen to rise in it, showing that the increased 

 pressure at the greater depth causes the air in the goblet to be more condensed. 

 If, on the other hand, the goblet be raised toward the surface, the cork will 

 be observed to descend toward the edge, showing that as it is relieved from 

 the pressure of the liquid, the air gradually approaches to its primitive dimen- 

 sions. 



These phenomena also prove that air has the property of elasticity. If it 

 were simply compressible, and not elastic, it would retain the dimensions to 

 which it was reduced by the pressure of the liquid ; but this is not found to be 

 the result. As the compressing force is diminished, so in the same proportion 

 does the air, by its elastic virtue, exert a force by which it resumes its former 

 dimensions. 



That it is the air alone which excludes the water from the goblet in the pre- 

 ceding experiments, can easily be proved. When the goblet is sunk deep in 

 the vessel of water, let it be inclined a little to one side until its mouth is pre- 

 sented toward the side of the vessel ; let this inclination be so regulated that 

 the surface of the water in the goblet shall just reach its edge. Upon a slight 

 increase of inclination, air will be observed to escape from the goblet, and to rise 

 in bubbles to the surface of the water. If the goblet be then restored to its 

 position, it will be found that the cork will rise higher in it than before the 

 escape of the air. The water in this case rises and fills the space which the 

 air, allowed to escape, has deserted. The same process may be repeated until 

 all the air has escaped, and then the goblet will be completely filled by the 

 water. 



Liquids are compressible by mechanical force in so slight a degree, thatt 

 are considered in all hydrostatical treatises as incompressible fluids. 



