THE ATMOSPHERE. 



199 



? Ins a limit, or that all material substances consist of ultimate constituent par- 

 ticles or atoms, which admit of no further subdivision, and on the mutual 

 relations of which the form and properties of the various species of bodies 

 depend. 



Now those ultimate particles of the air are endued with a certain definite 

 weight, because it is the aggregate of their weights which form the weight of 

 any mass of air. It is a fact, established by experiment, that in proportion as 

 air expands, its elastic force is diminished ; and therefore, if it continue to 

 expand, it will at length attain a state of attenuation in which the disposition 

 of its constituent particles to separate by their elasticity is so far diminished 

 as not to exceed the gravity of those constituent particles themselves. In this 

 state the two forces will be in equilibrium, and the elastic force being neutral- 

 ized, the particles will no longer be dilated. 



In these observations we have assumed a principle which is of the last 

 importance in pneumatics, and which, indeed, may be regarded as forming 

 the basis of this part of physical science, in the same manner as the power of 

 transmitting pressure is the fundamental principle of hydrostatics. This latter 

 principle, indeed, also extends to elastic fluids ; and all the consequences of 

 the free transmission of pressure which do not also involve the supposition of 

 incompressibility, are applicable to elastic fluids with as much truth as to 

 liquids. But the principle to which we now more especially refer, and which 

 may be looked upon as the chief characteristic of this form of body, and neces- 

 sary to render deh'nite the notion of their elasticity, may be announced as fol- 

 lows : 



" The elastic force of any given portion of air is augmented in exactly 

 the same proportion as the space within which it is enclosed is diminished ; 

 and its elastic force is diminished in exactly the same proportion as the space 

 through which it is allowed to expand is augmented." 



Fig. 5. 



Fig. 6. 



D 



Lie 



iLc 



To explain this, let A B C D, fig. 5, be conceived to be a cylinder, in which a 

 piston, A B, moves air tight, and without friction, and let us suppose the distance 

 of the lower surface, A B, of the piston, from the bottom, D C, of the cylinder, to 

 be 12 inches. Let air be imagined to be enclosed below the piston, and let us sup- 

 pose that the elastic force of this air is such as to press the piston with a force 

 of 16 ozs. From what has already been stated, it is clear that, to maintain the 

 piston in its place, it is necessary that it should be pressed downward with an 

 equivalent force of 16 ozs. Now let the force upon the piston be doubled, or let 

 the piston be loaded with a pressure of 32 ounces. The inward pressure pre- 

 vailing over the elasticity, the piston will immediately be forced toward D C, 

 but. will cease to move at a certain distance, A B, fig. 6, from the bottom. Now, 

 if this distance A D be measured, it will be found to be exactly 6 inches. The 

 air has, therefore, contracted itself into half its former dimensions. 



