17 PRESSURE OF GASES 33 



exert pressure on each other, as has been proved by mani- 

 f old.,ol>servations . 1 



\/The meaning of this law is simply that a mixture of two 

 or more gases possesses the same amount of kinetic energy 

 as its components taken together, and the correctness of 

 this fact is proved by the observation that if two gases at 

 the same temperature and pressure are mixed together there 

 is neither generation nor absorption of heat. 



18. Heating by Compression 



Eemembering that the pressure, energy, and tempera- 

 ture of a gas increase together in constant ratios, we have 

 at once an explanation of the fact that the temperature of 

 a gas is raised by compression and lowered by expansion. 

 Even without employing the assumptions of the kinetic 

 theory, it is not difficult to see that a diminution of volume 

 caused by heightened pressure must be bound up with an 

 increase of energy, and that part of this energy may be 

 transformed into heat ; expansion, on the other hand, requires 

 an expenditure of mechanical or heat energy to overcome 

 the opposing external pressure. All, however, is not ex- 

 plained by this general consideration. We obtain a deeper 

 insight into the nature of the phenomenon when we inves- 

 tigate more closely the nature of the molecular motion in a 

 gas that is being compressed or expanded. 



Let the gas be in a cylinder which is closed by a movable 

 piston. To keep this in equilibrium a pressure must be 

 exerted upon it which will balance the action of the mole- 

 cules impinging on it. But when an excess of pressure acts 

 on the piston from the outside, the piston is driven into the 

 cylinder; and during this motion of the piston the forces 

 that come into play in the collisions with the oppositely 

 moving molecules are increased. The molecules are, there- 

 fore, thrown back with greater vigour, and consequently 



1 For instance, Lament, Pogg. Ann. cxviii. 1863, p. 168; Schlomilchs 

 Zeitschrift, viii. 1863, p. 72, ix. 1864, p. 439; Bun sen, Gasometrische 

 Methoden, 1857, p. 209; 2nd ed. 1877, p. 267; 0. E. Meyer and F. 

 Springmiihl, Pogg. Ann. cxlviii. 1873, p. 540. 



D 



