OF ARTS AND SCIENCES. 113 



lisioiis against the sides of the containing vessel. The temperature 

 depends upon the rapidity of the motion, being represented by the 

 vis viva of the separate molecules. This theory has therefore a very 

 intimate relation with the mechanical theory of heat, and is in part de- 

 pendent on it. The truth of such a theory must be judged of from the 

 agreement of the results deduced from it with observed phenomena. The 

 following physical laws and pi'operties of gases follow from this theory. 



1. Mariotte's law. 



2. Equal volumes of all gases set free the same amount of heat when 

 compressed by the same fractional part of their volume, this amount being 

 the exact mechanical equivalent of the force employed in producing the 

 compression. Hence, if the compression is considerable, the amount of 

 heat will vary with the rapidity of the compression, being much greater 

 when the whole amount of heat set free is confined in the gas, than 

 when it is suffered to escape as rapidly as it is formed. 



3. Gases expand equally for equal increments of temperature. 



4. Equal volumes of all gases, measured at the same temperature 

 and pressure, contain the same number of particles.* 



5. Gases will diflFuse into each other in accordance with the follow- 

 ing laws. 



a. Gases at the same temperature and pressure will diffuse into each 

 other with a velocity directly as the square root of the specific gravity, 

 when measured by the mass which is diffused, and therefore inversely 

 as the specific gravity when measured by volume. 



/3. The temperature of the gas being increased, while its volume 

 remains constant, the diffusive power will increase as the square root 

 of the elasticity. 



y. The temperature varying, while the pressure remains constant, 

 tlie diffusive power will vary inversely as the volume. 



Or, in general, gases diflFuse into each other according to the laws 

 which regulate their flow into a vacuum. 



6. A body moving rapidly through a gas will be subject to an in- 

 crease of temperature, varying as the square of its velocity. For dif- 

 ferent gases this increase will be directly as the specific gravity of the 

 gas ; but the eflfect of radiation being eliminated, the increase of tem- 

 perature will in the case of the same gas be independent both of the 

 temperature and the density of the gas. 



* Maxwell, Philosophical Magazine, January, 1860. 

 VOL. V. 15 



