THE PROGRESS OF PHYSICS. 147 



KINETIC THEORY OF GASES. 



We have had occasion to refer to this important 

 theory in the chapter on Chemistry ; it will be enough 

 to recall two or three of the steps in its develop- 

 ment. 



Diffusion. — Every one is aware of the rapidity 

 with which an escape of coal-gas makes itself felt 

 through a house. Dalton theorised this in his sug- 

 gestion that a gas consists of particles which are 

 constantly flying about in all directions, spreading as 

 far as they can, and inter-penetrating another gas, 

 or mixture of gases in the case of air, until equilib- 

 rium of pressure is attained. 



A more precise study of the movements of gaseous 

 particles was subsequently undertaken by Graham, 

 who showed that the relative rates of diffusion of two 

 gases are inversely proportional to the square roots 

 of their densities. Thus hydrogen diffuses four times 

 more quickly than oxygen. 



Joule's Calculation of Velocity of Parotides. — In 

 1848 and 1857, Joule took another stride forward in 

 determining the mean translational velocity of the 

 particles, basing his calculations on the conclusion 

 that the pressure of a gas is proportional to the energy 

 of motion of its particles. " Thus it may be shown 

 that the particles of hydrogen at the barometrical 

 pressure of 30 inches, at a temperature of 60°, must 

 move with a velocity of 6225.54 feet per second in 

 order to produce a pressure of 14.714 lbs. on the 

 square inch.'' In other words, as Sir Henry Roscoe 

 expresses it, a molecular cannonade or hailstorm of 

 particles is maintained against the bounding surface 

 at a rate far exceeding that of a cannon ball. 



