90 SMITH'S INTERMEDIATE CHEMISTRY 







becomes exactly, or almost exactly, twice as great. This must 

 mean that, although the particles are becoming crowded, they 

 do not interfere with one another's motion, excepting of course 

 where actual collision causes a rebound. Only in the absence 

 of interference would doubling the number of molecules per unit 

 of volume give exactly double the number of impacts on the walls. 

 Hence the molecules must have practically no tendency to cohe- 

 sion. Finally, the molecules must be supposed to move in straight 

 lines between collisions. 



Boyle's law therefore adds four more details concerning molec- 

 ular behavior, namely, that the impacts of the particles produce 

 the pressure, that the crowding of the molecules represents the con- 

 centration of the material and that the particles move in straight 

 lines and show almost no cohesion, since pressure and concen- 

 tration are very closely proportional to one another. 



How, now, can we account for Charles' law (p. 46), according to 

 which an increase in volume (or in pressure, if the volume is kept 

 constant) results from heating a mass of rapidly moving mole- 

 cules? The action of a particle colliding with a surface is meas- 

 ured in physics in terms of its mass and its velocity. It is evident 

 that heating a cloud of molecules would not increase the mass 

 of each, and it must therefore increase the velocity. 



Diffusion. It is found by experiment that the lighter a gas is, 

 the faster its particles move by diffusion (p. 89) in any direction. 

 Exact measurement shows that the rate is inversely proportional 

 to the square root of the density of the gas. From the results ob- 

 tained for any particular gas, the average speed at which the 

 molecules of which it is composed are moving can be calculated. 

 For the hydrogen molecule at room temperature, this speed is 

 1840 meters per second, or faster than a rifle bullet. The speed 

 of the oxygen molecule, which is almost exactly 16 times as heavy, 

 is consequently the square root of one-sixteenth (that is, one quar- 

 ter) of this value, or 460 m. per sec. 



