ATOMS AND MOLECULES 23 



ceasing. They encounter the side of the jar in the 

 vastness of their number, and are continually beating and 

 pressing on it. They encounter each other, and billions 

 of collisions are taking place every instant; billions of 

 blows are being given and received. Everyone gives as 

 good as it gets, and gets as good as it gives, though all is 

 of the feeblest, and no clash of battle is heard. Great is 

 the commotion among them. Great is their activity and 

 multiplied their motions. They are not always moving 

 every one with the same velocity. The mean or average 

 velocity is, however, always the same in the same gas. 

 It is different in different gases. The measure of 

 difference is determined by the density the greater the 

 density, the less is the velocity ; the less the density, the 

 greater is the velocity. It is, however, to the square root 

 of the density that it is inversely proportional. The mean 

 velocity of hydrogen molecules, when the temperature 

 is at the freezing point and the barometer showing a 

 pressure of 30 inches, is 6097 feet per second. Every 

 molecule in a jar, therefore, is running to and fro with 

 great rapidity, and covering a distance of 1 mile 272 

 yards in a second. The density of oxygen is sixteen 

 times that of hydrogen. The square root of 16 is 4, 

 and the velocity of oxygen molecules is one-fourth that 

 of hydrogen, or 1524 feet per second. The density of 

 chlorine is 35 '5, say 36, and the square root is 6, and 

 therefore the velocity 1016 feet. In a jar of hydrogen 

 every molecule is travelling 69*27 miles per minute. 

 In a cubic inch the distance accomplished by all is 

 69-27 x 10 23 , and if myriads of jars were tested, and the 

 number of miles calculated, that would practically be 

 the proportional number. In oxygen and chlorine jars 

 the number of miles covered by a molecule would be 

 17-32 and 11 '5 4 respectively, and for those in a cubic 



