MOLECULAR MOTIONS AND TEMPERATURE 159 



Although the Brownian movement enables scientists 

 to observe by their effects the actions of individual 

 molecules and has thus extended our knowledge of 

 molecular physics, we shall direct our present attention 

 to the average behavior of a large number of mole- 

 cules. In other words, we shall treat the problem 

 which these haphazard motions present according to 

 what is called the " statistical method." You know, 

 for example, that if you flip a coin it is just as likely 

 to be " heads" as " tails." There are only two pos- 

 sible " events," as they are called, and the chance 

 or probability of one is the same as that of the other. 

 In the same way the chance that you are taller than 

 the average for your age and sex is equal to the chance 

 that you are shorter than the average. If one assumes 

 that you are taller his probability of being right is one 

 chance in two, or one half. On the other hand, what 

 is the probability that you are taller than your two 

 closest friends? There are three possible events, 

 namely, tallest, taller than one, or shortest, which are 

 all equally probable. The probability of any one of 

 them is 1/3. 



Consider then N molecules of a gas in a rectangular 

 box, of dimensions a, &, and c centimeters. (See Fig. 

 13.) What is the probability that any particular 

 molecule is moving more in the direction of the a 

 dimension than along the lines of b or c ? The events 

 are all equally likely and the probability of each is 1/3. 

 Of the N molecules, N/3 may be thought of as 

 moving in the direction of a. 



Not all the molecules, however, are moving with 

 the same velocity. It is just as probable, however, 



