36 MAXWELL'S LAW 77 



as is deduced from the observations that are discussed in 

 detail in 55, we obtain 



v = 0-74 fl, 



and, on substituting for O its value 447, as given in 28, 

 v = 332 metres per second. 



The ratio 0'74 calculated for air holds good equally for 

 the other so-called permanent gases, and is also approxi- 

 mately admissible for those that are condensable, so that in 

 general we may assume the ratio of the speed of sound to 

 the mean molecular speed to be about j in round numbers. 



According to the formula, the speed v of sound must 

 decrease with falling temperature, just as the molecular 

 speed H. This theoretical conclusion is confirmed by 

 experiment. 1 According to experiments made by Greely 2 

 in Arctic regions, at temperatures between 8 and 48 C., 

 the speed decreases by 0-603 metres per second with every 

 degree ; its value, therefore, may be represented as a function 

 of the temperature $ by the formula 



v = 332 + 0-603 3 



= 332 (1 + 0-00182 S), 



while the molecular speed is given by the formula 

 n = 447V(1 + 0-00367 3) 

 = 447(1 + 0-00183 S), 

 when 3 is small enough. 



37. Effusion of Gases 



One directly valuable result of the numerical calculation 

 of the mean speed with which the molecules of different 

 gases move is obtained from the fact that from these 

 numbers we can at once infer the speed with which gases 

 will issue through fine openings in a thin wall. We have, 

 therefore, to consider the phenomenon designated effusion 



1 Benzenberg, Gilb. Ann. xlii. 1812, p. 1. 



2 Meteorolog. Zeitschrift, 7. Jahrg. 1890, p. 6 (25. Jahrg. d. Zeitschr. d. ost. 

 Ges. fur Met.). 



