OCTOBEE 14, 1898.] 



SCIENCE. 



491 



cules by unobstructed impact on the walls 

 of the containing vessel ; recombination be- 

 ing more and more retarded as the pressure 

 is lessened, because the greater separation 

 of the molecules reduces the frequency of 

 collisions. Helium, perhaps because it is 

 monatomic and therefore incapable of dis- 

 sociation, has a nearly straight curve like 

 hydrogen ; and its ratio, given in the sixth 

 column, varies but little throughout the 



when mixed with a large proportion of 

 other gases, is something like a hundred 

 times that of hydrogen. I shall not be sur- 

 prised to find the conductivity of the pure 

 gas a thousand or more times greater than 

 that of hydrogen ; but let us be conserva- 

 tive, and for the present purpose call it a 

 hundred times and see what follows. I 

 have given it this value at the head of the 

 sixth column. 



Comparison of Gaseous Properties. 



2 _ 3 _ 4 If 



whole range observed, which was more than 

 a thousand millionths. I have taken the 

 density of helium as 2, but Professor Earn- 

 say finds it a little less than this when 

 purified as far as possible by repeated diffa- 

 sions. This makes its relative molecular 

 velocity a little more than .71, and brings 

 it still closer to the observed value of its 

 heat conductivity. 



From the foregoing, we may reasonably 

 conclude that the heat conductivity of gases 

 at low pressures, and their mean molecular 

 velocities, are closely related. Hence, if 

 we can learn the heat conductivity of an 

 otherwise unknown gas we can form some 

 idea of its mean molecular velocity ; and 

 from this, of its specific heat, density and 

 molecular weight. 



As before indicated, the heat conductivity 

 of the new gas at very low pressure, even 



If my inference that the heat conductivity 

 and molecular velocity of gases are directly 

 related is correct, then the molecular veloc- 

 ity of the new gas will be 100 times that of 

 hydrogen, as shown at the head of the fifth 

 column. As is well known, the mean molec- 

 ular velocity of a gas varies directly with 

 the square root of its absolute temperature, 

 and is independent of pressure. The mean 

 molecular velocity of hydrogen at the tem- 

 perature of melting ice has been found by 

 calculation to be 5,571 feet per second. 

 Hence the mean molecular velocity of the 

 new gas at the same temperature will be 

 557,100 feet, or more than 105 miles per 

 second. At anything like this molecular 

 velocity it would be quite impossible for a 

 gas to remain in the atmosphere, unless the 

 space above also contained it. A velocity of 

 ouly about seven miles per second, if uu- 



