434 Mr. T. Graham on the Molecular Mobility of Gases. 



experiments amounted to 0*4 and 0*32 per cent, respectively. In 

 7 minutes the carbonic acid observed was 1*02 and 0*90 per cent.; 

 mean 0*96 per cent. The effect of diffusion is now quite sen- 

 sible, and it may be said that about 1 per cent, of carbonic acid 

 has diffused to a distance of half a metre in seven minutes. 



A portion of carbonic acid has therefore travelled by diffusion 

 at an average rate of 73 millims. per minute. It may be added 

 that hydrogen was found to diffuse downwards, in air contained 

 in the same cylindrical jar, at the rate of 350 millims. per minute, 

 or about five times as rapidly as the carbonic acid ascended. 

 In these experiments the glass cylinder was loosely packed with 

 cotton wool, to impede the action of currents in the column of 

 air ; but this precaution was found to be unnecessary, as similar 

 results were afterwards obtained in the absence of the cotton. 

 To illustrate the regularity of the results, I may complete this 

 statement by exhibiting the proportion of carbonic acid found in 

 the upper stratum already referred to, after the lapse of differ- 

 ent periods of time. 





Carbonic acid per cent. 



Experiment 1. 



Experiment 2. 



Mean. 



After 5 minutes 



After 7 minutes 



0-4 



102 



1-47 



1-70 



2-41 



5-60 



8-68 



0-32 

 0-90 

 1-56 

 1-68 

 2-69 

 515 

 8-82 



0-36 

 096 

 1-51 

 1-69 

 2-55 

 5-37 

 8-75 



After 10 minutes 



After 15 minutes 



After 20 minutes 



After 40 minutes 



After 80 minutes 





In eighty minutes the proportion of carbonic acid had risen to 

 8*75 per cent., 10 per cent, being the proportion which would 

 indicate the completion of the process of diffusion. 



The same intestine movement must always prevail in the air 

 of the atmosphere, and with even greater velocity, in the pro- 

 portion of 1 to 1*176, the relative diffusion-ratios of carbonic 

 acid and air. It is certainly remarkable that in perfectly still 

 air its molecules should spontaneously alter their position, and 

 move to a distance of half a metre, in any direction, in the course 

 of five or six minutes. The molecules of hydrogen gas disperse 

 themselves to the distance of a third of a metre in a single 

 minute. Such a molecular movement may become an agency 

 of considerable power in distributing heat through a volume of 

 gas. It appears to account for the high convective power 

 observed in hydrogen, the most diffusive of gases. 



