Mr. T. Graham on the Molecular Mobility of Gases. 419 



It is worth observing; what result a plate of more open struc- 

 ture, such as stucco, will give in comparison with graphite. For 

 the graphite plate, a cylinder of stucco, 12 millims. in thickness, 

 was accordingly substituted, and gas allowed to percolate at both 

 low and high pressures, as in the former experiments with 

 graphite. 



1. Under a constant pressure of 100 millims. of mercury, 

 gas was allowed to enter through 100 millim. divisions of the 

 diffusiometer. 



With air, the time in two experiments was 515, and again 

 515 seconds. 



With hydrogen 178 seconds, and again 178 seconds : 



2. Under a pressure beginning with 710 millims. (28 inches) 

 and ending with 660 millims. (26 inches), the time with air was 

 374 and 375 seconds ; mean 374-5 seconds. The time with 

 hydrogen was 129 and 130 seconds; mean 129*5 seconds: 



374-5 



EOT- -8891 ' 



The stucco cylinder of the preceding experiments had been 

 dried over sulphuric acid, without the application of heat. It 

 was further desiccated at 60° C. for twenty-four hours, in order 

 to find whether the porosity would be altered. The ratio of the 

 time of hydrogen to that of air now became 1 to 2*788 at the 

 lower degree of pressure, and 1 to 2*744 at the higher degree of 

 pressure. 



It will be observed that the theoretical diffusion -ratio of 

 hydrogen to air, which is 1 to 3*80, is greatly departed from in 

 these experiments with stucco. The ratio appears to be tending 

 to the proportion of the transpiration- times of the same gases, 

 namely, 1 to 2*04. In an experiment recorded by Bunsen, the 

 ratio observed between the times of hydrogen and oxygen in 

 passing, under a small degree of pressure, through stucco dried 

 by heat was so low as 1 to 2*73, the stucco being probably less 

 dense than in the experiments before us. 



With stucco the permeation of gases under pressure appears to 

 be a mixed phenomenon — to some extent molecular diffusion into 

 a vacuum, such as holds with the plate of graphite, but princi- 

 pally capillary transpiration of gas in mass. 



The diffusiometer was now closed by a plate of white biscuit- 

 ware, 2*2 millims. in thickness. The time of fall at the constant 

 pressure of 100 millims., through a range of forty divisions of 



