480 Intelligence and Miscellaneous Articles. 



ON THE DIFFUSION OF VAPOURS THROUGH CLAY CELLS. 

 BY DR. J. PULUJ. 



The apparatus for the experiments in this investigation con- 

 sisted essentially of a clay cell enclosed in a tin-plate box, and 

 connected with a cooling-apparatus and a vertical glass tube. A 

 moderate current of air passed through the box, while the vapour 

 flowed into the cell, and, striking past at the walls of it, diffused 

 reciprocally with the air in the box. The surplus vapour, as well 

 as the air that had diifused through, passed into the cooling- 

 apparatus, where the former condensed, and the air, saturated with 

 vapour at the temperature of the room, flowed into the glass tube. 

 The volume of this air was measured by means of soap-films or 

 very thin disks of mica suspended by soap-water in the tube, by 

 which the pressure could be preserved equal on the two sides of 

 the cell. The outflowing air from the box passed through an ab- 

 sorption-apparatus, the increase in weight of which consequently 

 gave the quantity of vapour that had diffused through in a fixed 

 time ; and from this the volume of the vapour was calculated. 

 Two series of experiments, carried out with steam between 123°-8- 

 145°-3 and 136°'6-144°-9 C, gave the result, that, while the ratio 

 of the volumes of the transdiffused air and vapour remains constant 

 and is almost exactly equal to the square root of the reciprocal value 

 of the vapour-density , the velocity of the diffusion increases with the 

 temperature. Experiments with the vapours of alcohol and ether 

 gave for this ratio somewhat higher values than the numbers cal- 

 culated from the theoretical vapour-densities. The deviation which 

 had been already observed from Graham's law is not of equal 

 amount in the case of every vapour; and the author makes it 

 appear not improbable that the forces acting between the molecules 

 of substances and their vapours, which with some vapours are 

 even more striking, may modify the square-root ratio, and that a 

 case would not be inconceivable in which a vapour of greater 

 density would diffuse through a porous plate more rapidly than 

 one of less density, as is the case with absorbent films of liquid — a 

 reversal of the diffusion-ratio which has also been observed in the 

 osmosis of liquids. 



Meanwhile it is to be regarded as certain that the vapours investi- 

 gated diffuse through clay cells in nearly the inverse ratio of the 

 square root of their densities. 



In the appendix to his memoir, the author discusses Dufour's 

 experiments on the diffusion of dry and moist air through porous 

 plates, demonstrates the inadmissibility of Dufour's assumption 

 that dry air diffuses more rapidly than moist of which the density 

 is less than that of the former, remarks that certain experiments 

 made by Dufour himself must be left unexplained by that assump- 

 tion; and, starting from the presupposition of the result obtained 

 from the experiments he has described, that aqueous vapour diffuses 

 more quickly than air, gives a complete explanation of the experi- 

 ments of Dufour. — Sitzungsb. der Tc. AJcademie in Wien, math.- 

 naturw. Classe, 1877, No. vii. pp. 69-71. 



