PROFESSOR THOMAS GRAHAm's SCIENTIFIC WORK. 209 



colloid substrtuces as India rubber, and that their transmission throuuli 

 liquid and colloid septa is then effected by the agency of liquid and 

 not gaseous diffusion. Indeed, the complete suspension of the gaseous 

 function during the transit through colloid membrane cannot be kept 

 too much in view." Mr. Graham seems thus to have recognized at 

 least three distinct modes of gas transmission through a solid or semi- 

 solid septum : 



1st. By a sufficient degree of pressure gases might be forced bodily, 

 i. e. in masses, through the minute channels of a porous septum ; or, in 

 other words, might i)ass through such a septum by transj) initio a, of 

 course in the direction only of the preponderating total pressure. 



2d. As the channels of a porous septum became more and more 

 minute, their resistance to the bodily transmission of gas would be- 

 come greater and greater, and the quantity of gas forced through them 

 less and less, until at length the septum would be absolutely im- 

 permeaisle to transpiration under the particular pressure. But su^ch 

 a septum, of which the individual capillary channels were so small 

 as to offer a frictional resistan(;e to the passage of gas greater than 

 the available pressure could overcome, might nevertheless present a 

 cousiderable aggregate of interspace through which the diffusion i)roper 

 of gases, consequent on their innate molecular mobility, could take 

 place freely in both directions. 



3d. A septum might be (luite free from pores, of any kind or degree 

 of minuteness, and so far be absolutely impermeable to the transmis- 

 sion of gas in the form of gas ; but it might nevertheless permit a 

 considerable transmission of certain gases by reason of their prior 

 solution or liquefaction in the substance of the septum. And whereas 

 the mere passage of gas, by transpiration or diffusion through a porous 

 septum, would take place in thorough independence of the nature of the 

 material of the septum, in this last-considered action, the transmission 

 would take place by virtue of a sort of chemical affinity between the gas 

 and the material of the septum — the selective absorption of the gas by 

 the septum being a necessary antecedent of its transmission; whence 

 it might be said the gas was transmitted because it was first absorbed.' 

 Of course in certain transmissions two, or all three, modes of action 

 might come into play simultaneously. 



IX. 



Occlusion of f/ases hy metals. — The experiments of Devillc and Troost 

 having made known the curious fact of the permeability of ignited 

 homogeneous i)latiinun and ignited homogeneous iron to hydrogen gas, 

 and given some indication also of the permeability of ignited iron to 

 carbonic oxide gas, Mr. Graham, in 18GG, corroborated the results of 

 the French chemists in reference to platinum ; but, nn)difying their 

 method by letting the hydrogen pass into a space kept vacuous by the 

 Sprengel i)ump, instead of into an atmosphere of other gas, assimilated 

 14 s 71 



