﻿216 Prof. W. Ramsay on the Passage of 



dry wood and many minerals ; (2) pores through which gases 

 do not pass under pressure, but pass by their proper molecular 

 movement of diffusion as in artificial graphite ; and (3) pores 

 through which gases pass neither by capillary transpiration nor 

 by their proper diffusive movement, but only after liquefaction, 

 such as the pores of wrought metals and the finest pores of 

 graphite." 



It is noteworthy that Graham considers that gases are 

 liquefied in the pores of metals. But in the experiments here 

 described this can hardly be the case. For at 270° and at 

 higher temperatures hydrides of palladium do not exist, as is 

 conclusively shown byTroost and Hautefeuille ; just as water 

 does not exist in superheated steam. And yet it is at these 

 temperatures that palladium is permeable to hydrogen, and 

 not at temperatures at which hydride of palladium is stable. 



There are several facts which must be borne in mind in 

 seeking for an interpretation for the phenomenon of the 

 passage of hydrogen through palladium. 



First. The hydrogen in the act of passing is a reducing 

 agent, as shown by its behaviour towards the oxides of 

 nitrogen. At such temperatures as were here employed, 

 hydrogen is without action on the lower oxides of nitrogen. 



Second. Bellati and Lussano have shown (Atti R. 1st. 

 Ven. i. series vii. p. 1173) that hydrogen " diffuses " through 

 an iron plate which is used as a negative electrode on electro- 

 lysing dilute sulphuric acid. Their observation has been 

 confirmed and amplified by Shields (Chem. News, lxv. p. 195). 

 Shields has shown that neither lead nor platinum nor pal- 

 ladium allow hydrogen to pass under similar circumstances, 

 and experiments made by myself show that nickel does not 

 allow carbon monoxide to pass at temperatures at which that 

 compound is stable. 



Third. Iron and platinum, as shown by Deville and Debray 

 (Compt. rend. lvii. p. 965), are permeable at a red heat to 

 hydrogen. 



I think these considerations prove that it is necessary to 

 add a fourth class to the three classes suggested by Graham. 

 Graham's first class involves actual holes, that is, passages 

 large in comparison with the molecular diameter ; his second 

 implies pores small compared with molecular diameter, but 

 still greatly exceeding that diameter ; his third class would be 

 termed " solid solution '* in the present state of our knowledge, 

 i. e. when coal-gas passes through india-rubber, the latter 

 dissolves the gas on the side exposed to it, while the gas 

 evaporates from the other side, so as to render the pressure 

 of the dissolved gas equal on both sides of the membrane. 



