508 Mr. T. Graham on the Absorption and 



then, that hydrosulphuric acid is to be classed among the non- 

 penetrating gases. The result appears to be : — 



I. Gas capable of passing through a septum of platinum 1*1 mil- 



lim. in thickness at a full red heat. 



Hydrogen (211 cub. centims. per hour). 



II. Gases incapable of passing through a septum of fused plati- 



num 1*1 millim. in thickness at a full red heat. 



Oxygen . . . (not 



to the extent of 0*2 cub. 



centim. 



Nitrogen . . . 



a 



)) 



>} 



Chlorine . 



a 



» 



}> 



Hydrochloric acid 



f) 



> 



a 



Vapour of water . 



)i 



» 



}> 



Carbonic acid 



)> 



) 



)} 



Carbonic oxide . 



)> 



i 



>> 



Marsh-gas (C H 4 ) 



a 



) 



)) 



defiant gas . 



3) 



)} 



)> 



Hydrosulphuric acid 



)i 



)f 



)i 



Ammonia . 



i) 



i 



a 



It remains to be discovered whether a sensible passage of any 

 of these gases could be effected through a platinum septum much 

 reduced in thickness, or through the same septum under the in- 

 fluence of a considerably higher temperature. A fallacious ap- 

 pearance of permeation is sometimes occasioned by the escape 

 from the platinum itself of a small quantity of gas, particularly 

 of carbonic oxide and hydrogen, as will immediately appear. 

 The permeation is in consequence never unequivocal for the first 

 hour or two that the platinum septum is heated. 



One of the curious experiments of M. Deville was repeated, in 

 which hydrogen appears to escape from the platinum tube pretty 

 much as the same gas would escape from a graphite diffusiometer 

 — the platinum tube being full of hydrogen, while the annular 

 space between the platinum and outer porcelain tube was occu- 

 pied by atmospheric air. At the maximum temperature the 

 supply of hydrogen to the platinum tube was shut off, as that 

 gas entered at one end of the tube, while the other end of the 

 platinum tube was left in connexion with a barometer- tube dip- 

 ping into a cistern of mercury. Immediately the mercury began 

 to rise in the gauge tube from the passage of hydrogen outwards 

 through the walls of the platinum tube; and the latter in the 

 end became nearly vacuous from the complete escape of the 

 hydrogen. 



