416 ME. T. GRAHAM ON THE ABSORPTION AND 



" La permeability de la mati^re est d'une nature toute diiFerente dans les corps 

 homogenes, comrae le fer et le platine, et dans des pates plus ou moins discontinues, 

 resserrees par la cuisson ou la pression, comme la terre k creuset, la plorabagine, dont 

 M. Graham s'est sorvi dans ses memorables experiences. Dans les metaux, la porosite 

 rcsulto do la dilatation que la chaleur fait eprouver aux espaces interraoleculaires ; elle 

 est en relation avec la forme des molecules que Ton pout toujours supposer regulieres, 

 et avec leur alignement qui determine le clivage ou les plans de facile fracture des 

 masses cristallisees. C'est cet intcrvalle intermoleculaire que le phenomene de la porosite 

 des metaux purs et fondus accuse avec une evidence eclatante, c'est aussi par ce pheno- 

 mene qu'on peut esperer de calculer la distance des molecules solides aux temperatures 

 elevces oii les gaz peuvent s'y introduire." 



A new kind of porosity in metals is imagined, of a greater degree of minuteness than 

 the porosity of graphite and earthenware. This is an intermolecular porosity due 

 entirely to dilatation. The intermolecular porosity of platinum and iron is not sufficient 

 to admit any passage of gas at low temperatures, but is supposed by M. Deville to be 

 developed by the expansive agency of heat upon the metals, and to become sensible at 

 the temperature of ignition. Such a species of porosity, if it exists, may well be expected 

 to throw light on the distances of solid molecules at elevated temperatures, when gases 

 introduce themselves. The ready passage through platinum of some gases, particularly 

 of hydrogen, and the difficult passage of others render such molecular views the more 

 remarkable. 



The passage of hydrogen through the substance of heated platinum appears in its 

 most simple aspect when the gas is allowed to make its way through the metal into a 

 vacuous space. The experiment of M. Deville, where a tube of platinum charged with 

 nitrogen is placed within a large porcelain tube charged with hydrogen*, was modified 

 by placing the platinum tube, closed at one end, in communication by the other (open) 

 extremity with the Sprengel pump, so that a vacuum was substituted for the nitrogen. 

 It was then easy to observe that a vacuum in the platinum tube was preserved for hours 

 when the external gas admitted into the annular space between the porcelain and plati- 

 num tubes was either atmospheric air or hydrogen at the natural temperature. The 

 tubes being placed across an empty furnace, the latter was now lighted ; and it was seen 

 that, with air circulating outside the platinum, the vacuum remained undisturbed, even 

 when the temperature of the tubes rose to a bright red heat. But when dry hydrogen 

 was driven through the same annular space, the platinum, while continuing impermeable 

 at all temperatures below a dull red heat, began to admit hydrogen to the vacuum as 

 soon as the external porcelain tube became visibly red-hot. In seven minutes the 

 Sprengel pump now delivered 15-47 cub. centims. of gas, of which 15-27 cub. centims. 

 appeared, by explosion with oxygen, to be hydrogen. 



In a repetition of the last experiment, hydrogen dried by sulphuric acid was again 

 allowed to circulate in excess outside the platinum. After a vacuum was once obtained 



• Comptes Bendus, vol. Ivii. p. 965. 



