1868.] 



Hydrogen Gas hy Metals. 



427 



This gives for carbonic acid one twenty-thousandth part of the rate of 

 hydrogen. Whether it is a penetration of the same sort, although greatly 

 less in degree, or rather the consequence of a sensible porosity in the 

 palladium (of which it would become the measure), remains uncertain. 



The quantity of hydrogen held by the metal at these high tempera- 

 tures may become too small to be appreciated ; but I presume it is still 

 present, and travels through the metal by a kind of rapid cementation. 

 This extreme mobiUty is a singular property of hydrogen, which was in- 

 volved in the fundamental discovery, by MM. H. Sainte-Claire Deville and 

 Troost, of the passage of that gas through plates of iron and platinum at 

 high temperatures. 



The marked rapidity of the passage of the same gas through a thin 

 sheet of caoutchouc appears to be more capable of explanation on known 

 principles. Caoutchouc of less than O'l millimetre in thickness, if impreg- 

 nated with hydrogen, loses its gas entirely by the most momentary expo- 

 sure to the air. A tube of 2 millimetres in thickness, through which 

 hydrogen and carbonic acid were singly passed, each for an hour, was 

 found to retain — 



Of hydrogen 0*01 13 volume. 



Of carbonic acid 0*2200 „ 



The absorption, then, is in the proportion of 1 hydrogen to 20 carbonic 

 acid ; but the comparative rate of penetration of the two gases through a 

 sheet of caoutchouc is as 1 hydrogen to 2\ carbonic acid ; or the hydro- 

 gen moves eight times as rapidly as the density of its solution would 

 indicate. But these gases differ in diifusibility as carbonic acid 1 to 

 hydrogen 4' 7. The rapid passage of hydrogen through caoutchouc is 

 thus partly explained by the rapid manner in which that gas is brought to 

 one surface of the sheet and conveyed away from the other by gaseous 

 diffusion, xlgain, both substances travel through the substance of the 

 caoutchouc by their diflfusibility as liquids. Suppose hydrogen in that 

 form to be nearly as much more diffusive than the other substance as it is 

 when both are gaseous, then the observed rapid passage of hydrogen 

 through caoutchouc would appear to be fully accounted for. 



Liquid diffusion has also a bearing upon the rapid dissemination of 

 hydrogen through a soft colloid metal, like palladium or platinum, at a 

 high temperature. The liquid diffusion of salts in water is known to be 

 six times as rapid at 100° as at 0°. If the diffusion of liquid hydro- 

 gen increases with temperature in an equal ratio, it must become a very 

 rapid movement at a red heat. Although the quantity absorbed may be 

 reduced (or the channel narrowed), the flow of liquid may thus be in- 

 creased in velocity. The whole phenomena appear to be consistent with 

 the solution of liquid hydrogen in the colloid metal. The " solution 

 affinity " of metals appears to be nearly confined to hydrogen and car- 

 bonic oxide, so that metals are not sensibly penetrated by other gases than 

 these. 



