Dialytic Separation of Gases by Colloid Septa. 515 



bustion-furnace for one hour, the platinum gave off 8*6 cub. 

 centims. gas, of which 8*2 were hydrogen. The platinum there- 

 fore appears to have occluded altogether 3*79 vols, of hydrogen. 

 The preceding experiment appeared to show a complete sealing 

 up of the occluded hydrogen at low temperatures, seeing that, 

 although nearly four volumes of gas were present, none escaped 

 below a red heat. But to test the effect of time at the tempera- 

 ture of the atmosphere, the platinum, again charged with hy- 

 drogen, was sealed up hermetically in a glass tube, which it 

 nearly filled, and not opened for two months. The air in the 

 tube was then transferred and examined. It did not exhibit any 

 reduction of volume under the electric spark or a pellet of spongy 

 platinum. The air therefore appeared to contain no hydrogen; 

 the latter had not diffused out, but, it is to be presumed, was 

 retained by the platinum without loss. These experiments, 

 although related last, were the first performed in this inquiry. 

 The included hydrogen was never entirely extracted in an hour, 

 and is probably understated. The gas always came off gradu- 

 ally, more than one-half of the whole in the first twenty or thirty 

 minutes. The last results may be stated as follows : — 



1 vol. hammered platinum occluded 2*28 vols, hydrogen. 

 » a » 2"80 „ 



n » m o*79 „ 



The high absorbing-power of the hammered platinum, or 

 rather the low absorbing-power of the fused metal, was ascribed 

 to a mechanical difference between the two — to a more open tex- 

 ture in the former, permitting more free access of hydrogen 

 liquefied, as it may be, to the interior of the metal. 



8. The extrication of occluded hydrogen from platinum had 

 always required a degree of temperature verging upon a red heat, 

 even when aided by a vacuum ; and this remains true of hydro- 

 gen originally absorbed at or near a red heat. But the fact ap- 

 pears to be compatible with the absorption of the gas, under the 

 pressure of the atmosphere, at a considerably lower temperature. 

 Thin platinum-foil was first deprived of a little natural gas by 

 ignition in vacuo in the porcelain tube. The foil was afterwards 

 placed in a glass tube and heated again in a stream of hydrogen 

 to a temperature not exceeding 230° C, for three hours, by 

 means of an oil-bath, and further allowed to cool slowly in an 

 atmosphere of the same gas for several hours. A second glass 

 tube receiver, to which the platinum-foil was transferred, was ex- 

 hausted, as usual, at 20° C. without any sensible evolution of 

 gas. With a red heat superadded, gas came off in twenty minutes 

 (but nearly all in the first seven minutes) to the extent of 0*75 

 cub. centim., of which 0-56 cub. centim. proved to be hydrogen. 



2L2 



