522 Mr. T. Graham on the Absorption and 



of the mercury. The air was dried by sticks of potash ; but still 

 it did not penetrate the palladium. Dry hydrogen was then 

 conducted to the upper surface of the palladium disk, but still 

 without any penetration by that gas after several hours. Cotton- 

 wool moistened with ether was now placed upon the disk, when, 

 after eight minutes, the confined air within the tube began to 

 expand ; and in the course of an hour longer, the 40'5 volumes 

 of confined air increased to 90*4 (thermometer 18°'5, barometer 

 758), when the expansion ceased. The increase of volume ap- 

 peared to be due entirely to ether-vapour, absorbable by a pellet 

 charged with sulphuric acid. Why hydrogen proved to be inca- 

 pable of penetrating the palladium in such circumstances it is 

 difficult to say. It can only be imagined that the palladium-foil 

 may have previously condensed on its surface a minute film of 

 foreign matter, which rendered the palladium inactive to hydro- 

 gen but not to ether-vapour. 



On the other hand, the penetrating power of hydrogen, here 

 referred to the liquefaction of that gas, appears not to be solely 

 confined to metallic septa. There is reason to suspect that in 

 diffusing through a plate of graphite hydrogen passes in a small 

 proportion as a liquid, without any counterdiffusion of air. Hence 

 the constant excess observed of the diffusive coefficient of hy- 

 drogen, which came out 3'876, 3*993, and 4-067*, instead of 

 the theoretical number 3*8, corresponding to the density of the 

 gas referred to air. Such phenomena of gaseous penetration 

 suggest a progression in the degree of porosity. There appear 

 to be (1) pores through which gases pass under pressure or by 

 capillary transpiration, as in 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 ca- 

 pillary transpiration nor by their proper diffusive movement, but 

 only after liquefaction, such as the pores of wrought metals and 

 the finest pores of graphite. 



Osmium-iridium, 



A portion of small grains of osmium-iridium, amounting to 

 2'528 grms., was exposed to hydrogen through all descending 

 temperatures from a red heat, as the preceding metals had been 

 treated. The osmium-iridium was then heated again to redness 

 in the Sprengel vacuum, to extricate any hydrogen that might 

 have been absorbed. But only a bubble or two of gas, too 

 minute to be measured, passed over in 15 minutes, at a red 

 heat. Osmium-iridium, then, exhibits no absorbent power 



* Philosophical Transactions, 1863, p. 404. 



