Memoir of Thomas Graham. 119 



has since been eifected by purely chemical methods, Avliieh are 

 both cheaper and more rapid. 



These experiments on india-rubber naturally led to the 

 study of similar effects produced with metallic septa, which, al- 

 though to some extent previously (^li?erved in jnissniu' tinses 

 through heated metallic tubes, had hceu only iui}>ertect!y uimUt- 

 stood. Thus, when a stream of hyrli-(^-on or carhonu- oxi.h' is 

 passed through a red-hot iron tube, a no mconsiderablc j.ortu")n 

 of the gas escapes through the walls. The same is true to a 

 still greater degree w^hen hydrogen is passed through a red-hot 

 tube of platinum, and Graham showed that through the walls 

 of a tube of palladium hydrogen gas passes, under the same 

 conditions, almost as rapidly as water through a sieve. More- 

 over, our distinguished associate proved that this rapid Tnmsier 

 of gas through these dense metallic septa was .bu-. as m tho^ 

 case of the india-rubber, to an actual chemical c(nnbinati(>n ot 

 its material with the metal, formed at the snrtacc. wliciv the gas 

 is in excess, and as rapidly decomposed on the .'ipc-iti' i:u;o 

 of the septum. He not only recogniz(Ml as bchmoniL^ \n this 

 class of phenomena the very great absovi'tion ot hydntticn !\v 

 platinum plate and sponge in the familiar cx})eriincnt id the 

 Doebereiner lamp, but also showed that this gas is, m some 

 cases at least, a definite constituent of meteoric iron,— a fact ot 

 great interest from its bearing on the meteoric theory. 



We are thus led to Graham's last important discovery, which 

 was the justification of the theory we have been considering, 

 and the crowning of this long line of investigation. As may be 

 anticipated from what has been said, the most marked example 

 of that order of chemical compounds, to which the metallic 

 transpiration of aeriform matter we have been considering is 

 due, is the compound of palladium with hydrogen. Ijraham 

 showed that when a plate of this metal is made the negative 

 pole in the electrolysis of water, it absorbs neariy one thousand 

 times its volume Jf hydrogen gas,-a quantity approximately 

 equivalent to one atom of hydrogen to each atom of palla- 

 dium. He further showed that the metal thus becomes so pro- 

 foundly altered as to indicate that the product of this union is 

 a definite compound. Not only is the volume of the metal 

 increased, but its tenacity and conducting power for electricity 

 are diminished, and it acquires a slight susceptibility to magnet- 

 ism, which the pure metal does not possess. The chenaical 

 qualities of this product are also remarkable. It precipitates 

 mercury from a solution of its chloride, and m general acts as 

 a strong reducing agent Exposed to the action of cblorine. 

 bromine, or iodine, the hydrogen leaves the palladium and en- 

 ters into direct union with these elements. Moreover, although 

 the compound is readily decomposed by heat, the gas cannot be 

 expelled from the metal by simple mechamcal means. 



