ABSORPTION OF HYDROGEN BY PALLADIUM. 89 



by again reversing the current ; the second strip, that formerly 

 coiled up by virtue of its swelling through absorption of hydrogen, 

 now uncoils in consequence of the shrinking through removal of 

 hydrogen ; and vice versa with the other strip. 



It is remarkable that hydrogen, when thus dissolved in or 

 absorbed by palladium, becomes more chemically active than 

 before ; so that chemical changes may be produced in liquids by 

 immersing therein palladium containing occluded hydrogen, that 

 would not be produced by simply bringing the liquids into contact 

 with hydrogen gas. 



Expt. 83. To form Prussian Blue by means of Occluded 

 Hydrogen. Mix together solutions of perchloride of iron and 

 red prussiate of potash (ferricyanide of potassium not the same 

 compound as the ferrocyanide of potasssium used in Expts. 57 

 and 59) ; the mixture will become dark brown sherry colour if the 

 compounds be pure. Into this fluid insert a piece of palladium 

 foil or wire charged with occluded hydrogen; by and by the 

 surface of the metal will become covered with a blue film, arising 

 partly from the action of the occluded hydrogen on the ferri- 

 cyanide of potassium, converting it into ferrocyanide of potassium, 

 which then forms prussian blue with the perchloride of iron, as in 

 Expt. 57 ; and partly from an analogous action of the occluded 

 hydrogen on the perchloride of iron converting it into another 

 chloride of iron, which develops a blue pigment termed TurribuWs 

 blue, by reacting on the ferricyanide of potassium. In either case 

 a chemical reaction is brought about by the occluded hydrogen, 

 which ordinary gaseous hydrogen does not effect so readily, if at 

 all, showing that the chemical activity of the hydrogen is increased 

 by the physical process of absorption by the palladium. 



In consequence of the power possessed by solids of dissolving 

 gases, a remarkable property is possessed by thin plates of certain 

 kinds of solids ; viz., that if on one side of the plate the air in con- 

 tact therewith is continually removed by a pump, whilst the other 

 side is kept constantly in contact with a suitable gas, this gas 

 will dissolve in the solid, pass inwards, and finally make its way 

 to the other side, where it will evaporate again and be removed by 

 the pump. Thus if an india-rubber bag be filled with pieces of 

 wood or other solid objects to prevent its collapsing, and the air be 

 then pumped out of the interior, the outside air will slowly pass 

 through the rubber in this kind of fashion, so that the pump will 

 extract a small continuous stream of gases. Moreover, since the 

 air is a mixture of two gases, oxygen and nitrogen (Expt. 148), 

 and since the former of these dissolves in and passes through 

 rubber more readily than the latter, it results that the air thus 



