HYDROGEN AS A GAS AND AS A METAL. Z\}\) 



Witlioiit disturbiug the apparatus, we now connect, by means of the 

 binding-screw and a copper wire, the palladinra spiral with the zinc 

 phite of a galvanic battery — one or two cells are sufficient — and the tin 

 rod is connected with the opposite plate of the battery. Innnediately 

 on making contact, the water is decomposed by the electric current, hy- 

 drogen being liberated at the surface of the palladium, and oxygen on 

 the tin. The latter gas is set free in bubbles, but the hydrogen, instead 

 of being liberated in the gaseous form, is absorbed by the palladium ; 

 the volume of the metal is thereby increased, and the coil opens out as 

 if endued with vitality. In doing this the glass needle is made to travel 

 over several feet on the screen, and assumes the i)osition represented by 

 the dotted line b. 



In this way we are able easily to show the expansion of the ptilladiuui 

 then charged with hydrogen : but it is obvious that, on the removal of 

 the hydrogen, the needle should return to its original position. lu 

 order to take away the hydrogen from the palladium, we have simply to 

 reverse the poles of the battery ; that is to say, we now connect the tin 

 bar with the negative wire, and the palladium with the other pole. 

 Now, on passing the current, oxygen is set free at the surface of the 

 palladium spiral, and hydrogen on the tin. The oxygen, liberated in 

 Juxtaposition with the b^'drogenized palladium, unites with the hydro- 

 gen, and forms water, no material amount of oxygen appearing at the 

 surface of the palladium until all the hydrogen has been removed from 

 the latter. 



If we v/atch the effect of the change produced in this way on the 

 screen, we fiud that, according as the hydrogen is removed from the 

 palladium, the needle begins to move from the position it assumed at Z>, 

 when the spiral was charged with hydrogen, or alloyed with hydro- 

 genium, if we prefer the term, until it returns to the starting-point a. 

 Instciid of resting there, as it ought to do, however, it ti-avels on steadily 

 until it reaches the position c; thus demonstrating the singular fact 

 that the contraction of the palladium on losing its charge of hydrogen 

 is double the amount of its previous expansion. Proiessor Graham 

 accounts for this singular phenomenon by supposing that the particles 

 of the metal slide over each other in a lateral direction, so that the band 

 of the palladium is unusually shortened, though its density is not in- 

 light, so that tho latter may be reflected iu a vertical diroctiou, as indicated by the 

 dotted Hue. Immediately above this mirror is placed on a wooden support the flat 

 glass cell D, about an inch in depth, and of any required size. Above this vessel is 

 placed a double convex leus, E ; the height at which this must be placed above D will 

 depend on the focus of the lens, aud is readily fouud by ascertaining tho point at 

 which the image of the object placed iu D is most clearly defined on the screen. The 

 rays reflected from the first mirror, B, and having passed through D and E, now fall 

 on a second mirror, C, by which means they are projected on to the screen, aud, when 

 the focus is properly adjusted by shifting E, a well-defined image of any object placed 

 at D is shown erect on the sheet. 



