88 SCIENTIFIC AMUSEMENTS. 



simplest ways of causing palladium to absorb hydrogen is to 

 decompose acidulated water by means of a voltaic current, whereby 

 two gases are evolved hydrogen at one pole or electrode and oxygen 

 at the other. If the terminals of the wires leading from the 

 voltaic battery are made to be strips of thin palladium foil, the 

 strip attached as the " negative " electrode, at which the hydrogen is 

 disengaged, will absorb most of the liberated gas, and in so doing 

 will increase somewhat in dimensions, the metal swelling as it 

 absorbs the gas, much as water does when it absorbs ammonia 

 (Expt. 74). When a strip of palladium foil, or a thin palladium 

 wire, is fully charged with hydrogen in this way, a slight elevation 

 of temperature suffices to expel a little of the occluded gas ; in 

 consequence the charged metal can be used as a candle or torch ; 

 when the end is held for a moment to a light the heat expels some 

 of the hydrogen, which takes fire and burns ; the heat thus produced 

 causes more hydrogen to be liberated, so that the flame keeps 

 burning for some time, the metal being clothed with a very pale 

 flame, almost invisible in daylight, somewhat as though it had 

 been dipped in strong alcohol, and then fired before the spirit 

 evaporated. 



Expt. 82. Palladium Eels. A curious effect may be produced 

 by taking advantage of the fact that palladium expands when it 

 absorbs hydrogen. Varnish one side of each of two long narrow 

 strips of palladium foil with shellac or other impervious varnish, 

 and then connect them with the wires of a sufficiently powerful 

 voltaic battery, and immerse them in water to which a few drops 

 of sulphuric acid have been added ; the water will then be decom- 

 posed by the current, and the hydrogen evolved at the negative 

 pole absorbed by the palladium strip. As only one side of the 

 strip is in contact with the water, the other being protected by the 

 varnish, only one side becomes charged with hydrogen, and in 

 consequence this side only swells ; so that the strip by and by 

 becomes curved and bent round, ultimately twisting up into a sort 

 of coil or spiral on account of the expansion of the charged side 

 relatively to the other one. "When one strip is thus pretty fully 

 charged, reverse the direction of the current ; hydrogen will now be 

 evolved on the surface of the second strip, which will soon begin 

 to curl up like the first one ; but simultaneously oxygen will be 

 evolved on the surface of the first strip, and this will act chemically 

 on the hydrogen already absorbed, removing it, and consequently 

 discharging the gas dissolved in the metal, and causing the latter 

 to shrink again. The effect of this is that the one strip coils up 

 and the other uncoils, as though they were alive, When this 

 action has gone on a while, it may be renewed in a different way 



