216 



Mr. Graham on the Relation [Jan. 14, 



suffered, by four successive discharges of hydrogen from it, a permanent 

 contraction of 23*99 millims. ; that is, a reduction of 3*9 per cent, on its 

 original length. The contractions will be observed to exceed in amount 

 the preceding elongations produced by the hydrogen, particularly when 

 the charge of the latter is less considerable. With another portion of wire 

 the contraction was carried to 15 per cent, of its length by the effect of re- 

 peated discharges. The specific gravity of the contracted wire was 12*12, 

 no general condensation of the metal having taken place. The wire 

 shrinks in length only. 



In the preceding experiments the hydrogen was expelled by exposing 

 the palladium placed within a glass tube to a moderate heat short of red- 

 ness, and exhausting by means of a Sprengel tube ; but the gas was also 

 withdrawn in another way, namely, by making the wire the positive elec- 

 trode, and thereby evolving oxygen upon its surface. In such circumstances 

 a slight film of oxide of palladium is formed on the wire, but it appears 

 not to interfere with the extraction and oxidation of the hydrogen. The 

 wire measured, 



Difference. 



Before charge ...... 443*25 millims. 



With hydrogen 449*90 „ +6*65 millims. 



After discharge 437*31 „ —5*94 ,, 



The retraction of the wire therefore does not require the concurrence of 

 a high temperature. This experiment further proved that a large charge 

 of hydrogen may be removed in a complete manner by exposure to the 

 positive pole (for four hours in this case) ; for the wire in its ultimate 

 state gave no hydrogen on being heated in vacuo. 



That particular wire, which had been repeatedly charged with hydrogen, 

 was once more exposed to a maximum charge, for the purpose of ascer- 

 taining whether or not its elongation under hydrogen might now be facili- 

 tated and become greater in consequence of the previous large retraction. 

 No such extra elongation, however, was observed on charging the retracted 

 wire more than once ; and the expansion continued to be in the usual 

 proportion to the hydrogen absorbed. The final density of the wire 

 was 12*18. 



The wire retracted by heat is found to be altered in another way, which 

 appears to indicate a molecular change. When the gas has been expelled 

 by heat, the metal gradually loses much of its power to take up hydrogen. 

 The last wire, after it had already been operated upon six times, was again 

 charged with hydrogen for two hours, and was found to occlude only 320 

 volumes of gas, and in a repetition of the experiment, 330*5 volumes. The 

 absorbent power of the palladium had therefore been reduced to about 

 one-third of its maximum. 



The condition of the retracted wire appeared, however, to be improved by 

 raising its temperature to full redness by sending through it an electrical 



