of the Expansion of Palladium. 53 



uncoiling, the other rolls up on itself. These effects are com- 

 paratively slow at first ; but as the molecular state of the strips 

 is gradually altered, the evolutions are performed through a 

 large sweep with singular rapidity. 



The most striking experiment of all is afforded by the fact 

 that an electrodeposited film of extreme tenuity is capable of oc- 

 cluding hydrogen, and at the same time possesses sufficient 

 tenacity to produce by its expansion a very considerable amount 

 of motion. 



A thin strip of platinum-foil, 200 millims. long by 4 millims. 

 wide, was coiled into a circle (like a watch-spring), the external 

 periphery being varnished. Upon the exposed surface a thin 

 film of palladium was deposited by a small battery (^ litre Bun- 

 sen) from a solution of about 1*6 per cent, of the chloride of 

 palladium, the time of exposure being six minutes. The posi- 

 tive pole was represented by a fine platinum wire, a very small 

 portion of which was immersed. A grey coherent film was thus 

 obtained. The strip was then placed in acidulated water and 

 connected with the zinc end of a small battery. 



In consequence of its absorption, there was no evolution of 

 gas from its surface ; but the metal instantly uncoiled itself, the 

 unattached end passing through an arc of 65°. 



On reversing the direction of the current, the strip as rapidly 

 returned to its normal position. The tenacity of the film soon 

 becomes impaired. 



In order to give an estimate of the thickness of the film, a 

 sheet of platinum-foil, 20 millims. x 20 millims., having there- 

 fore on both sides a surface of 800 square millims., was accu- 

 rately weighed on a delicate assay-balance at the Mint and ex- 

 posed for six minutes, as in the case of the strip, to the chloride- 

 of-palladium solution. The foil, after washing in distilled water 

 and drying in vacuo, showed an increase in weight of 0*0009 grm. 



The following calculation gives the thickness of the film ca- 

 pable of producing so remarkable a result. 



grm. 



U{ ^°^l^ Wm ^ =00000763 cub. centim., 



or *0762 cubic millimetre. 



0-000095 ofamillim.thick, 



or yoJ^fi" °f a millimetre. 

 For the sake of comparison 



gold leaf =27sW mcn = Tolhre millimetre. 



better (before varnishing) to cover one side of the palladium strip with a 

 thin layer of solder, although the simplicity of the arrangement is to some 

 extent sacrificed. 



Sp. gr. assumed to be . 



11-8 



Surface 



•0762 

 800 



