104 On Electrical Evaporation. [June 11, 



below redness the deposited film of gold was easily removed from the 

 walls of the tube in the form of very brilliant foil. 



After having been subjected to electrical volatilisation, the appear- 

 ance of the residual piece of gold under the microscope, using a J-inch 

 object glass, was very like that of electrolytically deposited metal, 

 pitted all over with minute hollows. 



This experiment on the volatilisation of gold having produced 

 good coherent films of that metal, a similar experiment was tried, 

 using a brush of platinum as a negative electrode. On referring to 

 the table it will be seen that the electrical volatility of platinum is 

 much lower than that of gold, but it was thought that by taking 

 longer time a sufficient quantity might be volatilised to enable it to 

 be removed from the tube. 



The vacuum tube was exhausted to such a point as to give a dark 

 space of 6 mm., and it was found, as in the case of gold, that as a 

 coating of metal was deposited upon the glass the resistance rapidly 

 increased, but in a much more marked degree, the residual gas in the 

 tube apparently becoming absorbed as the deposition proceeded. It was 

 necessary to let a little air into the tube about every 30 minutes, to 

 reduce the vacuum. This appears to show that the platinum was 

 being deposited in a porous spongy form, with great power of 

 occluding the residual gas. 



Heating the tube when it had become in this way non-conducting 

 liberated sufficient gas to depress the gauge of the pump 1 mm., and 

 to reduce the vacuum so as to give a dark space of about 3 mm. 

 This gas was not re-absorbed on cooling, but on passing the current for 

 ten minutes the tube again refused to conduct, owing to absorption. 

 The tube was again heated, with another liberation of gas, but much 

 less than before, and this time the whole was re-absorbed on cooling. 



The current was kept on this tube for 25 hours; it was then 

 opened, but I could not remove the deposited metal except in 

 small pieces, as it was brittle and porous. Weighing the brush 

 that had formed the negative pole gave the following results : 



Grains. 



Weight of platinum before experiment 10'1940 



after experiment 8'1570 



Loss by volatilisation in 25 hours 2'0370 



Another experiment was made similar to that with gold and 

 platinum, but using silver as the negative pole, the pure metal being 

 formed into a brush of fine wires. Less gas was occluded during the 

 progress of this experiment than in the case of platinum. The silver 

 behaved the same as gold, the metal deposited freely, and the vacuum 

 was easily kept at a dark space of 6 mm. by the very occasional admis- 



