14 



NATURE 



[July 2, 1891 



It was sealed in a vacuum tube as the negative pole, an alumi- 

 nium Dole beins: at the other side. Part of the alloy, where it 

 joined ths olatinum wire passing through the glass, was closely 

 surrounded with a narrow glass tube. A. clean glass plate was 

 supported about 3 mm. from the rod of alloy. After good 

 exhaustion the induction current was passed, the alloy bemg 

 kept negative. Volatilization was very slight, but at the end of 

 half an hour a faint purple deposit was seen both on the glass 

 plate and on the walls of the tube. On removing the rod from 

 the apparatus it was seen that the portion which had been 

 covered by the small glass tube retained its original purple 

 appearance, while the part that had been exposed to electrical 

 action had changed to the dull white colour of aluminium. 

 Examined under the microscope, the whitened surface of the 

 Austen alloy was seen to be pitted irregularly, with no trace of 

 crystalline appearance. 



This experiment shows that, from an alloy of gold and 

 aluminium, the gold is the first to volatilize under electrical 

 influence, the aluminium being left behind. The purple colour 

 of the deposit on glass is probably due to finely-divided metallic 

 gold. The first deposit from a negative pole of pure gold is 

 pink : this changes to purple as the thickness increases. The 

 purple then turns to green, which gets darker and darker until 

 the metallic lustre of polished gold appears. 



If we take several liquids of different boiling-points, put them 

 under the same pressure, and apply the same amount of heat to 

 each, the quantity passing from the liquid to the gaseous state 

 will diff"er widely in each case. 



It was interesting to try a parallel experiment with metals, 

 to find their comparative volatility under the same conditions 

 of temperature, pressure, and electrical influence. It was 

 necessary to fix upon one metal as a standard of comparison, 

 and for this purpose I selected gold, its electrical volatility 

 being great, and it being easy to prepare in a pure state. 



An apparatus was made that was practically a vacuum tube 

 with four negative poles at one end and one positive pole at the 

 other. By a revolving commutator I was abh to make electrical 

 connection with each of the four negative poles in succession for 

 exactly the same length of time (about six seconds) ; by this 

 means the variations in the strength of the current, the 

 experiment lasting some hours, affected each metal alike. 



The exposed surface of the various metals used as negative 

 poles was kept uniform by taking them in the form of wires that 

 had all been drawn through the same standard hole in the 

 drawplate, and cutting them by gauge to a uniform length ; 

 the actual size used was o"8 mm. in diameter and 20 mm. 

 long. 



The comparison metal, gold, had to be used in each experiment ; 

 the apparatus thus enabled me to compare three different metals 

 each time. The length of time that the current was kept on 

 the revolving commutator in each experiment was eight hours, 

 making two hours of electrification for each of the four 

 negative electrodes ; the pressure was such as to give a dark 

 space of 6 mm. 



The fusible metals, tin, cadmium, and lead, when put into the 

 apparatus in the form of wires, very quickly melted. To avoid 

 this difficulty a special form of pole was devised. Some small 

 circular porcelain basins were made, 9 mm. diameter ; through 

 a small hole in the bottom a short length 'of iron wire, o"8 mm. 

 in diameter, was passed, projecting downwards about 5 mm. ; 

 the basin was then filled to the brim with the metal to be 

 tested, and was fitted into the apparatus exactly in the same way 

 as the wires ; the internal diameter of the basins at the brim 

 was 7 mm., and the negative metal filed flat was thus formed of 

 a circular disk 7 mm. diameter. The standard gold pole being 

 treated in the same wav, the numbers obtained for the fusible 

 metals can be compared with gold, and take their place in the 

 table. 



The following table of the comparative volatilities was in 

 this way obtained, taking gold as = 100 : — 



to the current. By dividing the number.'^ so obtained by the 

 specific gravity of the metal, the following order is found : — 



2-52 

 2 -02 

 I 29 

 071 

 0-47 



Aluminium and magnesium appear to be practically non- 

 volatile under these circumstances. 



The order of metals in the table shows at once that the 

 electrical volatility in the solid state does not correspond with 

 the order of melting-points, of atomic weights, or of any other 

 well-known constant. The experiment with some of the 

 typical metals was repeated, and the numbers obtained did not 

 vary materially from those given above, showing that the order is 

 not likely to be far wrong. 



It is seen in the above table that the electrical volatility of 

 silver is high, while that of cadmium is low. In the two earlier 

 experiments, where cadmium and silver were taken, the cadmium 

 negative electrode in 30 minutes lost 7-52 grs., whilst the silver 

 negative electrode in i^ hours only lost 0-19 gr. This apparent 

 discrepancy is easily explained by the fact (already noted in the 

 case of cadmium) that the maximum evaporation effect, due to 

 electrical disturbance, takes place when the metal is at or near 

 the point of liquefaction. If it were possible to form a negative 

 pole in vacuo of molten silver, then the quantity volatilized 

 in a given time would be probably more than that of cadmium. 



Gold having proved to be readily volatile under the electric 

 current, an experiment was tried with a view to producing a 

 larger quantity of the volatilized metal, A tube was made 

 having at one end a negative pole composed of a weighed brush 

 of fine wires of pure gold, and an aluminium pole at the other 

 end. 



The tube was exhausted and the current from the induction 

 coil put on, making the gold brush negative ; the resistance of 

 the tube was found to increase considerably as the walls became 

 coated with metal, so much so that, to enable the current to pass 

 through, air had to be let in after a while, depressing the gauge 

 \ mm, 

 ■ The weight of the brush before experiment was 35 "4940 grs. 

 The induction current was kept on the tube for 14I hours ; at 

 the end of this time the tube was opened and the brush removed. 

 It now weighed 32-5613, showing a loss of 2-9327 grs. When 

 heated 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 volatilization, the 

 appearance of the residual piece of gold under the microscope, 

 using a ^-inch object-glass, was very like that of electrolytically 

 deposited metal, pitted all over with minute hollows. 



This experiment on the volatilization 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 

 volatilized 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 this way non-conducting 

 liberated sufficient gas to depress the gauge of the pump i 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 



