July 2, 1891] 



NATURE 



213 



ing experiments are described by him in the American Journal 

 of Science and Arts. ^ The process has been much used for the 

 production of small mirrors for physical apparatus. 



This electrical volatilization or evaporation is very similar to 

 ordinary evaporation by the agency of heat. Cohesion in solids 

 vanes according to physical and chemical constitution ; thus every 

 kind of solid matter requires to be raised to a certain temperature 

 before the molecules lose their fixity of position and are rendered 

 liquid, a result which is reached at widely different temperatures. 

 If we consider a liquid at atmospheric pressure — say, for 

 instance, a basin of water in an open room — at molecular 

 distances the boundary surface between the liquid and the super- 

 incumbent gas will not be a plane, but turbulent like a a stormy 

 ocean. The molecules at the surface of the liquid dart to and 

 fro, rebound from their neighbours, and fly off in every direction. 

 Their initial velocity may be either accelerated or retarded, 

 according to the direction of impact. The result of a collision 

 may drive a molecule in such a direction that it remains part and 

 parcel of the liquid ; on the other hand, it may be sent upwards 

 without any diminution of speed, and it will then be carried 

 beyond the range of attraction of neighbouring molecules and fly 

 off into and mingle with the superincumbent gas. If a molecule 

 of the liquid has been driven at an angle with a velocity not 

 sufficient to carry it beyond the range of molecular attraction of 

 the liquid, it may still escape, since, in its excursion upwards, a 

 gaseous molecule may strike it in the right direction, and its 

 temporary visit may be converted into permanent residence. 



The intrinsic velocity of the molecules is intensified by heat 

 and diminished by cold. If, therefore, we raise the temperature 

 of the water without materially increasing that of the surround- 

 ing air, the excursions of the molecules of the liquid are rendered 

 longer and the force of impact greater, and thus the escape of 

 molecules into the upper region of gas is increased, and we say 

 that evaporation is augmented. 



If the initial velocities of the liquid molecules can be 

 increased by any other means than by raising the temperature, 

 so that their escape into the gas is rendered more rapid, the 

 result may be called " evaporation" just as well as if heat had 

 been applied. 



Hitherto I have spoken of a liquid evaporating into a gas ; 

 but the same reasoning applies equally to a solid body. But 

 whilst a solid body like platinum requires an intense heat to 

 enable its upper stratum of molecules to pass beyond the sphere 

 of attraction of the neighbouring molecules, experiment shows 

 that a very moderate amount of negative electrification super- 

 adds sufficient energy to enable the upper stratum of metallic 

 molecules to tly beyond the attractive power of the rest of the 

 metal. 



If a gaseous medium exists above the liquid or solid, it 

 prevents to some degree the molecules from flying off. Thus 

 both ordinary and electrical evaporation are more rapid in a 

 vacuum than at the ordinary atmospheric pressure. 



I have recently made some experiments upon the evaporation 

 of different substances under the electric stress. 



Evaporation of Cadmium. — A U-shaped' tube was made, 

 having a bulb in each limb. The platinum poles were at the 

 extremities of each limb, and in each bulb was suspended from 

 a small platinum hook a small lump of cadmium, the metal 

 having been cast on to the wire. The wires were each weighed 

 with and without the cadmium. The tube was exhausted, and 

 the lower half of the tube was inclosed in a metal pot containing 

 paraffin wax, the temperature being kept at 230° C. during the 

 continuance of the experiment. A deposit around the negative 

 pole took place almost immediately, and in five minutes the bulb 

 surrounding it was opaque with deposited metal. The positive 

 pole with its surrounding [luminosity could be easily seen the 

 whole time. In thirty minutes the experiment was stopped, and 

 after all was cold the tube was opened and the wires weighed 

 again. The results were as follows : — 



Original weight of cadmium 

 Weight after experiment ... 



Positive pole. Negative pole. 

 9'34grs. 9'38grs. 

 9-25 „ 1-86 „ 



Cadmium volatilized in 30 mins. ... o'og ,, 7'52 ,, 



Finding that cadmium volatilized so readily under the action 

 of the induction current, a large quantity, about 350 grs., of 

 the pure metal was sealed up in a tube, and the end of the tube 

 containing the metal was heated to a little above the melting- 

 Third Series, vol. xii. p. 49, January 1877 ; and vol. xiv. p. 160, 

 September 1877. 



NO. I 1 3 I , VOL. 44] 



point ; the molten metal being made the negative pole, in a few 

 hours the whole quantity had volatilized and condensed in a 

 thick layer on the far end of the tube, near, but not touching, 

 the positive pole. 



Volatilization of Silver.— Silver was the next metal experi- 

 mented upon. The apparatus was similar to that used for the 

 cadmium experiments. Small lumps of pure silver were cast on 

 the ends of platinum wires, and suspended to the inner ends of 

 platinum terminals passing through the glass bulb. The platinum 

 wires were protected by glass, so that only the silver balls were 

 exposed. The whole apparatus was inclosed in a metal box 

 lined with mica, and the temperature was kept as high as the 

 glass would allow without softening. The apparatus was ex- 

 hausted to a dark space of 3 mm., and the current was kept on 

 for li hours. The weights of silver, before and after the experi- 

 ment, were as follows : — 



. Positive pole. Negative pole. 



Origmal weight of silver ... 18-14 grs. 24 63 grs. 



Weight after the experiment ... 18-13 .. 24-44 » 



Silver volatilized in I i hours ... o-oi „ 0-19 ,, 



In this tube it was not easy to observe the spectrum of the 

 negative pole, owing to the rapid manner in which the deposit 

 obscured the glass. A special tube was therefore devised, of the 

 following character :— A silver rod was attached to the platinum 

 pole at one end of the tube, and the aluminium positive pole was 

 at the side. The end of the tube opposite the silver pole was 

 rounded, and the spectroscope was arranged to observe the light 

 of the volatilizing silver "end on." In this way the deposit of 

 silver offered no obstruction to the light, as none was deposited 

 except on the sides of the tube surrounding the silver. At a 

 vacuum giving a dark space of about 3 mm. from the silver, a 

 greenish-white glow was seen to surround the metal. This glow 

 gave a very brilliant spectrum. The spark from silver poles in 

 air was brought into the same field of view as the vacuum glow, 

 by means of a right-angled prism attached to the spectroscope, 

 and the two spectra were compared. The two strong green 

 lines of silver were visible in each spectrum ; the measurements 

 taken of their wave-lengths were 3344 and 3675, numbers which 

 are so close to Thalen's numbers as to leave no doubt that they 

 are the silver lines. At a pressure giving a dark space of 2 mm. 

 the spectrum was very bright, and consisted chiefly of the two 

 green lines and the red and green hydrogen lines. The inter- 

 calation of a Leyden jar into the circuit does not materially 

 increase the brilliancy of the lines, but it brings out the well- 

 known air lines. At this pressure not much silver flies off from 

 the pole. At a higher vacuum the luminosity round the silver 

 pole gets less and the green lines vanish. At an exhaustion of 

 about one-millionth of an atmosphere the luminosity is feeble, 

 the silver pole has exactly the appearance of biing red-hot, and 

 the volatilization of the metal proceeds rapidly. 1 



If, for the negative electrode, instead of a pure metal such as 

 cadmium or silver, an alloy was used, the different components 

 might be shot off to different distances, and in this way make an 

 electrical separation — a sort of fractional distillation. A negative 

 terminal was formed of clean brass, and submitted to the 

 electrical discharge in vacuo ; the deposit obtained was of the 

 colour of brass throughout, and on treating the deposit chemically 

 I could detect no separation of its component metals, copper and 

 zinc. 



A remarkable alloy of gold and aluminium, of a rich purple 

 colour, has been kindly sent me by Prof. Roberts- Austen. Gold 

 being very volatile in the vacuum tube, and aluminium almost 

 fixed, this alloy was likely to give different results from those 

 yielded by brass, where both constituents fly off with almost 

 equal readiness. The Au-Al alloy had been cast in a clay tube, 

 in the form of a rod 2 cm. long and about 2 mm. in diameter. 



' Like the action producing volatilization, the "red heat "is confined to 

 the superficial layers of molecules only. The metal instantly assumes, or 

 loses, the appearance of red heat the moment the current is turned on or 

 off, showing that, if the appearance is really due to a rise of temperature, it 

 does not penetrate much below the surface. The extra activity of the 

 metallic molecules necessary to volatilize them is, in these experiments, con- 

 fined to the surface only, or the whole mass would evaporate at once, as 

 when a metallic wire is deflagrated by the discharge of a powerful Leyden 

 jar. When this extra activity is produced by artificial heat one of the effects 

 IS the emission of red light; so it is not unreasonable to imagine that when 

 the extra activity is produced by electricity the emission of red light should 

 also accompany the separation of molecules from the mass. In comparison 

 with electricity, heat is a wasteful agent for promoting volatilization, as the 

 whole mass must be raised to the requisite temperature to produce a surface 

 action merely ; whereas the action of electrification does not appear to pene- 

 trate much below the surface. 



