62 



SCIENCE. 



[Vol. XVIII. No. 443 



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ELECTRICAL EVAPORATION.' 



It is well known that when a vacuum tube is furnished with 

 internal platinum electrodes, the adjacent glass, especially 

 near the negative pole, speedily becomes blackened, owing 

 to the deposition of metallic platinum. The passage of the 

 induction current greatly stimulates the motion of the re- 

 sidual gaseous molecules; those condensed upon and in the 

 immediate neighborhood of the negative pole are shot away 

 at an immense speed in almost straight lines, the speed vary- 

 ing with the degree of exhaustion and with the intensity of 

 the induced current. Platinum being used for the negative 

 pole, not only are the gaseous molecules shot away from the 

 electrode, but the passage of the current so affects the nor- 

 mal molecular motions of the metal as to remove some of 

 the molecules from the sphere of attraction of the mass, 

 causing them to fly ofiP with the stream of gaseous molecules 

 proceeding from the negative pole, and to adhere to any ob- 

 ject near it. This property was, I believe, first pointed out 

 by Dr. Wright of Yale College, and some interesting experi- 

 ments are described by him in the American Journal of 

 Science and Art (Third Series, xii. 49, xiv. 169). The pro- 

 cess 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 varies according to physical and chemical constitu- 

 tion ; thus every kind of solid matter requires to be I'aised 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 sur- 

 face between the liquid and the superincumbent gas will not 

 be a plane, but turbulent like a stormy ocean. The mole- 

 cules at the surface of the liquid dart to and fro, rebound 



1 Abstract of a paper read by Professor William Crookes, F.R.S., before 

 the Royal Society, London, on June 11 ; from Nature of July 3. 



from their neighbors, and fly off in every direction. Their 

 initial velocity may be either accelerated or retarded, accord- 

 iog 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 up- 

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

 carried beyond the range of attraction of neighboring mole- 

 cules, 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 surrounding 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 re- 

 gion of gas is increased^ and we say that evaporation is 

 augmented. 



If the initial velocities of the liquid molecules can be ia- 

 creased 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 "Iplatinum requires an intense heat to 

 enable its upper stratum of molecules to pass beyond the 

 sphere of attraction of the neighboring molecules, experi- 

 ment shows that a very moderate amount of negative elec- 

 trification super-adds sufficient energy to enable the upper 

 stratum of metallic molecules to fly 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 evapora- 

 tion of different subjects under the electric stress. 



Evaporation of Cadmium. — A jj-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, tke 

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

 weighed with and without the cadmium. The tube was ex- 

 hausted, and the lower half of the tube was inclosed in a 

 metal poflcontaining paraffine wax, the temperature being 

 kept at 230° C. during the continuance of the experiment. 

 A deposit around the negative pole took place almost imme- 

 diately, 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: — 



Positive pole. Negative pole. 



Original weight of cadmium 9.34 grains. 9.38 grains. 



Weight alter experiment 9.S5 " 1.86 " 



Cadmium volatilized in 30 minutes 0.09 " T.52 " 



Finding that cadmium volatilized so readily under the 

 action of the induction current, a large quantity, about 350 

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

 end of the tube containing the metal was heated to a little 



