1891.] On Electrical Evaporation. 89 



not only are the gaseous molecules shot away from the electrode, but 

 the passage of the current so affects the normal molecular motions of 

 the metal as to remove some of the molecules from the sphere of attrac- 

 tion of the mass, causing them to fly off with the stream of gaseous 

 molecules proceeding from the negative pole, and to adhere to any 

 object near it. This property was, I believe^jirst pointed out by 

 Dr. Wright, of Yale College, and some interesting 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 volatilisation or evaporation is very similar to 

 ordinary evaporation by the agency of heat. Cohesion in solids 

 varies 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 superincumbent gas will not be a 

 plane, but turbulent like a stormy ocean. The molecules at the 

 surface of the liquid dart to and fro, rebound from their neigh- 

 bours, 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 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 the 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 tem- 

 porary 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 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. 



* Third Series, vol. 12, p. 49, January, 1877, and vol. 14, p. 169, September, 1877. 



