160 Mr. Shelf ord Bidwell on the 



rebound but coalesce, and the entire stream of water, both 

 ascending and descending, appears to become coherent. Now 

 it seems to me certain that the innumerable minute particles 

 of water generated in the steam-jet, each consisting perhaps of 

 only a few molecules, must necessarily come into frequent 

 collision with one another ; for we cannot suppose that they 

 all travel with equal velocities and in exactly the same direc- 

 tion ; and there is no reason why they should not behave 

 just in the same manner as the larger drops of the water-jet, 

 rebounding w T hen they are not electrified, coalescing into 

 larger drops when they are. It is true that the degree of 

 electrification required to produce the phenomenon in the 

 case of the steam-jet is apparently much greater than in the 

 other, a rubbed stick of sealing-wax being altogether inopera- 

 tive. Perhaps this is a consequence of the different sizes of 

 the drops in the two cases. That the actual electrification of 

 the particles in the steam-jet is really very small indeed, is 

 proved by the fact that if two electrified steam clouds are 

 generated in close proximity to each other, they exhibit 

 little, if any, evidence of mutual repulsion when they are 

 similarly electrified, or of attraction when their electrifications 

 are of the opposite kind. 



Lord Rayleigh shows cause for believing that if the elec- 

 trifications of the drops in the water-jet were strictly equal, 

 the phenomenon in question would not occur ; and the reasons 

 why they are not, in fact, equal would just as well apply to 

 the case of the steam-jet. How unequal charges of electricity 

 of the same name are operative in bringing about coalescence 

 Lord Rayleigh does not explain, nor am I prepared to hazard 

 a conjecture on the point. 



Lord Rayleigh concludes his paper by referring to the 

 importance of the investigation from a meteorological point 

 of view. I may do the same. It seems certain that the 

 steam-jet experiments go far towards explaining the cause of 

 the intense darkness which is characteristic of thunder-clouds, 

 as well as of the lurid yellow light by which that darkness is 

 frequently tempered. 



I had made the above described experiments and drawn 

 the above stated conclusions concerning them, and had just 

 requested the Secretary of the Physical Society to accept the 

 present communication, when Prof. Silvanus Thompson, 

 whose knowledge of scientific history is proverbially encyclo- 

 paedic, was good enough to bring to my notice the fact that 

 experiments upon the electrification of a steam -jet had been 

 recently made in Germany by the late Robert Helmholtz. 

 An account of these experiments, which are of the greatest 



