1893.] on Fog, Clouds and Lightning. 101 



the nozzle the continuous stream is found to break up into separate 

 drops, which collide with one another, and again rebounding, become 

 scattered over a considerable space. But when the jet is exposed to 

 the influence of an electrified substance, such as a rubbed stick of 

 sealing wax, the drops no longer rebound after collision, but coalesce, 

 and the entire stream of water, both ascending and descending, becomes 

 nearly continuous. Look at the shadow of the jet upon the screen 

 and notice what a magical effect the electrified sealing wax produces. 



There is one other point to which I wish to direct your particular 

 attention. If the sealing wax, or better, the knob of a charged 

 Leyden jar, is held very close to the jet, so that the electrical 

 influence is stronger, the separate drops do not coalesce as before, but 

 become scattered even more widely than when no electrical influence 

 was operating. They become similarly electrified and, in accordance 

 with the well-known law, repel one another. 



We will now remove the water jet, and in its place put a little 

 apparatus for producing a jet of steam. It consists of a half-pint tin 

 bottle, through the cork of which passes a glass tube terminating in a 

 nozzle. When the water in the bottle is made to boil a jet of steam 

 issues from the nozzle, and if we observe the shadow of the steam jet 

 upon the screen we shall see that it is of feeble intensity and of a 

 neutral tint, unaccompanied by any trace of decided colour. A 

 bundle of needles connected by a wire with the electrical machine is 

 placed near the base of the jet, and when the machine is worked 

 electricity is discharged into the steam. A very striking effect 

 instantly follows. The cloud of condensed steam is rendered dense 

 and dark, its shadow at the same time assuming the suggestive 

 yellowish-brown colour. 



I at first believed that we had here a repetition, upon a smaller 

 scale, of the phenomenon which occurs in the water jet. The little 

 particles of condensed water must frequently come into collision with 

 one another, and it seemed natural to suppose that, like Lord Kay- 

 leigh's larger particles, they rebounded under ordinary circumstances, 

 and coalesced when under the influence of electricity. The great 

 majority of the small particles ordinarily formed consisted, I thought, 

 of perhaps only a few molecules, which were dispersed in the air 

 and again converted into vapour without ever having become visible, 

 while the larger particles formed by their coalescence under electrical 

 action were of such dimensions as to impede the more refrangible 

 waves of light. Hence the brownish-yellow colour. 



Other explanations have been proposed. There is the molecular 

 shock theory of the late R. Helmholtz (who, as it turned out, had 

 studied electrified steam jets before I made my own experiments) ; 

 I shall refer to his speculation later. And there is the dust-nucleus 

 theory, which no doubt appears a very obvious one. 



Though I knew that my own hypothesis was not quite free from 

 objection, neither of these alternative ones commended itself to me as 

 preferable ; and so the matter rested until a few months ago, when 



