204 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



have fallen on the siiifaee of the liijuid, where they are seen floating. 

 If now we take a piece of brown paper, or any convenient material, and 

 rub it sliglitly and hold it over the cup, the ''scum'' will disappear at 

 once, and be replaced by other drops when the electrified body is 

 removed. As in Lord Kayleigh's experiments, a very feeble electrifica- 

 tion is sufficient to cause the absorption of the drops into the body of 

 the licpiid. It is therefore not because there is sujjposed to be any 

 difference in the action of electricity on large and on very small drops 

 that a different conclusion from Mr. BidwelFs has been arrived at, but 

 because all the experiments to be described point to the conclusion 

 that the dense form of condensation is not due to an increase in the 

 size of the drops, but to an increase in the number, accompanied of 

 course by a diminution in the size. 



We may suppose the following to be something like the manner in 

 which the electricity acts on the jet: In a steam jet the rapid move- 

 ments of the drops give rise to frequent collisions, and these result in 

 the coalescence of many of the drops, so that each diop in ordinary 

 condensation is made up of a number; but when the jet is electrified, 

 the electrification prevents the i)articles coming into contact, as they 

 repel each other, and the consequence is we have a greater number of 

 particles in a dense and electrified jet than in an ordinary one. 



Lord Eayleigh's experiments on the action of electricity on water 

 jets support this view. He has shown that in order to produce (coales- 

 cence the electrification must be very slight, and he also points out 

 that the coalescence does not seem to be so much due to electrification 

 as to a difference of electrification, which would appear to cause a dis- 

 charge of electricity to take place between the drops, which ruptures 

 the films, so causing contact. Further, he has shown that when the 

 electrification is strong and the conditions are such that the drops 

 become electrified, the effect is diametrically the opposite, and instead 

 of coalescence the particles now scatter far more than the unelectrified 

 dro])s. Now from the conditions of the experiments with electrified 

 steam jets it is evident that the drops are electrified and are in the 

 same condition as the electrified scattering water jet. We are there- 

 fore entitled to expect that the electricity will prevent and not aid the 

 coalescence of the small drops in the steam jet. 



Other considerations also point to the increase in the density of the 

 jet being due to an increase, and not to a diminution in the number of 

 drops. We know that if we blow steam into air, that the fewer dust 

 nuclei there are in the air the thinner is the condensation; and when 

 the dust is nearly all out of the air, only a fine rain falls which can 

 scarcely be detected by the unaided eye. Further, the evidence from 

 condensation produced by expanding moist air points to the same con- 

 clusion, namely, that the more dust particles there are in the air the 

 denser is the condensation when cooled by expansion, and the purer 



