620 Professor Lodge [May 28, 



WEEKLY EVENING MEETING, 



Friday, May 28, 1886. 



Sir Frederick Pollock, Bart. M.A. Vice-President, in the Chair. 



Professor Oliver Lodge, D.Sc. 



The Electrical Deposition of Dust and SmoJce, with special reference to 

 the collection of metallic fume, and to a possible purification of 

 the atmosphere. 



The research of which I have the honour to speak in this theatre to- 

 night, takes its origin in an observation made by Dr. Tyndall, 

 frequently shown by him in this place, and of which he has published 

 an account in your ' Proceedings' for 1870. 



The phenomenon discovered by Dr. Tyndall was a dark or dust-free 

 space formed over a hot body when held in strongly illuminated dusty 

 air. A ilame or hot poker held beneath the concentrated beam of an 

 electric lantern shows it perfectly, and the appearance is as of wreaths 

 of black smoke. [Experiment.] 



That it is not smoke, but anti-smoke, is perhaps rendered most 

 evident by observing the utterly diiferent appearance of smouldering 

 brown paper held in the same place. [Experiment.] In fact it is 

 manifest that what we see in a sunbeam is not the beam itself but the 

 dust illumined by it. It is somewhat misleading, though customary, 

 to speak of the motes as rendering a sunbeam visible ; it is really the 

 beam which renders visible the motes. And the more motes there 

 are, the more there is to see, naturally ; hence smoke in the beam 

 greatly increases its luminosity, while anything which removes or 

 expels the dust-particles destroys its luminosity, leaving a clear dark 

 channel. 



Now what is the cause of this dust-free space ? The first idea 

 was that the dust is burned and destroyed by the flame. But this 

 explanation is negatived by using a moderately warm body, say a hot 

 kettle, and noticing that above this also the dark space is perfectly 

 distinct, though narrow. 



Dr. Frankland next suggested that much of visible dust consisted 

 of moisture, and that it was dried and rendered invisible by warmth. 

 Both explanations can be at once disproved by replacing common 

 miscellaneous dust of unknown origin by a carefully chosen smoke, 

 say that of burnt magnesium, which is certainly neither volatile nor 

 combustible. 



Finally Dr. Tyndall suggested an ingenious mechanical explana- 

 tion, that the air was dragged up in convection currents faster than 

 its supported dust, which consequently got left behind ; and with this 

 ho was content. 



