82 Sir Oliver Lodge [March 24, 



composed of clean steam blown from a boiler into a bell jar or globe, 

 so as to make a cloud, or clean country mist ; and in that case when 

 the point is electrified the drops are seen to coalesce, and therefore 

 rapidly to grow in size, under the action of the electricity, until it 

 becomes a Scotch mist or fine rain, too heavy for suspension, and so 

 rapidly falls as it does in nature. Or the air of the jar may be 

 rendered foul beforehand with dust or other impurity, or it may be 

 contaminated with burnt sulphur, before admitting the steam, so as 

 to imitate the effect of water- vapour mingled with the products of 

 the combustion of coal ; and then the country mist becomes a town fog 

 — thick, dense, and yellow ; but the electricity clears it just the same. 

 Instead of blowing steam in from a boiler, the moisture of the 

 atmosphere itself may be used in the following way : — Take a large 

 glass globe with a bottle neck, fit the neck with a plug through 

 which a tube passes leading to a compression-pump, such as a bicycle- 

 tyre pump, a strong bottle of water being interposed, so that any air 

 supplied has to ])ubble through the water ; it is well also to rinse the 

 inside of the globe with water, because otherwise it becomes misty 

 and is soon difficult to see through. Things being thus prepared, 

 burn a scrap of sulphur inside the vessel, which may be conveniently 

 done by having a little cotton wick previously soaked in melted 

 sulphur, and then, when cold and dry, lighted for a few seconds in 

 the mouth of the globe ; only a trace of burnt sulphur is wanted, 

 otherwise it will itself make a smoke ; then put the bung into the 

 neck of the globe and slowly pump air into it (which, by bubbling 

 through the water, becomes fairly damp), until the pressure has risen 

 as much as is safe. Wait a short time, say a quarter of a minute 

 or so, for the heat of compression to escape, and then remove the 

 plug. It will indeed blow itself out when slightly loosened, and the 

 compressed air will instantly expand, thereby chilling itself and 

 depositing its vapour in the visible form of mist or fog — town fog in 

 this case, because of the presence of the sulphur-combustion products — 

 the thick, dense, yellow variety. There it will hover and remain for 

 a long time, completely filling the globe as an opaque cloud, which is 

 very visible ; but to study its detail and behaviour it may be illu- 

 minated by the beam of an electric arc, or in the daytime it may be 

 well seen by looking through it at a window behind the globe. Then 

 introduce a point — an insulated point — through the neck into the 

 interior of the globe, and supply the electricity by momentary 

 excitation of the coil. The instant the electricity is supplied the fog 

 is seen to be in motion, black or dust-free spaces arise in it, the 

 particles begin to coalesce and cling together, and in a very short 

 time the whole of the fog has disappeared, being deposited upon the 

 sides of the vessel as a dirty wet deposit, or, if dry smoke has been 

 used, as a kind of black snow. With such a smoke as burnt mag- 

 nesium, the effect of an imitation snow-storm in the air is very soon 

 produced by the electricity, and the walls and floor become coated 

 over with white. 



