very beautifully; electrify further, and the air is perfectly clear.  Next burn turpentine, and electrify gently; the dense black smoke coagulates into black masses over an inch long; electrify further, and the glass is covered with soot, but the air is clear.  Turpentine smoke acts very well, and can be tried on a larger scale; a room filled with turpentine smoke, so dense that a gas-light is invisible inside it, begins to clear in a minute or two after the machine begins to turn, and in a quarter of an hour one can go in and find the walls thickly covered with stringy blacks, notably on the gas-pipes and everything most easily charged by induction.  Next fill a bell-jar full of steam, and electrify, paying attention to insulation of the supply point in this case.  In a few seconds the air looks clear, and turning on a beam of light we see the globules of water dancing about, no longer fine and impalpable, but separately visible and rapidly falling.  Finally, make a London fog by burning turpentine and sulphur, adding a little sulphuric acid, either directly as vapor or indirectly by a trace of nitric oxide, and then blowing in steam.  Electrify, and it soon becomes clear, although it lakes a little longer than before; and on removing the bell-jar we find that even the smell of SO2 has disappeared, and only a little vapor of turpentine remains.  Similarly we can make a Widnes fog by sulphureted hydrogen, chlorine, sulphuric acid, and a little steam.  Probably the steam assists the clearing when gases have to be dealt with.  It may be possible to clear the air of tunnels by simply discharging electricity into the air—­the electricity being supplied by Holtz machines, driven say by small turbines—­a very handy form of power, difficult to get out of order.  Or possibly some hydro-electric arrangement might be devised for the locomotive steam to do the work.  I even hope to make some impression on a London fog, discharging from lightning conductors or captive balloons carrying flames, but it is premature to say anything about this matter yet.  I have, however, cleared a room of smoke very quickly with a small hand machine.

It will naturally strike you how closely allied these phenomena must be to the fact of popular science that “thunder clears the air.”  Ozone is undoubtedly generated by the flashes, and may have a beneficial effect, but the dust-coagulating and dust-expelling power of the electricity has a much more rapid effect, though it may not act till the cloud is discharged.  Consider a cloud electrified slightly; the mists and clouds in its vicinity begin to coagulate, and go on till large drops are formed, which may be held up by electrical action, the drops dancing from one cloud to another and thus forming the very dense thunder cloud.  The coagulation of charged drops increases the potential, as Prof.  Tait points out, until at length—­flash—­the cloud is discharged, and the large drops fall in a violent shower.  Moreover, the rapid excursion to and fro of the drops may easily have caused them to evaporate so fast as to freeze, and hence we may get hail.