Page:Philosophical Transactions - Volume 024.djvu/667

scended some quicker than others, according to their Different Magnitudes. That the Descending Globules of Light (as by a strict Observation I have since made) did not slide down the sides of the Glasses, but were carried round by their own weight, as if they were turn'd on an Axis: Which is very well worth notice, That the Descending Globules had a Double Motion, the one Perpendicular, the other as a Rotation on an Axis. And in that motion the Adhering parts of Quicksilver were continually tearing from the sides of the Glasses, producing an Apt form, which in such a medium, from such a Body, exhibits Light. That the smaller Globules, whose weight were not sufficient to cause their Descent, remain'd opake, there being (in this as well as all other Mercurial Experiments) no Light to be obtain'd without Motion. That the same Motion given to the like Globules of Quicksilver in common Air, produce not the same Effect, as I have lately try'd, by forcing Mercury through Leather, by Condensing Air strongly on the surface of it. From all which it seems to appear very Plain, that there is Requir'd the Concurrence of a Proper Figure, Medium, and Motion, to Produce the Mercurial Phosphore.

To try whether so thin a Medium as a Vacuum, or the Nearest Approach to it, was Absolutely necessary in the Production of such a Light as is Discoverable in the Barometer, by putting the Mercury in motion, I made use of the Gage belonging to my Air Pump, as (I thought) the most Proper Instrument for that Purpose. (Which Gage being so well known to this Society, I need not here trouble you with a Description of it.) Upon the Plate of the Pump I plac'd a small Receiver, the Air from which beingwith