YOL. XXIV.] PHILOSOPHICAL TRANSACTIOV8. 255 



the quicksilver into small particles, descending all around the exposed sides of 

 both the glasses, and appearing like a shower of fire in a very surprising manner. 

 The form of the receiver, as well as the included glass, was very distinguishable 

 by its light, and continued so till all the mercury had entered through the fun- 

 nel. What more occurred to my observation was, that the descending mercury 

 resembled more the falling of snow, by the slowness of its motion, than that of 

 rain: that none of it appeared luminous but what descended contiguous to the 

 sides of the glasses: that some of the globules of mercury descended quicker 

 than others, according to their different magnitudes: that the descending glo- 

 bules of light did not slide down the sides of the glasses, but were carried 

 round by their own weight, as if they revolved on an axis : and, what was re- 

 markable, that the descending globules had a double motion, the one perpendi- 

 cular, the other as a rotation on an axis ; and, in that motion, the adhereing 

 parts of quicksilver were continually tearing from the sides of the glasses, pro- 

 ducing 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 de- 

 scent, remained opaque, there being (in this, as well as all other mercurial ex- 

 periments) no light to be obtained 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 tried, by forcing mercury through leather, by condensing air 

 strongly on its surface. From all which it seems to appear very plain, that 

 there is required the concurrence of a proper figure, medium, and motion, to 

 produce the mercurial phosphorus. 



ExPER. III. Showings that it requires not so thin a Medium, as is made by 

 the Weight of the Mercury in the Torricellian Experiment, to produce the Mer- 

 curial Phosphorus. — 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 gauge belonging to my air-pump. Upon the plate of the pump 

 I placed a small receiver, the air from which being exhausted, the quicksilver 

 in the gauge was elevated to 29 inches and a half. Then permitting some air to 

 re-enter the receiver by the cock, the mercury in the gauge descended, and 

 made several vibrations before it became stagnant: in all which it appeared lu- 

 minous only while descending, till the quicksilver was purposely broken by a 

 violent agitation of it: then the separate parts appeared light on their under 

 surfaces, which became concave during their ascending, as the other were when 

 they exhibited their light in descending: the convex surfaces being always 

 opaque. These appearances continued on every admission of air, till near the 

 half was admitted. But after that, though the mercury had the same motion 



