VOL. XXIV.] PHILOSOPHICAL TRANSACTIONS. 273 



was made, to repeat the experiment at greater rarefactions, the sparks produced 

 still diminished in their quantity and light; till at last, when the receiver was 

 well exhausted of air, then, although a more violent motion was given to the 

 steel than before, yet not the least spark appeared to be struck from it : but a 

 small continued light was visible on the edge of the flint, that was rubbed by 

 the steel. On admitting a little air, some sparks, on the motion given, were 

 discovered of a dull gloomy hue; but on letting in a little more air, I know not 

 by what accident, the whole quantity insinuated, and then on repeating the 

 wheel's motion, the sparks appeared as numerous and as vivid as at first. 



Ex. PER. III. On the Production of a Purple Light, on the Attrition of 

 Glass on Woollen in Vacuo. With the various Phcenomena of the same Expe- 

 riment at several Trials, &c. — The glass used in this experiment was globular, 

 about 4 inches diameter, having a passage through the middle, to receive the 

 spindle, to which it was fastened with corks and a screw. The woollen, against 

 which it was to rub, was such as is commonly sold for gartering, the coarser 

 sort of which I purposely chose for its harshness, thinking it likely to improve 

 the phaenomenon, beyond the list of cloth formerly used for the same purpose. 

 This woollen was wrapped about the arms of the brass spring, described in 

 former experiments, and being screwed down to its place, gently embraced the 

 glass globe, as fig. 25. Being thus prepared, and the large receiver put over 

 all, the pump was wrought, and in a little time the receiver was exhausted. 

 The large wheel being then turned, it gave 15 revolutions to the included 

 globe, to one of its own; in which swift motion, rubbing on the woollen, a 

 fine purple light ensued, the included apparatus being distinguishable by it ; 

 and it continued so during the attrition. On admitting a little air, both the light 

 and its colour diminished. And as the air was at several times permitted to 

 re-enter the receiver, so the light became manifestly more pale, and less vivid. 

 And even when the receiver was replete with air, a small faint light discovered 

 itself on the same attrition given, as at first. What is farther observable in 

 this experiment is, that the purple light seemed to be about the breadth of half 

 an inch, and about one inch high, being no where visible but on each arm of 

 the brass spring, where the glass in its motion rubbed on the woollen. That 

 the light remained steady without the least undulation, notwithstanding the 

 motion of the glass was considerably swift. 



The first time I made this experiment, its success was much the same as I 

 have just now related : but when I came to repeat it twice or thrice, with the 

 same glass, no purple light appeared, a pale one then succeeded in its stead ; 

 nor could all the means I used recover it, till I took a new glass, which after I 

 had used for the same purpose two or three times, served me as the former ; 



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