VOL. XXIV.] PHILOSOPHICAL TRANSACTIONS. 2/5 



motion, gave no encouragement to hope for any increase of it. I expected 

 to have found the woollens at least discoloured by the friction, which was 

 sometimes made more than moderate. On the contrary, not the least sign 

 of any such thing appeared. The light totally disappeared, on the re-admis- 

 sion of less than a quarter of the recipient's natural contents of air, althoiigh 

 the attrition then made, was as great as it had been at any time before. I 

 cannot discover that the various colours of woollen contributes any thing to 

 the different colours of light, exhibited in any of these experiments. 



ExPER. IV. On the Production of a considerable Light, by the Attrition of 

 Glass on Glass in Facuo, and in common Air. — In this experiment I made use 

 of a globular glass, about 3 inches diameter, fixed on the spindle, as in the 

 last, and to the two arms of the brass spring were tied two slips of thin board, 

 to each of which were first fastened two pieces of glass tube, by putting some 

 small nealed wires through their cavities ; which wires likewise passing through 

 some holes of the board made for that purpose, kept them tight in their places, 

 as in fig. 26. In this manner, being covered with a large receiver, and fitted 

 in all respects as usual, the pump was wrought ; which in a little time had ex- 

 hausted the air from it. Then upon turning the large wheel, a swift motion 

 was given to the included glass globe, in which motion, rubbing on the said 

 tubes, a considerable light was exhibited; so that the whole included apparatus 

 became perfectly distinguishable by it, and would have been much more so, 

 had not the day-light prevented: it being then but very little after 5. P.M. 

 with a clear horizon, and the experiment made in an open room. The colour 

 of the light produced, resembled melted glass ; not only just on the parts where 

 the friction was\nade, but seemingly likewise at the extremities of the tubes 

 A, B, where the globe in its motion did not touch them. 



It was further observable in this experiment, that upon suffering the air to 

 re-enter the recipient at several times, at each time, the motion being given 

 as at first, no sensible decay of light or its colour appeared ; excepting at last, 

 when the tubes by much rubbing became worn, and consequently lessened in 

 their spring, the light (in proportion to the abatement of their force on the 

 moving body) was diminished ; as I have often observed, when the motion has 

 been made for some small time, either in vacuo, or in common air. And had 

 the experiment been begun where it finished, the lesser quantity of light, from 

 that cause, would then have been exhibited in vacuo. Hence it follows, that 

 the different mediums did no ways contribute to the augmentation or diminu- 

 tion of the light produced, but seems wholly to proceed from the weakening 

 of the spring, by the wear of their bodies, which is caused by the violent attri- 

 tion of one upon the other in either medium. I do not find that the polish or 



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