532 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



bears to t!ie velocity of the others. Now, to make use of such a theorem, if 

 p and q be made equal to 4^ and -l-s., which are the proportions between the sines 

 of incidence and refraction for the red and violet rays, the ratio of the velocities 

 sought will come out in even numbers, that of 45 to 44, which differs entirely 

 from Mr. Melvil's. 



Thus, if Mr. Short's observations have led him to conclude, from Mr. Melvil's 

 principles, that the difference of refrangibility cannot be caused by the difference 

 of velocities, when the motion of light is performed in the manner of a projectile, 

 how surer may not his assertion be according to M. de Courtivron's calculation, 

 since they give a difference of time considerably greater? 



XCIII. On some New Electrical Experiments. By John Canton, M. A., 



F.R.S. p. 780. 



The resinous and vitreous electricity of Mr. Du Fay, which arose from his 

 observing bodies of the one class to attract, what those of the other would repel, 

 when each were excited by attrition , received no light till the publication of the 

 second part of Mr. Franklin's experiments ; where it appears, that the one kind 

 of bodies electrify positively, and the other negatively; that excited glass throws 

 out the electric fire, and excited sulphur drinks it in. But no reason has yet 

 been assigned, why vitreous bodies should receive, and resinous bodies part with 

 this file, by rubbing them. Some persons indeed, of considerable knowledge in 

 these matters, have supposed the expansion of glass, when heated by friction, t© 

 be the cause of its receiving more of the electric fluid than its natural share ; 

 but this supposition cannot be made with regard to bodies of the other sort, such 

 as sulphur, sealing-wax, &c. which part with it when treated in the same man- 

 ner. The following experiments, first made at the latter end of December 1 753, 

 and often repeated since, may perhaps cast new light on this difficult subject. 



Having rubbed a glass tube with a piece of thin sheet-lead and flower of 

 emery mixed with water, till its transparency was entirely destroyed; after making 

 it perfectly clean and dry, Mr. C. excited it with new flannel, and found it act 

 in all respects like excited sulphur or sealing-wax. The electric fire seems to 

 issue from the knuckle, or end of the finger, and to spread itself on the surface 

 of this tube, in the beautiful manner represented at a and b in fig. 1, pi. 14. 



If this rough or unpolished tube, be excited by a piece of dry oiled silk, espe- 

 cially when rubbed over with a little chalk or whiting, it will act like a glass 

 tube with its natural polish. And in this case, the fire a}3pears only at the 

 knuckle, or end of the finger; where it is very much condensed before it enters; 

 as at A and b in fig. 2. 



But if the rough tube be greased all over with tallow from a candle, and as 

 much as possible of it wiped off with a napkin, then the oiled silk will receive a 



