316 Royal Society. 



These numbers are suggestive in many ways ; but I will only 

 remark the very high refractive energy of lithium, the practical iden- 

 tity of nickel and cobalt, and the remarkable fact that the specific 

 refractive energy of the metals are (with one or two exceptions) in 

 the inverse order of their atomic weights. 



' c On a Method of making a Direct Comparison of Electrostatic 

 with Electromagnetic Force ; with a Note on the Electromagnetic 

 Theory of Light." By J. Clerk Maxwell, F.R.SS.L. & E. 



The experiments described in this paper were made in the laboratory 

 of Mr. Gassiot, who placed his great battery of 2600 cells of bichlo- 

 ride of mercury at the disposal of the author. Mr.Willoughby Smith 

 lent his resistance-coils of 1,102,000 Ohms; Messrs. Forde and 

 Fleeming Jenkin lent a sensitive galvanometer, a set of resistance- 

 coils, a bridge, and a key for double simultaneous contacts ; and 

 Mr. C. Hockin undertook the observation of the galvanometer, the 

 adjustment of the resistances, and the testing of the galvanometer, 

 the resistance-coils, and the micrometer-screw. The electrical ba- 

 lance itself was made by Mr. Becker. 



The experiments consisted in observing the equilibrium of two 

 forces, one of which was the attraction between two disks, kept at a 

 certain difference of potential, and the other was the repulsion be- 

 tween two circular coils, through which a certain current passed in 

 opposite directions. For this purpose one of the disks, with one of 

 the coils attached to its hinder surface, was suspended on one arm 

 of a torsion-balance, while the other disk, with the other coil behind 

 it, was placed at a certain distance, which was measured by a micro- 

 meter-screw. The suspended disk, which was smaller than the fixed 

 disk, was adjusted so that in its position of equilibrium its surface 

 was in the same plane with that of a " guard-ring," as in Sir W. 

 Thomson's electrometers, and its position was observed by means of 

 a microscope directed on a graduated glass scale attached to the disk. 

 In this way its position could be adjusted to the thousandth of an 

 inch, while a motion of much smaller extent was easily detected. 



An exactly similar coil was placed at the other end of the torsion- 

 balance, so as to get rid of the effects of terrestrial magnetism. 



It was found that though the suspended disk and coil weighed 

 about half a pound, a very slight want of equality between the op- 

 posing forces could be detected, and remedied by means of the mi- 

 crometer. 



The difference of potential between the disks was maintained by 

 means of Mr. Gassiot's great battery. To measure this difference of 

 potential, it was made to produce a current through Mr. Willoughby 

 Smith's resistance-coil, and the primary coil of the galvanometer 

 shunted with a variable resistance. 



The current in the coils was maintained by a Grove's battery, and 

 was led through the secondary coil of the galvanometer. 



One observer, by means of the micrometer-screw, altered the dis- 

 tance of the disks till the suspended disk was in equilibrium at zero. 

 At the same time the other observer altered the shunt till the gal- 



