TRANSACTIONS OF SECTION A. 635 



2. The current density. For electrolytic analysis Classen gives a current 

 density of 0'05 to 1*0 ampere per 100 sq. cm., and S. P. Thompson U'08 to 0'25 

 ampere per 100 sq. cm. for steel facing. We find that it is advisable to strike 

 the deposit with a density of 0'2 ampere per 100 sq. cm., then reduce to 0'15 to 

 0'18 ampere per 100 sq. cm., and that this should not he exceeded, though 

 densities as low as O'OS can be used. 



The potential ditierence between the terminals we used was 0'7 volt, and this 

 was obtained by placing a single storage cell, voltage 2, in series with a dilute 

 sulphuric acid cell with lead electrodes and a small external resistance. 



3. The electrodes. The cathode must present a perfectly clean surface. "VVe 

 used thin copper sheet and found the best method of cleaning was to flush it with 

 nitric acid and then scrub it with excess of a strong solution of potassium cyanide. 



All parts of the cathode except that on which the deposit is to form must be 

 insulated from the solution. This may be done by coating the necessary parts 

 with Briuiswick black, on which, if perfectly dry, the solution has no action. 



The anode was a sheet of rolled Swedish iron, and to prevent the impurities it 

 contained from mixing with the electrolyte it was enclosed in a porous cell. The 

 accumulation of sulphuric acid in the electrolyte was prevented as much as possible 

 by changing the solution in this cell twice daily. The solution used contained 

 one per cent, ferrous chloride. The surface of the cathode is covered with small 

 conical cavities, due to the formation of microscopic gas bubbles ; in the early stages 

 of the deposition great care must be taken to remove these by periodically exposing 

 the surface to the air and in addition rubbing the surface. By taking these pre- 

 cautions the process can be carried on without interruption, and a firm, coherent 

 deposit obtained of great purity. 



6. On a Magnetic Field Tester. 

 By Professor W. E. Ayrton, F.R.S., and T. Mather. 



7. On idc Velocity of Light in Rarefied Gases through ivhich an Flectrical 

 Discharge is passing, i??/ Edwin Edser, -d.^.C/S*., «W(i Sydney G. 

 Starling, A.R.C.S. 



Lord Rayleigh has published the result of a research relative to the velocity of 

 light in an electrolyte through which an electric current is passing. Dilute 

 sulphuric acid was used, and the conclusion reached was that the velocity was 

 unaffected by the passage of the current. 



It seemed to us worth while to perform a similar experiment to the above, sub- 

 stituting a rarefied gas for the electrolyte. The conditions are somewhat difi'erent, 

 for though it has been shown by Professor J. J. Thomson that the ordinary stratified 

 discharge can be assimilated to a current passing through an electrolyte by con- 

 sidering each stratification to correspond to a Grotthus chain, yet there is left still 

 outstanding the phenomenon known as the kathode rays. Professor J. J. Thomson 

 considers these rays to consist of a number of atoms, each carrying its tubes of 

 force, and moving with a velocity comparable with 2 x 10'' cm. per second. If 

 these tubes of force consist of or comprise a number of vortex filaments of the 

 ether, we might reasonably hope to detect some diSerence in the velocity of the 

 light, according as it moves with or against this ether current. 



The method used by us is essentially that employed by Professor Michelson in 

 re-performing Fizeau's experiment, two vacuum tubes with plane glass ends being 

 substituted for the tubes through which in his experiment water was caused to 

 flow. A discharge was produced in these tubes by a RuhmkorS" coil, and the 

 interference bands were carefully observed. No alteration could be detected on 

 the discharge being reversed. The experiment was repeated at various pressures 

 (determined by the McLeod gauge) with the same result. 



A valid objection which might be raised to the above experiments is, that on 

 account of the extremely short duration of these discharges, which succeeded each 



