ON ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 343 



The half-silvered central plate would be at its best if it reflected light of the 

 same intensity as it transmits. I have generally found the reflection on the side 

 next the air more powerful than upon the side next the glass ; so that the ideal 

 would require the geometric meau of the two reflections to be equal to that of the 

 two transmissions. A very slight .silvering is all that is wanted, such as from its 

 want of coherence and brilliancy would be useless for other purposes ; and the bands 

 appear tolerably black, even though the interfering lights are of decidedly unequal 

 intensities. There is, of course, a reflection from the unsilvered surface of the plate. 

 Owing to a want of parallelism in my apparatus, this image was distinctly separated 

 from the other. The two back reflectors were of flat glass, silvered by the milk- 

 sugar process, and used as specula. 



' The imperfections of the surfaces disturbed the formation of the bands from full 

 accordance with theory. The definition was usually better when the pencils were 

 limited, as by the screens employed to define the incident ray, than when allob- 

 struction was removed. The final adjustments for the distinctness and desired 

 width of bands were made with the eye at the telescope by shifting the reflecting 

 prism, and occasionally by slight displacements of one of the other reflectors. 



The tubes enclosing parts of cd, fe, and containing the electrolyte (diluted sul- 

 phuric acid of nearly maximum conductivity), were closed at the ends by plates of 

 parallel glass. The current entered by lateral attachments, so arranged that liquid 

 (or gas) rising or falling from the platinum electrodes would not at first enter the 

 operative part of the tubes. The diameter of the tubes was about J inch, and the 

 eflective length about 11 inches. 



It will be suificient to give the details of one experiment. The two tubes were 

 connected in multiple arc, and of course in such a manner that the current travelled 

 in opposite directions. The magnitude of the whole current (say, from eight Grove 

 cells) was 1-5 ampere ; so that the current density, in amperes per sq. cm., was 



'^ ^ =-26. 



IT X -38- X 2-54'2 



Now, one of the interfering rays travelled 22 inches, or 56 centimetres, with the 

 current, and the other ray the same distance against the current. On reversal oi 

 the current no shift of the bands could be perceived, under conditions where a shift 

 of '^ ^ of a band must have been evident. Hence we may conclude that a current 

 of the above-mentioned density does not accelerate (or retard) the propagation of 

 light in the ratio of y^^ X to 224 cms. In the liquid we may take X = 4 x 10"* cm. ; 

 and if we reduce the result so as to correspond to density unity, we may say that 

 in dilute sulphuric acid a current of one ampere per square centimetre does not 

 alter the velocity of light by 1 part in 13 millions, or by 15 metres per second. 



It would probably be possible to carry the test ten or fifteen times further by 

 the use of much larger tubes and a more powerful battery, but there seems to be no 

 sufficient encouragement at present to make the attempt. The case would, of 

 course, be very different were anyone to show by a priori argument a reason for 

 expecting an efl'ect of this order of magnitude. 



Electro-Chemical Thermo-Dynamics. (Letter from Professor Willard 



Gibes to the Secretary of the Electrolysis Committee of the British 



Association.^ 



New Haven: November 21, 1887. 



Professor Oiiver J. Lodge, 



Dear Sir, — As the letter which I wrote you some time since concerning the 

 rendement of a perfect or reversible galvanic cell seems to have occasioned some 

 discussion, I should like to express my views a little more fully. 



It is easy to put the matter in the canonical form of a Carnot's cycle. Let a 

 unit of electricity pass through the cell producing certain changes, ^^"e may sup. 



' Probably I might say Jg, but it is best to be upon the safe side. When the con- 

 tact was maintained a slight shift was observed, but in a direction independent of 

 that of the current. 



