may vanish, and that a plate of any metal at that temperature would be about 04 volt positive to a standard gold plate at 16° C.

$78. The approximate numerical values of the temperaturevariations are given in the following table :

It must be clearly understood that these are true temperaturevariations and not permanent changes in the plate caused by exposure to a high temperature.

X. Elimination of Metal-Air Potentials by Solid Non-conducting Films on the Metallic Surfaces.

79. As very great differences of opinion seemed to exist as to the part played by the layer of air which is close to the metallic surface, I devised a method in which it should be removed and a film of solid non-conducting material of a very different chemical nature put in its place. A copper plate which had been polished on glass-paper was filed with a clean dry file which had not been used for any other metal. Its potential with the standard gold plate being

+0.045 volt.

A zinc plate was prepared in an exactly similar way, and with the copper plate gave

+0.655 volt.

The copper plate was now gently heated with a bolt until it was hot enough to melt paraffin-wax; the temperature, about 50° C., required for this is not sufficient to cause sudden permanent change of the copper surface (see § 74). Paraffin-wax was then poured on, and the plate was filed with its own file while covered with molten wax. Thus the fresh surface exposed by the filing came directly into contact with the wax. More wax was poured on and the filings drained off, the plate remaining well covered with wax all the while. It was then allowed to cool, and gave with the bare zinc plate

+0.555 volt,

zinc being positive as before. Thus the change due to substituting paraffin-wax for air next the copper is not more than +0100 volt. I now waxed the zinc in exactly the same way. When it was quite cool it gave with the waxed copper

+0.602 volt,

showing that waxing the zinc had raised its potential

+0.047 volt.

So, on the whole, the substitution cf wax for air on both copper and zinc had only decreased their mutual potential by

+0.053 volt

and the potential of the waxed plates remained nearly constant for several hours. The changes due to waxing the plates as given above were confirmed by the independent comparison of each plate with the standard gold plate. It does not follow that even the small changes which did occur were due solely to the substitution of wax for air, for they may have been caused by slight changes in the surface on account of the filing.

§ 80. An aluminium plate was coated with wax in the way described in § 79, a knife being used to scrape the surface under the molten wax. When cold, this waxed plate gave with a bare zinc one +0.36 volt,

which is about the usual value for bare aluminium and zinc. The removal of the air had therefore not appreciably altered the potential. § 81. I now warmed the waxed aluminium slightly. Its potential with the zinc varied as follows::

which is nearly the same as the temperature-variation of bare aluminium in air.

§ 82. A plate of silvered glass used with the glass side facing a standard plate gave almost the same potential as clean silver in air. This plate, which was practically silver coated with glass, gave a temperature-variation larger than that of silver in air. In this case we have glass in contact with the silver surface instead of air, hut the change does not alter the potential. It may be mentioned that the back of the silver film was painted black, and not coated with glass; but this is of small consequence, since it has been proved by experiment that the condition of the back of a plate does not sensibly affect the volta contact-potential; or, more generally, that if parts of a plate be in different conditions, the potential observed will be the mean of the potentials of the different parts, the importance of each part being proportional to its capacity.

§ 83. On account of the great attraction of sodium for oxygen, it seemed of interest to measure its potential in circumstances which excluded that gas from the surface of the metal. In order to effect this two pieces of thin sheet-glass, each about 6 cm. square, were put into a dish of melted paraffin-wax together with some clean sodium, and a large drop of the sodium was put between the plates of glass and squeezed out into a small plate of 2 or 3 square cm. area. The glass plates, with sodium between them, were taken out of the melted wax and allowed to cool. Since the glass plates were of much larger diameter than the sodium, the edges of the latter were protected by the wax which filled up the space between the plates not occupied by sodium. The flat faces of the sodium were apparently in contact with the glass. The sodium was connected to the electrometer by a fine copper wire. The greater part of the sodium surface was bright or only slightly tarnished, and it remained in almost the same condition for many days, being protected by the glass plates and by the wax which filled the space between them unoccupied by sodium. The first measurements gave sodium

2.86 volts

positive to a tarnished zinc plate, i.e., about

3.56 volts

positive to the standard gold plate. This potential gradually decreased.

§ 84. Experiments were made to make sure that the result was not due to temporary electrification of the glass. For instance I breathed on the glass, causing a conducting layer of impure water to form on its surface. Repeated measurements, made by the usual method, showed that the potential at once fell to a small fraction of

VOL. LXII.

M

a volt, but slowly rose again to nearly its original value as the film evaporated. This shows that the electrification was not a temporary one of the glass surface, for that would not have returned to a definite value. Heating the plate by radiation or washing the glass with benzol caused the potential to rise further, but in no case was the potential quite so high as when the plate was first formed. An even more convincing proof that the potential measured was really that of the sodium, was found in the fact that the sensibility of the apparatus was such as would be given by a plate the size of the sodium. If the electrification had been on the whole surface of the glass, the sensibility, on account of the larger surface, would have been at least ten times as great as that observed.

§ 85. The experiments described in this chapter show that (i) when two metals are coated with the same non-conductor, such as wax or glass, their potential is not sensibly different from that of the bare metals in air; (ii) that temperature-variation still takes place, though air be excluded. These results seem to prove that gaseous films play no essential part in the phenomenon.

"On the Rotation of Plane of Polarisation of Electric Waves by a Twisted Structure." By JAGADIS CHUNDER BOSE, M.A., D.Sc., Professor of Physical Science, Presidency College, Calcutta. Communicated by Lord RAYLEIGH, F.R.S. Received February 14,-Read March 10, 1898.

In my previous papers* I have given accounts of the double refraction and polarisation of electric waves produced by various crystals and other substances, and also by strained dielectrics. An account was there given of the polarisation apparatus with which the effects were studied. In the present investigation effects had to be studied which were exceedingly feeble. The apparatus had, therefore, to be made of extreme sensitiveness; but the secondary disturbances became at the same time more prominent, and the great difficulty experienced was in getting rid of these disturbances.

In one of my communications I alluded to the fact that these secondary disturbances are to a great extent reduced when the radiators are made small. The advantage of a large radiator is the comparative ease with which the receiver can be adjusted to respond to the waves, but this advantage is more than counterbalanced by the increased difficulty with the stray radiation and other disturbances.

"On the Polarisation of the Electric Ray by Double-refracting Crystals," 'Journal of the Asiatic Society of Bengal,' May, 1895, and "On a New Electropolariscope," "The Electrician,' December 27, 1895.