can see. I leave it to itself now, resting on its paraffin supports and not touching the zinc, and the spot of light goes back to where it was; showing about three-quarters of a volt positive.

§ 42. I now take this copper wire, which is metallically connected with the zinc plate and the sheath of the electrometer, and bring it to touch the under side of the copper shelf on which the uranium is supported by its paraffin insulators. Instantly the spot of light moves towards the metallic zero, and after a few vibrations settles there. I break the contact; instantly the spot of light begins to return to its previous position, where it settles again in less than half a minute. You see, therefore, that if I re-make and keep made the metallic contact between the zinc and copper plates, a current is continuously maintained through the connecting wire, by which heat is generated and radiated away, or carried away by the air, as long as the contact is kept made. What is the source of the energy thus produced? If we took away the uranium, and sent cool fumes from a spirit-lamp, or shed Röntgen rays or ultra-violet light, between the zinc and copper, the results of breaking and making contact would be just what you see with uranium. So would they beyou have already, in fact, seen them (§ 5)-without either Röntgen rays or ultra-violet light, but with the copper and zinc a little closer together and with a drop of water between them and so would they be with dry ice, or with hot glass, between and touched by the zinc and copper. In each of these six cases we have a source of energy; the well-known electro-chemical energy given by the oxidation of zinc in the last-mentioned three cases; and the energy drawn upon by the cooled fumes, or by the Röntgen rays or ultra-violet light, acting in some hitherto unexplained manner, in the three other cases. We may conjecture evaporations of metals; we have but little confidence in the probability of the idea. Or does it depend on metallic carbides mixed among the metallic uranium? I venture on no hypothesis. M. Becquerel has given irrefragable proof of the truth of his discovery of radiation from uranium of something which we must admit to be of the same species as light, and which may be compared with phosphorescence. When the energy drawn upon by this light is known, then, no doubt, the quasi electrolytic phenomena, induced by uranium in air *, which you have

Experiments made in the Physical Laboratory of the University of Glasgow [§ 33 of Kelvin, Beattie, and Smolan, Proc. R. S. E.; also Nature,' March 11, 1897, and Phil. Mag. March 1898] show this electrolytic conductivity to be produced by uranium to nearly the same amount

seen, will be explained by the same dynamical and chemical principles as those of the previously known electrolytic action of cooled fumes from a spirit-lamp, and of air traversed by Röntgen rays or ultra-violet light.

APPENDIX *.

On a Method of Measuring Contact Electricity. IN my reprint of papers on Electrostatics and Magnetism (§ 400, of original date, January 1862) I described briefly this method, in connexion with a new physical principle, for exhibiting contact electricity by means of copper and zinc quadrants substituted for the uniform brass quadrants of my quadrant electrometer. In an extensive series of experiments which I made in the years 1859-61, I had used the same method, but with movable disks for the contact electricity, after the method of Volta, and my own quadrant electrometer substituted for the gold-leaf electroscope by which Volta himself obtained his electric indications.

I was on the point of transmitting to the Royal Society a paper which I had written describing these experiments, and which I still have in manuscript, when I found a paper by Hankel in Poggendorf's Annalen for January, 1862, in which results altogether in accordance with my own were given, and I withheld my paper till I might be able not merely to describe a new method, but if possible add something to the available information regarding the properties of matter to be found in Hankel's paper. I have made many experiments from time to time since 1861 by the same method, but have obtained results merely confirmatory of what had been published by Pfaff in 1820 or 1821, showing the phenomena of contact electricity to be independent of the surrounding gas, and agreeing in the main with the numerical values of the contact differences of different metals which Hankel had published; and I have therefore hitherto published nothing except the slight statements regarding contact electricity which appear in my Electrostatics and Magnetism.' As interest has been recently revived in the subject in common air, oxygen, and carbonic acid; and to about one-third of the same amount in hydrogen, at ordinary atmospheric pressure; but only to about of this amount in each of these four gases at pressures of two or three millimetres. There seems every reason to believe that it would be non-existent in high vacuum, such as that reached by Bottomley in his Volta-contact experiments (§ 14 above).

6

*First published in the British Association, Swansea meeting, August 1880, and Nature,' April 4, 1881.

of contact electricity, the following description of my method. may possibly prove useful to experimenters. The same method has been used to very good effect, but with a Bohnenberger electroscope instead of my quadrant electrometer, in researches on contact electricity by M. H. Pellat, described in the Journal de Physique for May 1880.

The apparatus used in these experiments was designed to secure the following conditions :-To support, within a metallic sheath, two circular dises of metal about four inches in diameter in such a way that the opposing surfaces should be exactly parallel to each other and approximately horizontal, and that the distance between them might be varied at pleasure from a shortest distance of about one-fiftieth of an inch to about a quarter or half an inch. This part of the apparatus I have called a "Volta-condenser." The lower plate, which was the insulated one, was fixed on a glass stem rising from the centre of a cast-iron sole plate. The upper plate was suspended by a chain to the lower end of a brass rod sliding through a steadying socket in the upper part of the sheath. An adjustable screw on this stem prevents the upper plate from being let down to nearer than about one-fiftieth of an inch, or whatever shortest distance may be wanted in any particular case. A stout brass flange fixed to the lower end of this rod bears three screws, one of which S is shown in the drawing, by which the upper plate can be adjusted to parallelism to the lower plate. The other apparatus used consisted of a quadrant électrometer, and in my original experiments an ordinary Daniell's cell, in my later ones a gravity Daniell's cell of the form which I described in Proc. R. S., 1871 (pp. 253–259), with a divider by which any integral number of per cents. from 0 to 100 of the electromotive force of the cell could be established between any two mutually insulated homogeneous metals in the apparatus.

Connexions. The insulated plate was connected by a brass wire passing through the case of the Volta-condenser to the electrode of the insulated pair of quadrants. The upper plate was connected to the metal sheath of the Volta-condenser, and to the metal case of the electrometer, one pair of quadrants of which were also connected to the case. One of the two terminals of the divider, connected to the poles of the cell, was connected to the case of the electrometer. To the third terminal (the bar carrying the slider) was attached one of the contact wires, which was a length of copper wire having soldered to its outer end a short piece of platinum. The other contact surface was a similar short piece of platinum fixed to the insulated electrode of the electrometer. Hence it will be seen that metallic connexion between the two plates

was effected by putting the divider at zero and bringing into contact the two pieces of platinum wire.

Order of Experiment. The sliding piece of the divider was put to zero, and contact made and broken, and the upper plate raised then the deflexion of the spot of light was observed. These operations were repeated with the sliding piece at different numbers on the divider scale, until one was found at which the make-break and separation caused no perceptible deflexion. The number thus found on the divider scale was the percentage of the electromotive force of the Daniell cell, which was equal to the contact electric difference. of the plates in the Volta-condenser.

[Addendum, November 23, 1880.-Since the communication of this paper to the British Association, I have found that a dry platinum disc, kept for some time in dry hydrogen gas, and then put into its position in dry atmospheric air in the apparatus for contact electricity, becomes positive to another platinum disc which had not been so treated, but had simply been left undisturbed in the apparatus. The positive quality thus produced by the hydrogen diminishes gradually, and becomes insensible after two or three days.]

P.S.-On December 24, 1880, one of two platinum plates in the Volta-condenser was taken out; placed in dried oxygen gas for forty-five minutes; taken out, carried by hand, and replaced in the Volta-condenser at 12.30 on that day. It was then found to be negative to the platinum plate which had been left undisturbed. The amount of the difference was about 33 of a volt. The plates were left undisturbed for seventeen minutes in the condenser, and were then tested again, and the difference was found to have fallen to 29 of a volt. At noon on the 25th they were again tested, and the difference found to be 18. The differences had been tested from time to time since that day, the plates having been left in the condenser undisturbed in the intervals. The following table shows the whole series of these results :

Mr. Rennie, by whom these experiments were made during the recent Christmas holidays, had previously experimented on a platinum plate which had been made the positive pole in an electrolytic cell with an electromotive force of one volt, tending to decompose water acidulated with sulphuric acid; the other pole being a piece of platinum wire. After the plate had been one hour under this influenco in the electrolytic cell he removed it, and dried it by lightly rubbing it with a piece of linen cloth. He then placed it in the Voltacondenser, and found it to be negative to a platinum plate in ordinary condition; the difference observed was 27 of a volt. This experiment was made on October 21; and on November 8 it was found that the difference had fallen from 27 to 07. Mr. Rennie also made similar experiments with the platinum disc made the negative pole in an electrolytic cell, and found that this rendered the platinum positive to undisturbed platinum to a degree equal to about '04 of a volt. The effect of soaking the platinum plate in dry hydrogen gas, alluded to in my first postscript, which also was observed by Mr. Rennie, was found to be about 11 of a volt. Thus in the case of polarization by hydrogen, as well as in the case of polarization by oxygen, the effect of exposure to the dry gas was considerably greater than the effect of electro-plating the platinum with the gas by the electromotive force of one volt.

[K.]

VI. On the Ratio of the Velocities of the Two lons produced in Gases by Röntgen Radiation; and on some Related Phenomena. By JOHN ZELENY, B.Sc., Assistant Professor of Physics, University of Minnesota*.

IT

T is the object of this paper to show that the positive and negative ions, which take part in the conduction of gases exposed to Röntgen rays, move with different velocities when in the same electric field; to determine the ratio of these velocities in several gases; and to consider various phenomena which are consequences of or are affected by the difference in

the two values.

The subject matter will be treated under the following subdivisions:

§ 1. The method for determining the ratio of the two velocities.

§ 2. The apparatus used.

§ 3. The form of tube used for constancy of radiation.

*Communicated by Professor J. J. Thomson.