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plate of mica was longer and wider than the gold leaves, and was connected with a small piece of iron wire, capable of moving up and

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down a tabe sealed into the top of the bulb. By means of an outside magnet the mica plate could thus be lowered between the gold leaves or raised ont of their way, as desired. The tube was exhausted to about the millionth of an atmosphere, the mica plate being held quite above the leaves. One side of the bulb was then heated, and the

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leaves permanently charged by means of the excited ebonite. The mica plate was now carefully lowered. As it came between the gold leaves they diverged further apart, and kept so as long as the mica plate was between them. On removing the plate the leaves reassumed their former divergence. This could be repeated any number of times.

A similar piece of apparatus (fig. 3) was made, only instead of a mica plate coming between the leaves, a mica cylinder, a, capable of being raised and lowered outside the divergent leaves, was employed. I was not able to get entirely concordant results with this, owing to the friction of the mica developing electricity on the inner surface of the glass tube; but in all cases, when the cylinder was raised until it covered the electrified leaves, it had the effect of diminishing the angle which they formed with each other.

The following experiments were also tried :—the leaves being separated about 160°, as at fig. 4, A, one side of the tube was slightly

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heated by a spirit flame. The leaf on that side fell to a vertical position, and remained so when all was cold, the other leaf sticking out as before, as at B. This would seem to show that the divergence of the leaves in this case was not so much due to their mutual repulsion, as to an attraction exerted on each of them by the inner surface of the glass tube. The remaining divergent leaf could be slightly lowered when the glass tube above it was warmed with a bunch of cotton wool dipped in hot water. On cooling the leaf rose again to its original position. When this side of the tube was also heated with a lamp, the leaf was repelled down, but not so readily as the other had been, and when the tube got cold, it rose to nearly its former position. This was repeated several times with uniform results. When the leaf was repelled down, the vertical leaf also moved away, so as to keep the same angle between them. It is therefore evident that the leaves themselves were also charged.

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Fig. 4, C, shows the two positions of the leaves, aa before applying heat to the side c of the tube, and bb after heating the glass at c.

The tube was now heated on both sides, causing the leaves to come nearer together as shown at fig. 4, D. While the glass was warm the cylinder was raised so that it surrounded the leaves : this caused them to get a little closer together, and they kept in this position, shown at E, after the whole apparatus was quite cold.

After remaining thus for some time, the cylinder was lowered, and the leaves widened out and took up the position shown at bb, fig. 4, C. They did not return to the position aa, showing that their divergence was now owing to their own mutual repulsion, and not to an attraction of one or other to the electrified glass.

In December, 1877, I totally immersed one of these exhausted glass bulbs in a vessel of water; the gold leaves having previously been charged, and standing at an angle of 112° from one another, as at fig. 5. The water was connected electrically with “earth,” and the whole was set aside in a cabinet on the 1st of January, 1878.

At the present time, after having remained in this condition for thirteen months, the leaves form exactly the same angle with one another which they did when they were first put in the cabinet.

VOL. XXVIII.

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From this experience I think we may consider that at an exhaustion of a millionth of an atmosphere, air is an absolute non-conductor of statical electricity. It is, therefore, legitimate to conclude that the vacuum of interstellar space offers equal obstruction to the discharge of electrified bodies, without necessarily interfering with their mutual repulsion if similarly electrified. It is possible that in these facts an explanation may be found of some obscure celestial phenomena.

II. “On the Reversal of the Lines of Metallic Vapours.” No. IV.

By G. D. LIVEING, M.A., Professor of Chemistry, and J.
DEWAR, M.A., F.R.S., Jacksonian Professor, University of
Cambridge. Received February 12, 1879.

In the experiments described in the following communication, instead of introducing the substances to be observed in the metallic form into our tubes, we have endeavoured to overcome, to some extent, the diffi. culty of the presence of impurities by making use of reactions which should generate the metallic vapours within the tubes. For this purpose we have generally employed the great reducing power of carbon and of aluminium at high temperatures.

In a former communication (“Proc. Roy. Soc.," vol. xxvii) we described the reversal of the two blue lines of cæsium and the two violet lines of rubidium by the vapours of those metals, produced by heating their chlorides with sodium in glass tubes. It might be doubtful from these experiments whether the absorption were due to the metals or to the chlorides. To decide this question, we first tried cæsium chloride by itself, heated in a tube such as we used before. No absorption lines could be seen, although a good deal of the chloride had been vaporized and distilled to the cool part of the tube. The experiments were next repeated, both with rubidium and cæsium

chlorides along with metallic lithium. The two violet lines of rubidium and the two blue lines of cæsium were reve

eversed, as when sodium was used instead of lithium, and as the lithium gave no sensible vapour, the observations could easily be continued for a much longer time with the same tubes. No other absorption lines could be discerned. It may be observed, however, that it is not easy to obtain a source of light sufficiently rich in the least refrangible red to allow of observations on the absorption of light so little refrangible as the red rubidium lines. A platinum wire, heated nearly to fusion by an electric current, appeared to give the brightest light in this part of the spectrum, but of that light no definite absorption by the rubidium could be observed in the red. We then had some mixtures of carbonate of cæsium with carbon, and of carbonate of rubidium with carbon, prepared by charring the tartrates; and observed the results of heating these mixtures in narrow porcelain tubes, placed vertically in a furnace, as described in our first communication on this subject (“Proc. Roy. Soc.," vol. xxvii). A small quantity of the cæsium mixture, introduced into a tube at a bright red heat, showed instantly the two blue lines reversed and so much expanded as to be almost in contact. The width of the dark lines decreased as the cæsium evaporated, but they remained quite distinct for a very long time. A similar effect was produced by the rubidium mixture, only it was necessary to have the tube very much hotter, in order to get enough of violet light to see the reversal of the rubidium lines. In this case the two lines were so much expanded as to form one broad dark band, which gradually resolved itself into two as the rubidium evaporated. The reversal of these lines of cæsium and rubidium seems to take place almost or quite as readily as that of the D lines by sodium, and the vapours of those metals must be extremely opaque to the light of the refrangibility absorbed, for the absorption was conspicuous when only very minute quantities of the metals were present. The red, yellow, and green parts of the spectrum were carefully searched for absorption lines, but none due to cæsium or rubidium could be detected in any case. It is perhaps worthy of remark that the liberation of such extremely electro-positive elements as cæsium and rubidium from their chlorides by sodium and by lithium, though it is probably only partial, is a proof, if proof were wanting, that so-called chemical affinity only takes a part in determining the grouping of the elements in such mixtures; and it is probable that the equilibrium arrived at in any such case is a dynamical or mobile equilibrium, continually varying with change of temperature.

Our next experiments were with charred cream of tartar in iron tubes, arranged as before. In this case a broad absorption band appeared, extending over the space from about wave-length 5,700 to 5,775, and in some cases still wider, with edges ill-defined, especially the more refrangible edge. By placing the charred cream of tartar in

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