For the electrification experiment, a third and insulated disk was clamped between the two steel disks and kept electrified to sparking tension. The arrangement is shown diagrammatically on a smaller scale in Fig. 19. [Illustration: FIG. 19. Arrangement for electrifying a third or middle steel disk to sparking potential while spinning.] The electrification test was exceptionally easy to apply, by connecting the insulated charging pin to a Voss machine in action: because when the disks were spinning and the bands in good condition, the electrification could be instantaneously applied, taken off, reversed, or whatever was desired; and the effect of the sudden lowering of potential by sparks passing between the revolving plates could be exactly looked for. The conclusion of my second _Philosophical Transactions_ paper--that of 1897--is that _neither an electric nor a magnetic transverse field confers viscosity upon the ether, nor enables moving matter to grip and move it rotationally_. QUESTION OF A POSSIBLE LONGITUDINAL MAGNETIC DRIFT. Later I tried a longitudinal magnetic field also; arranging a series of four large electric bobbins or long coils along the sides of a square inscribed at 45° in the optical square, Figs. 11 and 13; so that the light went along their axes. The details of this experiment have been only partially recorded, but the salient points are to be found stated in the _Philosophical Magazine_ for April, 1907, pages 495-500. The result was again negative; that is to say, a magnetic field causes no perceptible acceleration in a beam of light sent along the lines of force. The extra velocity that could have been observed would have been 1/9th of a millimetre per second, or 16 miles per hour, for each C.G.S. unit of field intensity. Another mode of expressing the result is that the difference of magnetic potential applied, namely, a drop of two million C.G.S. units of magnetic potential, does not hurry light along it by so much as 1/50th part of a wave-length. There may be reasons for supposing that some much slower drift or conveyance than this is really caused in the ether by a magnetic field; but if so, the ether must be regarded as so excessively dense that the amount of such a drift for any practicable magnetic field seems almost hopelessly beyond experimental means of detection. CHAPTER VI ETHERIAL DENSITY This leads us to enter upon the question of whether it is possible to determine with any approach to accuracy the actual density or massiveness of the ether of space, compared with those forms of matter to which our senses have made us accustomed.