Page:Proceedings of the Royal Society of London Vol 69.djvu/426

408 An addition to the apparatus was made, a supplementary tube sealed on containing a grain of corrosive sublimate. This was used as being non-volatile at the ordinary temperature, but easily vaporised by heat. The experiment last described was continued, and immediately sifter the phosphoric blue edge appeared fresh hydrogen was let in and exhaustion continued till the faint blue was eliminated. The mercury salt was then heated, when immediately a rich blue edging appeared on the face of each pink stratification and the yellow lines of mercury shone out distinctly. Mercury blue is of a fuller colour than that of the phosphoric blue.

In the 'British Association Reports' for 1865 (Abstracts, p. 15), Mr. Gassiot describes the changes produced in the colours of the stratifications by introducing a water resistance in series with the vacuum tube. Having shown that the blue components of FIG. 6. the stratifications are due to the presence of a trace of mercury with the hydrogen, experiments were commenced to ascertain what difference in the strength of the induced current would be necessary to alter the relative intensities of the pink and blue strata. Accordingly, I fitted up a resistance, shown in fig. 6, consisting of a glass tube, A A, 3 feet long and inch diameter nearly filled with distilled water. Through a cork, B, at the upper end of the tube a copper wire, C C, passes, and by raising or lowering the wire either the whole resistance of the water or any fraction of it can be thrown into the circuit. The upper part of the wire is connected with one pole of the coil and the water is connected with the other pole by means of a small platinum wire sealed through the bottom of the tube.

The wire was pushed down until it touched the platinum at the bottom, thus cutting out the water resistance. The strength of hammer-spring and the exhaustion were arranged to show good pink and blue discs. The wire was then gradually withdrawn, when the blue components gradually faded, and at a resistance of 6 inches of water the stratifications were all pink. Spectroscopic examination showed that in the parti-coloured state mercury was strongly present in all the blue components, together with the C hydrogen line ; the mercury spectrum, however, being in excess. But when the water resistance was put in, and the buttons were all pink, mercury was still to be detected, but the hydrogen spectrum was more prominent. The green line of mercury was always the first to appear,