cases occurred the phenomena of the pink and blue strata at one exhaustion, changing to all-pink with a concentration of the blue to the front on increasing the vacuum ; the single blue button showing the mercury spectrum. Continued pumping diminished the hydrogen, but apparently did not affect the mercury to the same extent. It appears that the great diffusibility of the hydrogen causes it to be readily pumped out, whilst the mercury is continually being re- plenished by diffusion from the pump.
A slight difference is produced in the purity of the colours of the strata according as aluminium or platinum poles are used. A pair of vacuum tubes was made, one having the usual shaped aluminium poles, the other having platinum poles of a special construction. Each terminal was of double wire, at one terminal bent into the form of a ring, and at the other a straight pole. The ends of the wires forming the poles were sealed through the tube close together but not touching, and terminated in loops outside, so that they could be raised to red or white heat by connecting them with a few battery cells. The arrange- ment will be readily understood by reference to the accompanying drawings (fig. 3). Thus heat could easily be applied during exhaustion,
FIG. 3. (Full size.)
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first to one pole and then to the other, even while the induction spark was passing. At first much gas was liberated from the platinum, but by repeated heating, pumping, and passing the spark, all the occluded gas was abstracted, and then the fillings with hydrogen and subsequent operations were commenced.
The general plan of the apparatus is shown in the drawing (fig. 4). At the end furthest from the pump is the hydrogen generator, A, consisting of a U-shaped tube filled with dilute sulphuric acid, having in one leg a plate of amalgamated zinc, B, and in the other a sheet of platinum, C. Both the platinum and the zinc are connected metallic- ally to platinum wires sealed through the glass. A funnel with a stopper, D, sealed to the outer limb of the generator admits dilute acid when required. A tap, E, on the other limb enables the reservoir of hydrogen to be disconnected from the rest of the apparatus. Following this tap is a battery of three tubes, one F, containing small lumps of
