27. _Mercury Vapour Lamp._--This is an interesting variety of arc light in which the vapour of mercury takes the place of the vapour of carbon. _G_, in Fig. 105, represents a glass tube from which the air has been exhausted. The wires of the lighting circuit are fused into the ends of the tube. At one end, and in contact with one of these wires, is a small pool of mercury. By pulling the cord _c_ the tube is tilted on the pivot _p_, so that a stream of mercury flows along the whole length of the tube and closes the electric circuit. When the tube falls back into its normal position, as represented in the figure, the electric arc persists upon the mercury vapour. Incandescent mercury vapour gives light strong in green, blue, and violet, but deficient in red and yellow. It, therefore, gives nothing its natural appearance but casts a ghastly hue over everything. This lamp was invented in 1902, by Peter Cooper-Hewitt, grandson of the founder of Cooper Union in New York City. It gives a very suitable light for making photographic prints, and is much used for that. This lamp operates upon the 110-volt circuit. It is the longest step yet taken toward getting light without heat, but perhaps shows what we must expect when we reach that goal, namely, unsatisfactory colour values in the light. Probably such is the case with the firefly. 28. _The Moore Light._--In 1896 Prof. D. McFarland Moore brought out his vacuum tube light (Fig. 106). We visited an ordinary dry goods store which had been equipped with this. Glass tubing is put together very much as one would put up a stove pipe or a job of plumbing. The joints are fused and made air-tight by playing a flame upon them after the pipe is up in place. This pipe is led around into all nooks and corners where there would be dark places. The air is pumped out of this tube and a trifling amount of some vapour is introduced, the kind varying according to the tint of colour which is desired. [Illustration: Fig. 106] Metal terminals are fused into the ends of this tube. The tube we saw was seventy-five feet long. A 1000-volt alternating current is applied to the terminals and the vapour becomes incandescent, filling the whole tube full of light. The first thing that the boys remarked was that although the room was brilliantly lighted no object cast a shadow. It seemed as though light was everywhere and there was no chance to screen it off.