The book of lantern ; being a practical guide to the working of the optical (1888)

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70 THE BOOK OF THE LANTERN. has a removable perforated shelf fitted to it, and the box itself is so arranged that it can be wedged tightly in its place. The shelf is to hold the necessary supply of scrap zinc, and as. the acidulated water attacks the metal, hydrogen is rapidly given off, until the water is forced by the pressure of the gas below the shelf, and the action ceases. It is again renewed when gas is drawn off from the tap, for then the water again rises to the zinc, and a fresh supply is generated. This apparatus is clearly a modification of the Dobereiner lamp, in which the gas generated in this manner impinges upon and renders red-hot a -pellet of spongy platinum. At a recent lantern exhibition at the Crystal Palace, where a 30-foot screen was used, pure hydrogen from a bottle fed the lime-light. On one occasion ordinary coal-gas was substituted, with a loss of light which was estimated by those well qualified to form a judgment, at no less than 25 per cent.