The illumination possible with this light is almost unlimited, and for really large halls it is, as remarked before, the only substitute for the electric arc. It consists essentially of a blowpipe flame, composed of oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &c., or acetylene air blast, heating to incandescence a block of lime, or other refractory material, and the essential feature is that one at least of these gases must be under pressure. Thirty years ago this was usually achieved by storing the gas in rubber bags, and obtaining the requisite pressure by means of heavy weights; but except in a very few outlying districts this method has now been completely superseded by the use of compressed gas cylinders. The earlier editions of this work contained very full directions for manufacturing gas for storage in bags, but it is so exceptional now to find an operator who uses this method that it seems hardly necessary to devote much space to it, and the same may be said of automatic oxygen 'generators.' The present work will therefore deal chiefly with compressed gas cylinders.
Most elaborate precautions are now enforced by the Board of Trade to ensure the absolute safety of these, and any doubt existing from occasional accidents of years ago may be promptly dismissed. Humanly speaking, an accident nowadays cannot happen, except by such wilful negligence on the part of the maker or filler as would almost render the culprit subject to criminal proceedings.
Compressed gas cylinders are painted a distinctive colour, oxygen for example being black and coal gas or hydrogen red; the screw connections to the pumps, and all nozzle
and regulator fittings, are made with a totally different screw and therefore cannot be interchanged; the cylinders themselves are bound by law to be reannealed and retested under hydraulic pressure at regular intervals; the steel itself has to be of a guaranteed quality; and, in fact, every possible risk is guarded against.
The most usual sizes of cylinders supplied for lantern exhibitions are those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in wooden or hemp cases.
Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size without case being 22 in. by 4 in. This size cylinder will supply an average limelight jet for just over two hours. The extra powerful jets as used for cinematograph work or for illuminating a very large screen take a good deal more, but for the usual apparatus as supplied for ordinary lantern purposes this is a pretty safe figure.
A 12-foot cylinder is therefore the favourite size for a lantern exhibition lasting from an hour to one and a half hours, as it leaves a fair margin for gas used in adjusting the instrument, &c., and a 20-foot cylinder will usually suffice for two such exhibitions.
The price of gas per cubic foot varies with the size of the cylinder, being less for large cylinders than for small ones, and the cost of transit is also less in proportion—hence it is frequently an economy to hire a large cylinder and retain it for several exhibitions. On the other hand most suppliers charge a small rent if a cylinder is retained beyond a definite
time, so this is a question to be decided by each user on its own merits.