seating; in its correct position the wing piece when clamped down should leave a turn or two of its thread exposed, in order to ensure that the cone does bed properly.
Care should be taken that the nozzle of the cylinder is free from dust before attaching any of these fittings: the best plan is first to blow into it, and finally wipe it round with the finger. Most professional operators hammer the wing piece home with a spanner or other convenient implement a barbarous method and really unnecessary if the cones are in good condition, but, nevertheless, almost always adopted in practice.
Pressure Gauges.—These are useful in determining the amount of gas remaining in a cylinder and are of a very usual type; they may either be screwed on to the cylinder before commencing to work and taken off again to screw on the regulator, or they can be supplied fitted to the regulator itself, in which case they can be observed during the course of the exhibition (Fig. 13). As the same gauge may be used for cylinders of different sizes (though never for those containing
different gases), they simply register in atmospheres, and knowing that a full cylinder shows a pressure of 120 atmospheres, the requisite calculation must be made to determine how many cubic feet are unused.
In the case of oxygen cylinders an approximate idea of the amount of gas remaining can be got by weighing it carefully when known to be either absolutely full or absolutely empty, and re-weighing it when information is required. Oxygen weighs approximately 1.4 oz. per cubic foot, and this is easily detected by an average scale. Coal gas is too light to be gauged in this way.
Gas-Bags and Generators.—It has already been remarked that there are two alternative methods of obtaining gas under pressure for limelight purposes, viz. gas-bags and generators (the latter for oxygen alone: there is no good hydrogen generator that I know of). In both these cases the oxygen is generated by heating a mixture of chlorate of potash and manganese black oxide. In the case of gas-bags the gas is prepared beforehand and passed through suitable purifiers into a rubber gas-bag. With a generator the oxygen is evolved during the exhibition itself; but this method has never come into very general use.
Coal gas or hydrogen is very seldom home generated; a gas-bag can, if necessary, be filled a few miles away and brought full to the place of exhibition, or filled on the spot if gas is laid on; or, failing this, acetylene or ether, or even methylated spirit may be utilised instead.
The bags in use are placed between double pressure boards (if both gases are required under pressure) and weights sufficiently heavy placed on the top (Fig. 14), or with a 'blow-through' jet only the oxygen need be stored in a bag and the coal gas used from the supply main.
Cylinders have, however, so universally superseded these appliances, that space is hardly warranted in fully describing them, especially as any operator wishing to adopt