Alternatively, of course, cylinders can be purchased, and the question of rent does not then come in; also gas is supplied a little cheaper in a customer's own cylinder than if sent on hire. If purchase is decided on it is frequently an economy to buy two, or two of each gas, if coal gas cylinders are required as well.
The whole contents of the cylinders can then be used up without waste, as if a cylinder should become exhausted during the course of a lecture, it is only a matter of a minute or two to change over to the spare one, whereas the compressors are required by law to empty out every cylinder returned to them for refilling, and any remaining gas is thereby wasted.
It is extremely tantalising, to say the least of it, to find the pressure gauge indicating that there is, say, 8 feet of gas remaining in a cylinder, and to be compelled to waste this or else risk running short for the next exhibition, and duplicate cylinders are the only way of avoiding the loss.
The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb. per square inch, and are closed by strong screw nozzles. The keys for opening or closing these are of three types, viz. the 'T' pattern, 'Spanner' pattern, and that known as the 'Double Lever' type. This latter is so made that in closing the valve it shuts up to half its length and
opens out to double the leverage when being used to open the cylinder (Fig. 9). The idea is to avoid the possibility, which has been known to occur, of the cylinder valve being screwed down by a powerful wrist and defying the efforts of the despairing lanternist to open it.
Cylinder nozzles are unfortunately not yet standardised, but those most frequently met with in this country are those adopted by the British Oxygen Company, both oxygen and coal gas cylinders being fitted with corresponding internal screws ⅞ inch diameter, those for oxygen being right-handed, and those for coal gas left-handed, and in each case terminated at the bottom by a hollow metal cone.
As such an internal screw cannot obviously be connected to a piece of rubber tubing, some type of screw connector must be employed, and this may take one of three forms: (1) A simple connecting nozzle, (2) a fine adjustment valve, or (3) a regulator. The first is seldom used in practice for lantern work, for the reason that the direct pressure of a full cylinder (120 atmospheres) cannot be checked or controlled by a tap on the jet, as the intervening rubber tubing would either burst or blow off, and must therefore be regulated at the cylinder nozzle itself, and gradually readjusted as the pressure diminishes.
To achieve this regulation with the ordinary cylinder key is difficult, though possible to a careful operator, but for a slight extra expense a combined nozzle and fine adjustment valve (Fig. 10) can be obtained, and regulation with this is