The illumination possible with this light is almost unlimited, and for really large halls it is, as remarked before, the _only_ subst.i.tute 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pressure by means of heavy weights; but except in a very few outlying districts this method has now been completely superseded by the use of compressed gas cylinders.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 s.p.a.ce 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 {17} 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.

[Ill.u.s.tration: FIG. 8.--Oxygen Cylinder in hemp cover.]

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 {18} time, so this is a question to be decided by each user on its own merits.

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.

[Ill.u.s.tration: FIG. 9.--Double Lever Key.]

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 {19} 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.

[Ill.u.s.tration: FIG. 10.--Fine Adjustment Valve.]

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 7/8 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 {20} infinitely easier. The best plan of all, however, is to use an automatic regulator, which not only reduces the pressure so as to permit of the required adjustments being made at the jet-taps, but also maintains a practically steady supply as the cylinder empties, thereby obviating continual readjustments. Regulators are now so inexpensive that they have come into almost universal use, and are generally reckoned an indispensable part of a limelight lantern equipment.

The form of regulator in most common use is that usually known as 'Beard's,' having been originally designed and patented by Messrs. R. Beard & Sons, though as the patent has now expired it is open to any firm to make the same article if they desire.

[Ill.u.s.tration: FIG. 11.--Construction of Beard's Regulator.]

The construction of Beard's Regulator is shown in Fig. 11. The gas enters from below into a rubber bag, C, from which it can emerge through the nozzle.

Any acc.u.mulation of gas raises the bellows against the pressure of a spiral spring pressing it down, and this brings into action an arrangement of so-called 'Lazy Levers,' which in turn presses down a small conical valve and closes the supply from the cylinder, this valve re-opening immediately the pressure diminishes.

The outward form of this regulator is shown in Fig. 12, {21} which incidentally also ill.u.s.trates the usual form of connection to the cylinder, referred to later on.

In Beard's Regulator the pressure at which the gas can be delivered is determined by the strength of the spiral spring, and can only be altered by changing this spring.

[Ill.u.s.tration: FIG. 12.--Beard's Regulator.]

In practice Beard's Regulators are supplied set to a low pressure for ordinary mixed or 'blow-through' jets and for a higher pressure (14-16 inches) for 'injector' jets. At this latter pressure the rubber tubing used must be fairly thick and strong and well tied on, and even so the taps of the jet should not be turned entirely off unless the gas at the cylinder is likewise turned off immediately afterwards. The British Oxygen Company make a regulator which can be set to any desired pressure, but it is not quite so delicate in its action as Beard's, and Messrs. Clarkson also make a pattern regulator which is widely used and well spoken of. The attachment of any of these fittings to the cylinder is a somewhat peculiar one, as will be seen on reference to Fig. 10 or Fig. 12. The regulator or nozzle ends at its lower extremity in a screw and cone, the latter being intended to make a gas-tight connection with the internal cone on the cylinder, and over this screws a loose wing piece with another outer screw, this latter fitting the thread in the cylinder.

In making the connection care must be taken that the wing piece is not screwed too far down the inner screw, or the cone will not reach down and make a tight fit on its {22} 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.

[Ill.u.s.tration: FIG. 13.--Regulator and Gauge.]

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 {23} 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 pa.s.sed 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 s.p.a.ce is hardly warranted in fully describing them, especially as any operator wishing to adopt {24} the process can obtain full directions from any responsible dealer.

LIMELIGHT JETS.--These are of three general types, viz. the 'Blow-through,'

the 'Mixed,' and the 'Injector.'

Of these the 'Blow-through' is now very little made, having been largely superseded by the 'Injector' pattern, but, as there are hundreds in common use in this country, they cannot yet be regarded as a thing of the past.

[Ill.u.s.tration: FIG. 14.--Gas-bags.]

The exact design of this jet varies considerably, but all are alike in this, that a jet of coal gas is burned at the orifice of a more or less open nozzle, and a stream of oxygen _blown_ _through_ it on to a cylinder of lime which it thereby renders incandescent. Fig. 15 represents the various designs chiefly adopted for this jet, that marked A being perhaps the most usual, though C is also frequently met with.

In light-giving power there is not much to choose between the various types; probably D on the whole is the best in this respect, but so much depends upon the exact position of the two nozzles, and the _smoothness_ or otherwise of that {25} provided for the oxygen blast, that exact comparisons are difficult.

[Ill.u.s.tration: FIG. 15.--'Blow-through' Nozzles.]

'Blow-through' jets are the weakest form of limelight as used at the present day, and may be taken roughly as some 50 per cent. better than acetylene, or in other words, sufficient to illuminate a 12-foot picture at a distance of some 40 to 50 feet; but their advantage is, or was, that they only required one gas (oxygen) under pressure, the coal gas supply being obtained from the ordinary house main.

[Ill.u.s.tration: FIG. 16.--'Blow-through' Jet.]

This advantage is now shared by the more recently introduced 'Injector'

jets, which give a far better light, and have therefore rendered the 'Blow-through' type nearly extinct.

The general construction of a 'Blow-through' jet is shown in Fig. 16, and it will be seen that a short vertical spindle is {26} provided to carry the lime cylinder, and that this can be rotated from the back by means of bevelled gear wheels, which at the same time screw the spindle up and down.

A lime cylinder of the usual pattern being placed on this spindle can be rotated from time to time to expose a fresh surface, as that in use gradually becomes 'pitted' by the blast, while the screw provides sufficient vertical movement to ensure that a complete rotation does not bring round the same position again.

Some arrangement is also generally provided by which the distance between the lime spindle and the jet can be adjusted. The exact position of this does not matter within a reasonable margin, but limes vary in size, and 'Pastilles,' and other subst.i.tutes for limes, which will be referred to later, vary still more, at any rate as regards this adjustment. The average distance which gives the best result is usually about half an inch, and once set need not be altered with that particular jet unless a lime of different size is employed; minor variations due to limes being drilled slightly out of centre, &c., do not seriously matter.

There is no accepted rule for colouring jet-taps in accordance with the cylinders, and although jets are sometimes met with painted in this way, _i.e._ red for coal gas and black for oxygen, it is more usual to find coal gas taps _black_ and oxygen _bright_, or sometimes both black or both bright. Care must therefore be taken that the right cylinder is connected to the right tap on the jet, but there should be no difficulty in telling which is which, and fortunately any mistake, even if it be made, is quite harmless.

THE MIXED-GAS OR DOUBLE-PRESSURE JET.--This jet is fundamentally different from the 'blow-through' form, inasmuch as the two gases are combined in one mixing chamber before combustion, and burn in their correct proportions at one nipple.

It is usually stated that this jet necessitates both gases being under equal or approximately equal pressure, but this {27} is not literally accurate, and I have given many a lantern exhibition with one of these jets, using coal gas from the ordinary supply, and oxygen from a cylinder.

To use a mixed jet in this way needs care, as a very slight excess of oxygen puts the light out with a 'pop' which, although not dangerous, is disconcerting, while the light obtained under these conditions is very little better than with a 'blow-through' jet, and far inferior to the 'Injector' jets to be described next.