[Ill.u.s.tration: FIG. 17.--Mixed Jet.]

[Ill.u.s.tration: FIG. 18.--Mixed Jet, Gwyer pattern.]

The mixed-gas jet is intended then to be used with both gases under pressure, and is the _only_ jet to be seriously {28} considered in cases where a really powerful light is required. The power of this jet is indeed almost unlimited, and those made with large bores, such for example as used for cinematograph work, provide a light amounting often to some two or three thousand candles, and consume an enormous amount of gas; but the ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an inch bore, and using some 5 feet of each gas per hour, or perhaps slightly more for the coal gas, will suffice for all ordinary work.

[Ill.u.s.tration: FIG. 19.--Mixing Chamber of Jet.]

The mixed-gas jet, like the 'blow-through,' is made in many forms, but these may be roughly divided into two main types, viz. those with small mixing chambers immediately below the nipple (Fig. 17), and those with larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern (Fig. 18).

The construction of the mixing chamber itself varies also, but that advocated by my father, the original author of this work, is generally followed, the chamber being packed with alternate discs pierced as in Fig.

19, which ensures a thorough mixture of the gases. A layer or two of gauze is often introduced as well by way of further improvement. The distance between the lime and nipple is much less than with the 'blow-through' jet, and the adjustment has to be more exactly made. About 1/8 inch is approximately correct for a jet of moderate power, and rather more for a bigger bore; also care must be taken to turn the lime frequently, as the latter 'pits' pretty quickly with these jets, and if it is neglected the jet may spurt back out of the hole, which is gradually formed, and crack the condenser. {29}

There is still an erroneous opinion extant that these jets are dangerous, and if the operator is working with the now obsolete gas-bags it is certainly a fact that an accident in careless hands is _possible_; but with cylinders there is, so far as I know, no possibility even of an accident under ordinary conditions.

It is true that if too much oxygen is turned on the jet may suddenly go out with a loud snap or pop, and this is in reality a miniature explosion in the mixing chamber; but it can in any case hardly be serious enough to matter, though I have found after such a snap that the gauze packing, inside the chamber above referred to, has been pierced right through, and, when first lit afterwards, the jet has for a few minutes burnt with a characteristic green flame, denoting the presence in the gas of fine copper or bra.s.s particles.

To obtain a good light with these jets, and in fact with _all_ jets, great care must be taken that the nipple is absolutely smooth, otherwise the flame is bound to hiss. The simplest plan is to slightly roughen a suitable sized needle with emery paper and to burnish the inside of the nipple from time to time with this. Especially if there has been one of the 'snaps'

referred to is it desirable to see that the inside of the nipple is thoroughly smooth and polished.

MANIPULATION OF THE MIXED-GAS JET.--On this point there is not much to be said. A good hard stone lime must be used--'soft' limes are useless for this jet--and the coal gas flame should be lit first, and the lime thoroughly heated with this before the oxygen is slowly turned on. As the oxygen increases the flame will gradually disappear and the light increase, until it is at a maximum for that particular amount of coal gas. This latter can then be turned on a little more, and more oxygen pa.s.sed to balance it until the jet begins to 'roar,' when we are getting the maximum light for that particular sized nipple. When the two gases are, however, in the proper proportion to give the best light, there will always be a slight excess of coal gas flame visible playing about the lime. {30}

THE INJECTOR JET.--This is essentially a mixed jet, and in outward appearance differs but little from one of the ordinary type (Fig. 20), but is so constructed that the pressure of oxygen 'sucks' coal gas into the mixing chamber, and so obviates all necessity for the latter being under pressure.

With this jet there is little or no danger of the jet 'snapping' out through a surplus of oxygen, as the greater the flow of this gas, the greater the suction on the coal gas side.

The light is not quite equal to a good mixed jet, but very nearly so, and therefore this jet is deservedly gaining in favour every day.

[Ill.u.s.tration: FIG. 20.--'Injector' Jet.]

One point must be noted: the oxygen itself must be under greater pressure than with the ordinary mixed jet if the best light is to be obtained, and therefore a special regulator must be used, or one of ordinary type modified (which can easily be done by the maker), and rubber connections must be securely tied both on to jet and regulator, as the pressure required to work this jet to advantage, while not enough to burst a rubber tube, is enough to blow it off an easy fitting connection.

THE OXYETHER LIGHT.--This is practically similar to the oxyhydrogen, except that ether vapour is used in place of the hydrogen or coal gas. The method adopted consists essentially of pa.s.sing a stream of oxygen through a vessel packed with some porous material (such as cotton wool or cotton gauze) which is saturated with ether. The oxygen {31} becomes saturated with ether vapour, and the mixture is then used in place of the coal gas supply in a double-pressure jet, an additional supply of free oxygen being still required through the ordinary oxygen tap.

The arrangement is cheap, as it dispenses with the necessity for a coal gas cylinder, and effective, as the light is little, if at all inferior to the oxyhydrogen, but differs from the latter in this, that with careless handling an accident is _possible_.

In competent hands there is no danger, and I have used ether saturators myself scores of times without one single contretemps; but it should _not_ be entrusted to any chance amateur.

The use of the ether light has a curious history. In the earlier days before the proper construction of ether saturators was understood, and gas-bags were still in vogue, it was largely condemned on the score of danger. Modern improvements in apparatus rendered it perfectly safe against anything but gross carelessness or bungling, and the London County Council and other similar bodies immediately supplied it broadcast to elementary schools (in disregard of warnings offered by myself and others), where it was often entrusted to incompetent operators or even senior boys. So far as I know no serious accident ever resulted, a pretty conclusive proof that the light is really safe, but in time the London County Council realised that the universal adoption of this illuminant was not advisable, and I believe _now_ prohibit it altogether in halls licensed by them for entertainments.

In time, no doubt, they will learn to adopt a sane policy between the two extremes, but at present the official att.i.tude in many localities has placed ether saturators out of the running, and before purchasing one the would-be operator should ascertain that he will be allowed to use it.

Ether saturators as made at the present day may be divided into two princ.i.p.al patterns, viz. those in which saturator and jet are combined in one piece of apparatus {32} which fits bodily into the lantern, and saturators which are used outside and connected by means of tubing to any ordinary oxyhydrogen double-pressure jet.

Both forms have their advantages and disadvantages; the first pattern tends to become too warm from its position in the lantern and generates ether vapour too quickly, while the second has the fault of becoming too cold (owing to evaporation of the ether) and therefore not vaporising quickly _enough_.

[Ill.u.s.tration: FIG. 21.--'Gridiron' Saturator.]

Writing at the present date, when manufacturers are slowly beginning to resume their normal occupations after the stress of war work, it is impossible to say exactly what models will or will not be made, but I will mention one typical example of each pattern as made in pre-war days.

The first of these is the 'Gridiron' (Fig. 21), adopted largely by the London County Council in the days I have referred to, and certainly one of the best designed saturators ever put on the market.

In the 'Gridiron' saturator there are three taps: two at the {33} rear and one in front, between the saturator and the mixing chamber. Between the rear taps is the inlet for the oxygen, which divides into two channels, that on the left pa.s.sing upwards through the U tube shown in the ill.u.s.tration (the corresponding tube on the right is merely a dummy), and thence through the saturator and out through the horizontal tube and tap into the mixing chamber, whence the saturated stream of oxygen finally pa.s.ses to the nipple, and the combination burns with a whitish flame closely resembling that produced by coal gas.

[Ill.u.s.tration: FIG. 22.--'Pendant' Saturator.]

The other channel for the oxygen is to the right, down the vertical tube shown there (the lower vertical tube on the left is also a dummy), underneath the saturator, and finally coming up into the mixing chamber from below, transforming the white flame into an intensely hot blowpipe exactly as it does with a coal gas jet. The front tap controls the supply of saturated ether to the mixing chamber, and whereas at first a good stream of oxygen is needed to pick up enough ether, by degrees as the instrument warms in the lantern, the oxygen pa.s.sing through the saturator can be cut off entirely, and even then the front tap must be gradually closed down to prevent the hot ether coming off too fast.

There is a disagreeable feeling of 'sitting on the safety-valve' in doing this, but in reality the pressure is never likely to become great enough to cause danger.

Of saturators for use outside the lantern the best-known is probably the 'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to the inlet marked A; B goes {34} direct to the oxygen tap of any ordinary mixed-gas jet; while C, from whence issues the saturated stream, is connected to the coal gas tap of the jet. Whichever pattern is used, the essential thing is to keep a good supply of oxygen well saturated. If the lime becomes incandescent without any free oxygen, or it is found that this requires gradually turning off, it indicates that the saturation is becoming defective, and to continue is to risk the jet snapping out. In the case of an outside saturator such as the 'Pendant,' this may even blow off the connecting tubes with a loud report, though no worse accident is likely to happen, and for this reason an outside saturator should be placed _as close_ to the jet as possible, so that the rubber tube may be kept short, and incidentally this keeps the saturator warm and accelerates vaporisation.

As ether vapour usually contains a certain amount of moisture which does not vaporise to any great extent, this gradually acc.u.mulates and the capacity of the instrument becomes reduced. It is therefore usually necessary to return a saturator to the makers every now and again for repacking.

The only real danger with a modern saturator is not in using but in _filling_. This should be done if possible in the open air, and at any rate never near a light. Ordinary sulphuric ether of specific gravity 720-730 is usually considered the best, and a quarter of a pint will keep an ordinary small-bore jet going for nearly two hours.

More precise directions are usually sent out by the makers, and as the various patterns of saturator in use are pretty numerous, it would be useless here to attempt more detailed instructions for working.

OXY-ACETYLENE JETS.--Any good mixed gas jet may be used with acetylene instead of coal gas, provided that it is under pressure more or less corresponding to that from an oxygen cylinder, and at the present day there is no difficulty in obtaining this, in civilised countries at all events, by {35} means of compressed or, to speak more correctly, 'dissolved'

acetylene cylinders, referred to later on.

With an 'Injector' jet there is no need for the acetylene gas to be under pressure at all, and a simple generator such as described on page 12 will answer perfectly, though in practice very seldom used. With such a generator the pressure is so low that in many cases the jet will not even burn until _some_ oxygen is turned on; but this introduces no real difficulty, as with a good 'Injector' a snap is practically impossible, provided the generator is large enough to evolve sufficient acetylene. It is far better in every way, however, to use the acetylene from a cylinder, just as with coal gas. Only in this case the cylinder is completely filled with a porous material, and this again filled with liquid acetone or other suitable fluid, in which the acetylene is dissolved as rapidly as it is pumped into the cylinder.

To compress acetylene in the ordinary way is neither safe nor practicable; but these 'dissolved' cylinders are now used extensively for both oxy-acetylene welding and motor car lighting, and may be entirely relied upon.

The D.A. (Dissolved Acetylene) Company were the pioneers in this country of the industry, and their methods of business are peculiar and ingenious. The user is requested in the first place to purchase a cylinder, and he then becomes the owner of _a_ cylinder, but not of one _particular_ cylinder. A list is supplied to him of various depots in the country where the Company's cylinders are stored, and when empty he can, on payment of a fixed sum, exchange his empty cylinder for a full one, which then becomes _his_ cylinder _pro tem_.

This saves the delay and expense of returning a cylinder to London, and incidentally clears the customer of any question of deterioration, this being obviously covered by degrees with each individual exchange. The system was first introduced in connection with the lighting of cars and only applies to the standard size for this purpose, viz. 20 cubic feet capacity, {36} but as this is, on the whole, the most convenient size for lantern work also, the limitation is not a disadvantage. The arrangement is also in vogue to a less extent with cylinders of 6 feet capacity (a size sometimes used for motor _cycles_), but the depots of exchange are at present far fewer for this size.

The oxy-acetylene blast is much _hotter_ than the ordinary oxyhydrogen, and therefore produces a more intense light. I have therefore used it with success on occasions when even the ordinary limelight would fail, and the choice has lain between an oxyhydrogen jet of enormous bore (and, of course, corresponding consumption of gas), and the oxy-acetylene.

For this very reason great care must be taken only to use the hardest limes, and even then to use the lime-turning movement frequently, or the lime will pit or crack and a broken condenser follow.

THE FALLOT ACETYLENE LIGHT.--This light consists of a jet of acetylene under pressure, without oxygen, but producing its own _air blast_ from the atmosphere by suction, much as the 'Injector' jet does, but the reverse way round.

The light is better than with an ordinary acetylene jet, though not quite so good as with a 'blow-through' jet; but as it only requires a cylinder of dissolved acetylene, or even a 'Pressure' generator, it is fast coming into favour.

The peculiarity of the Fallot apparatus is that, instead of providing a direct beam of light in the direction of the screen, it projects the beam _backwards_ on to a concave mirror, and it is the reflected light from this that is used (Fig. 23).

Instead of a lime is used a spherical 'Pastille' of peculiar composition, and before use each pastille must be burnt off exactly like an incandescent gas mantle, after which it is extremely fragile and difficult to handle.

To use this illuminant one lens of the condenser must be removed, the curvature of the mirror taking its place, and it will be seen at once that the pastille itself will get in its own {37} light and throw a shadow, which actually happens, but it is hardly perceptible unless specially looked for.

[Ill.u.s.tration: FIG. 23.--Fallot Air Blast.]

A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve, pressure gauge and burner, with two spare pastilles, is shown in Fig. 24, but if preferred a regulator, such as previously described for oxygen, can be used instead of the fine adjustment valve.

[Ill.u.s.tration: FIG. 24.--Fallot Air Blast, and Cylinder.]

{38}