Part 3

There are three kinds of jet used for the lantern. Firstly, a jet in which a stream of oxygen is forced through the flame of a spirit-lamp on to a cylinder of lime. This form is perfectly safe, and although it presents an immense improvement upon any form of oil arrangement, does not afford sufficient light to illuminate a screen more than about nine feet in diameter. Secondly, there is what is known as the blow-through jet. In this case the jet is connected with the house gas, and the oxygen meets it at the point of ignition, and is blown through it on to the lime. This form of jet is also safe; indeed, it is often called ‘the safety jet,’ and affords plenty of light even for professional use. It is the one that I recommend the amateur to work with. Thirdly, there is the mixed jet, in which the two gases in separate bags are both under pressure, and mix together before reaching the external orifice. This form of lime-light gives the most light of any; but such care is requisite in dealing with it, that I shall say no more about it, considering it unfit for casual acquaintance.

A beginner might fancy that it would be a simple thing to mix the two gases in one bag, put it under pressure, and use them thus. So it would, and a beautiful light would be the result. Probably a beautiful explosion would be another result, for the two gases mixed form a most terrible compound, and a bag so charged would be almost equal in danger to a live shell.

With the spirit jet, or the ‘blow-through,’ the operator need have no fear of danger. Oxygen is not an explosive, and, although the best supporter of combustion, is not itself capable of being ignited. The spirit jet I should not recommend, except in situations where coal gas is not obtainable, so in my directions for using the lime-light, let it be understood that I am considering the employment of the blow-through, or safety jet.

Let us suppose, then, that we are preparing for an exhibition. The gas is made, and is at hand in the gas-bag; our lantern--and I will, for simplicity’s sake, consider it a single lantern only--is raised on its stand-table, or other support, at a convenient height from the ground, and we wish to have a private rehearsal in order to see that all is right. The first thing is to attach the hydrogen terminal of the jet, marked H, by flexible tubing to the nearest gas-bracket. Now take a cylinder of lime from its box, clear out the hole in the centre with a bit of wire, or a match, and place it on the pin provided for it above the jet. See that it turns freely, and so adjust the pin that the lime is distant from the jet nozzle about the sixteenth of an inch. Light the gas, and turn it down so that the flame is about an inch high. This can be done with advantage an hour or more before the lantern is really required, for the lenses and body of the lantern will by this means get thoroughly warmed, and any moisture upon the glasses, which would show as a blemish on the sheet, will be removed.

In all cases the hydrogen should be lighted first, if only to thoroughly warm the lime. Now the oxygen-bag can be put between the pressure-boards, and connected by another tube to the tap marked O. A half-hundred-weight having been put on the boards, the gas-bag tap may be turned on to the full. We may now attend to the jet. Turn on the hydrogen so that it flames up some inches over the lime, then gradually turn on the oxygen. At first it is air only that comes away, but presently with a characteristic little snap the two gases come together, and the brilliant lime-light is produced. A little care in adjustment of each tap alternately will soon show us the amount of gas from each which will give the best result.

It will soon be apparent to the operator that the spot of light on the lime must be exactly in the axis of the system of lenses, or the effect upon the sheet will be spoiled. The vertical movement of the jet is governed by a little screw, which holds it to the post on which it is fixed, which post stands upon an iron tray gliding between grooves. We must raise or lower the jet until the right place is found, when the screw can be brought home, and the jet is fixed. Even now, most probably, we shall find that the outer margin of the disc is ill-defined. This shows that the light is either too near or too far from the condenser. By moving the jet on its tray bodily backwards and forwards, we shall soon be able to find its correct position, and when found, that position will hold good to the end of the exhibition. We can now put a picture on the slide-stage, and focus it by means of the screw on the front lens. If the lime is properly adjusted and centred, our picture will be illuminated equally well in every part.

DISSOLVING VIEWS.

So far, we have considered the production of a brilliantly illuminated picture with a single lantern. For many years after the lime-light came into use, this was all that was required of it. But suddenly the beautiful effect known as dissolving views was contrived--an effect which, at the time of its introduction, made a wonderful sensation--and the method of producing which was for a long time kept secret. The old way of dissolving one picture into another, and the way which must still be adopted if oil-lanterns are in question, was to use two lanterns, side by side, and by a kind of see-saw arrangement in front of the lenses, to gradually uncover one nozzle whilst the other was being closed. This was easily done by furnishing each end of the see-saw with a screen of tin, the edge of each being cut into teeth like a comb. The one picture was thus caused to mingle with the other until the first lantern was quite closed, when the new design became perfectly disclosed. It soon became evident that the same effect could be produced more simply and economically with the lime-light lanterns, by contriving a special form of gas-tap which would slowly turn off the gases supplied to one lantern, while it as slowly admitted the gases to the other. By this arrangement nearly half the gas supply is saved, and therefore very little more is wanted for a dissolving-view apparatus than for a single lantern.

Dissolving view, or biunial lanterns, as they are generally termed, are now made in very compact form. The two systems of lenses, one above the other--not side by side as of old--are fitted into one mahogany case, lined with tin, and furnished with doors, so that the lights can be tended when necessary. At the back of the arrangement is placed the dissolving-tap, which is connected by indiarubber tubing with the lime-jets, and has two nozzles by which the hydrogen and oxygen respectively can be supplied to it. The dissolver is also furnished with by-passes, so that when the gases are turned from one lantern, just sufficient remains to keep the jet turned down ‘to the blue.’

The management of a pair of lanterns like this is, of course, much more onerous than that of a single lantern; but when once understood presents no difficulty. Before lighting up, be quite sure that all connections--and there are many--are quite secure, and that each place where indiarubber tubing fits on to metal is secured with twine. Now light the hydrogen in both lanterns, which you can do by placing the dissolving-tap in an upright position. Move the dissolver until the upper jet seems on the point of going out; but prevent it doing so by opening the hydrogen by-pass, and adjust this little tap so that the flame remains about one inch high when the other lantern is being used. Move the dissolver backwards and forwards a few times, so as to see that it works well, and that the hydrogen flares up in each lantern alternately.

We can now pay attention to the oxygen supply. First see that the bag is properly adjusted between the pressure-boards, and that the weight--one 56 lb. weight is sufficient to begin with--is in its place, above the upper board. Attending to the lower lantern first, turn up the hydrogen, and very gently admit the oxygen by turning the tap attached to the jet. When the light has been satisfactorily established, the oxygen by-pass must be turned in the same way that the other by-pass was just now treated, so as to admit a small amount of gas passing to the lantern not in use. If this be not done, the sudden inrush of the oxygen is sure to cause a small explosion, which will blow the light out. This does not indicate any danger whatever, but is inconvenient and undesirable during an exhibition. The upper lantern may now be attended to with the same precautions; and if all has been done well, the light will move from jet to jet alternately, as the dissolver is worked.

In using a double lantern, it is necessary to make both pictures coincident on the sheet. If one picture overlaps the other at every change, it has a most slovenly appearance. Such an error can be avoided by manipulating certain screws fitted to the metal front of the lantern, by which the two discs thrown upon the sheet can be adjusted until they quite coincide. These screws allow the lower nozzle to be pointed slightly upwards, and the upper one to point downwards, so that the images cast by each may be made to meet at the screen.

The double lantern is mainly employed for dissolving views; but its use does not stop here. A great many beautiful “effects” can be compassed by its means, a few of which I may here describe. Statuary slides form very beautiful pictures if photographed direct from the marble; but their effect is much enhanced if, by means of the auxiliary lantern, a glow of colour is thrown upon the screen at the same time. A few squares of differently tinted glasses, each mounted like an ordinary slide, with a mask of oval or round shape, are all that is needed. Again, a wintry scene in one lantern may be much improved by the effect of falling snow, produced by working a special form of slide in the other lantern. This slide consists merely of a frame containing a roller at the top with the handle projecting outside. As the handle is rotated, a long ribbon of black paper pierced with needle-pricks is rolled up upon it, and each prick makes a descending spot of light upon the screen, which together look exactly like falling snow. Sunset and moonlight effects, windows lighted up in night-scenes, ripples upon water, can all be managed by specially-devised ‘effect’ slides in the second lantern. These effects can be much extended when a triple lantern is employed; but as this instrument is not often found--out of the hands of professional operators--we need not further allude to it.

But the double lantern can be used with great advantage from an educational point of view, in a manner that was first suggested by the present writer. I mean in the demonstration of the main features of Spectrum Analysis. Let me cite one example. A slide prepared and coloured so as to represent the continuous solar spectrum, marked with the principal Frauenhofer lines, is placed, say, in the lower lantern. We now wish to show the bright lines given by an incandescent metal, say sodium. A slide, all blackened out but the double D line due to sodium, and so placed on the glass that it will exactly register with the D line in the coloured spectrum slide, is placed in the upper lantern. The dissolver is now brought into play, and can be so adjusted that while the continuous spectrum has all but faded away, the two brilliant sodium lines stand out boldly in their proper place. The spectra of all the other metals can be treated in exactly the same manner, keeping the continuous spectrum in the lower lantern for constant comparison and reference.

MAKING OXYGEN GAS.

There is very little danger incurred in the use of the lime-light if only the operator be intelligent in its employment. But there are certain dangers connected with the preliminary making of the oxygen gas, so that that part of the work should only be entrusted to a careful worker, and one who, knowing where the greatest care is required, will take every precaution against disaster. I have myself made several thousand feet of oxygen on different occasions, and have never yet met with any mishap; but as I know of cases where serious accidents have occurred, I have endeavoured to find out why they have happened, and I now consider myself forewarned, and therefore forearmed, against their repetition. The articles required for making the gas comprise a retort in which to generate it, a gas-stove to furnish the necessary heat, a wash-bottle or purifier, an indiarubber gas-bag, and several feet of tubing. Beyond these is wanted the mixture of chlorate of potash and peroxide of manganese in powder, which forms the charge of the retort, from which the gas is generated on the application of heat.

I have found that the best proportions of chemicals to use are four parts (by weight) of chlorate to one of manganese. Before being mixed together, both should be most carefully picked over, and most probably both will yield a small crop of bits of straw and wood-chips, which are not only injurious, but would in sufficient quantity prove actually dangerous if permitted to remain. The larger crystals of the chlorate, and any lumps which may be found in the manganese, should be rubbed down to powder by gentle pressure with the wooden spoon used for mixing the compound, about two pounds of which will yield sufficient gas for an evening’s show. The mixture can now be funnelled into the retort, the nozzle of which, after being blown through to see that the passage is clear, can be screwed into its place.

The purifier, or wash-bottle, is made of tin, or may consist of a wide-mouthed bottle with an indiarubber cap, fitted with inlet and outlet tubes of metal or glass. In either case the vessel is three-parts filled with water, the inlet tube dipping some inches below the surface. This tube is subsequently connected by three or four feet of indiarubber tubing with the retort. As the gas is given off, it bubbles through this water, which not only cools it, but catches the particles of solid matter which are sure to be blown from the retort. The outlet tube is connected with the gas-bag. In the annexed illustration the relative positions of the retort, purifier, and gas-bag are shown, but in practice certain modifications are desirable. It is as well, for instance, to stand a chair between the retort and purifier, upon which the connecting rubber tube may rest, otherwise the water evolved from the crystals of potash will condense in the tube and obstruct the passage of the gas. Again, the gas-bag, instead of being on the same level, should be laid on a table, so that any water accidentally driven from the purifier may not be forced into it. The rubber tubes should have an orifice of nearly half an inch, and should on no account contain an inside coil of wire. Previous to making gas the bag should be warmed for an hour or so in front of a fire, so as to make it soft and pliable. The cock should then be opened, and the bag rolled tightly up so as to expel any air. The tap should then be closed.

Diagram showing relative positions of g b, Gas-bag; w b, Wash-bottle; and r, Retort.]

When all is ready, and every junction made except that between the bag and the pipe leading from the purifier, the gas-stove may be kindled, but turned down almost to its lowest point. If this precaution be not observed, the oxygen will come off with such rapidity as to be quite out of control, and will probably blow the connections away. In two or three minutes the gas will commence to bubble through the purifier, at first intermittently, but the bubbling will gradually become more regular, until it is evident that oxygen is coming over in earnest. The brightening of a spark at the end of a blown-out match, held to the end of the purifier-tube, will announce the fact that all the air has been driven out of the pipes, and that gas has taken its place. Now is the time to connect the gas-bag with the purifier. With a quick movement the indiarubber tube should be slipped over the nozzle of the bag with one hand, while the tap is turned on with the other. The gas should now come over regularly until the bag is nearly half full, and this will occupy about fifteen minutes. The action will then probably flag a little, when the flame feeding the stove may be increased. When the bag is nearly full the flame can be raised, if occasion should require, to its utmost.

Some care is requisite in finishing the operation. When the bag is quite full and as tight as a drum, it should be disconnected before anything else is done. Of course at the moment of taking the tube from it, the tap must be turned. The next thing is to detach the retort from the purifier before the gas is extinguished in the stove. This is a most important point, for if the retort were cooled by putting out the gas first, the water might be sucked into the retort from the purifier, and a small steam-boiler explosion would be the result. In an hour or so the retort will be cool enough to handle, when the nozzle can be unscrewed and the vessel washed out with several changes of hot water. It can then be dried and put away for the next occasion. The retort-nozzle and its indiarubber tube should also be washed out, or it may get clogged.

Particular care must be taken to purchase the chlorate and manganese of some reliable dealer. Accidents have been reported owing to lamp-black, bone-black, etc., having been supplied in error for the manganese, which is a black powder much resembling them. I find that the best plan is to buy several pounds of each at a time, and to test a small quantity of the mixed ingredients in a test-tube over a spirit-flame. If the gas comes off with a slight sparkling of the ingredients all is right; but if there is anything approaching to an explosion, the manganese is at fault. One accident which I heard of was due to connecting the retort to the wrong side of the purifier; the gas had no outlet, and the retort exploded, breaking all the windows of the room in which it was.

Retorts are made either of copper or iron. I have tried both, and much prefer the latter, which have also the advantage of being cheaper than the copper ones. A good retort should serve for at least fifty charges, and even then a new bottom can be put into it, and it will do duty for another fifty. But no retort will last long unless properly treated. I am quite satisfied that the spent charge if allowed to remain has the effect of eating into the metal; it should, therefore, be washed out soon after use. The thickness of a retort after much use can be gauged by tapping it with the back of a knife when empty.

The oxygen gas is most usually stored in an indiarubber bag. These vary very much in quality, and the cheapest kinds are to be avoided. The rubber should be soft and pliant, and should be covered outside with stout twill. A good bag will almost last a lifetime if it be only used occasionally. I have had one for many years, which looks shabby enough with constant use, but seems to be as gas-tight and as efficient generally as the first day I had it. Even then it had been in use for some months by a friend.

Iron bottle for compressed oxygen gas.]

A bag full of gas is not the most portable thing that could be named, and various attempts have been made to replace it by some contrivance of a more convenient character. There is, too, the danger of accidental injury, which can readily occur if the bag is carelessly handled. In very cold weather, for example, some bags get quite stiff and hard, and have to be almost thawed before they can be used. Neglect of this precaution will lead to cracks, which will eventually turn into holes. Metal gas-holders--miniature copies of the big reservoirs seen at gas-works--which serve as travelling-boxes for the rest of the apparatus--are recommended by some, but are seldom used. Perhaps the most convenient arrangement--which, however, is only suitable for a single lantern--is to employ an iron bottle in which the oxygen is greatly compressed. Such a bottle charged with gas can be obtained from several dealers, and I have had some little experience of their use. A bottle measuring three feet in length, and about seven inches in diameter, will hold about eight feet of compressed gas--sufficient for an ordinary evening’s work. At the end of the bottle is an opening closed with a tap which can only be opened by a special form of key. Into this orifice is screwed a nozzle, upon which an indiarubber tube can be readily fitted. When a bottle of this kind is used, the amount of oxygen which reaches the lime must be directly controlled by this tap, and not by the tap attached to the jet, which must remain fully opened. The reason of this is that the gas issues with such force, that, unless controlled in the way I have stated, it will blow off the tubing by its pressure. I found it so difficult to regulate the supply, that I had an independent screw-tap made to fit on to the bottle. This screw-tap is of the same pattern as those used on steam engines to control the supply of steam to the cylinder, and with it I can adjust the oxygen supply with the greatest nicety. My chief objections to the bottle system--which presents many advantages in the matter of portability--are, firstly, that you cannot charge the bottle yourself--it must always go back to the dealer for that purpose--and you are therefore dependent for one of your first requisites upon others; and secondly, you do not know how much gas you are using, for you cannot see it diminish as you can when you employ a bag. For these and other reasons I still prefer bags to store my gas, and shall continue to do so until some improvement comes about.

Gas-bag and pressure-boards.]

The gas-bag is wedge-shaped, and the thin edge of the wedge has in its centre a gas-cock for the entrance and exit of the gas. The object of the bag being in this form is, that it can conveniently be placed between the pressure-boards. These, in their simplest form, consist of two boards strongly put together--the size of the bag which is used--which are hinged together at one end. At the centre of this end, and between the hinges, is cut an oval opening through which the gas-cock is thrust. A shelf on the upper board serves as a support for the weights. At least two weights are required--one being sufficient at first, when the bag is full; the other being added later on when the pressure becomes reduced. The weighted gas-bag should always be placed as near the lantern as circumstances will allow--in order that it may be under the eye of the operator--and the tube from it which supplies the lantern should be guarded, so that it is not accidentally stepped upon.