The magic lantern and its management

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.

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.

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.

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.

The lime-cylinders are sold in air-tight tin canisters holding one dozen each. They are of two qualities—hard and soft. For work where large pictures are required—necessitating the use of the mixed jet with both gases under heavy pressure—the hard limes must invariably be used. But where only a small sheet has to be covered, and one gas only is subjected to the unusual pressure, the soft limes will afford perhaps a better light than the harder ones. Occasionally the lime, be it hard or soft, will split. In such a case it ought to be immediately replaced by a fresh one, otherwise the flame may possibly be diverted towards the condensing lenses, and will fracture that nearest to the light. It is advisable always to keep a spare lime within the lantern, lying on the tray to which the jet is attached. By this means it gets warmed, and ready for use if required. Lime is most absorbent of moisture, and will, if kept where air can get to it, swell to double its former size, and finally crumble to powder. For this reason the tin canister containing the cylinders should have stretched over it, when once opened, a broad indiarubber band which will cover the junction between lid and box. If this precaution be neglected, the limes will swell with a force sufficient to break the tin-containing vessel asunder. A compressed lime cartridge, to which water is applied by artificial means, has lately been successfully introduced to supersede explosives hitherto used for mining purposes. This will show the immense power of expansion of which the substance is capable.