THE LIME-LIGHT.
The lime-light consists of a jet of mixed hydrogen and oxygen gases under pressure, ignited, and forced upon a cylinder of lime, which it renders white-hot. The heat given by these gases is second only to the heat of the electric arc, and will melt that most refractory of metals, platinum. Even the lime block cannot withstand the great heat to which it is exposed, but is quickly pitted under the action of the flame. Hence lime-light jets are furnished with an arrangement by which the lime cylinder can be turned at frequent intervals, so as to offer a fresh surface for the gases to play upon.
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.