PhotoHistory

From: THE BRITISH JOURNAL OF PHOTOGRAPHY
No. 1392. VOL. XXXIV. JANUARY 7, 1887.

ACCESSORIES OF THE LANTERN

CHAPTER I - GAS BAGS

By ALBERT WILLIAM SCOTT

Every lantern operator knows what an important part of his outfit the gas bags are. Supposing he is using the mixed gas jets he requires two bags, and the cost of these accessories will be nearly as much as the price of the biunial, especially if the latter is a cheaply made instrument. The gas bags are not only expensive as regards their first cost, but they also form the largest item in the cost of maintenance. The lantern with its fittings and the slides may keep in good order during years of constant use, and cost little or nothing in repairs, but the gas bags, more especially those used for oxygen, deteriorate so rapidly that the work of a single winter season often wears them out completely. The life of a gas bag in constant use being so short, any means by which it can be lengthened will naturally be of interest to those whose purses are affected by the matter.

The material of which gas bags are made is rendered air-proof by one or two films of india-rubber, by which the cloth or twill, which is used in two or three thicknesses, is cemented together. Upon the substance of this india-rubber depends the longevity of the gas bag. In the best bags the rubber is over a sixteenth of an inch thick, and of course resists the corrosive action of the gases for a much longer time than cheaper bags made air-tight by a mere skin of rubber.

When a gas bag begins to leak it is not so much the cloth which is defective as the layer of rubber attached to it, which has become rotten. Hence it is real economy to purchase a first class gas bag in preference to a cheaper one of the same size; the extra cost will be amply compensated by increased durability.

All new bags should be thoroughly tested, to ascertain if there is any leakage. This may be done by filling the gas bag with air or coal gas, and placing it between the pressure boards with a half-hundredweight thereon. If the top board only sinks one or two inches during a period of twelve hours the bag may be considered sound; if the bag is only half full at the end of that time it is still serviceable, as the leakage during the two hours of an exhibition would not be very serious, still, such a loss should by no means be tolerated in a new bag. Sometimes the leakage is caused by the plug of the tap not being screwed up tight enough, or from not being properly greased with tallow. If the leakage is not connected with the tap the bag should be returned to the vendor, with a request for a sound bag to be given in exchange.

As gas bags are used for both oxygen and coal gas, and as it is extremely dangerous for these gases to be mixed together in one bag, it is necessary to use each bag constantly for the same gas. As a general rule, a bag used for oxygen should never on any account be filled with coal gas and vice versa; the bag which is used for coal gas should be a complete stranger to oxygen. The non-observance of this rule is the cause of perhaps one-half of the explosions that occur in connection with lantern work. It is a good plan to paint in large white letters “O” on each side of the oxygen bag, and “H” on the sides of the coal gas or hydrogen bag.

It will usually be found in the case of two similar bags devoted respectively to oxygen and hydrogen that the one used for oxygen will be quite worn out and unserviceable through leakage before the other bag shows any deterioration whatever. Hence it is obvious that oxygen has a greater corrosive action on the india-rubber than coal gas. This destructiveness is said to be due to the chlorine gas which is evolved from the chemicals in the retort in company with the oxygen, and careful operators hence add powdered chalk, lime, or potash to the water contained in the “cooler,” with the view of absorbing this obnoxious chlorine. I believe that the water which is carried over from the cooler into the gas bag, when the gas is generating very fast and the liquid in the purifier is in furious commotion, helps the bag to decay. The water once in the bag cannot evaporate, and it keeps the interior surface permanently damp. It is a good plan when making oxygen to have the bag on a table, and the cooler on the floor. Any drops of water that may be forced from the cooler into the rubber tube by the pressure of gas will then flow back into the vessel when the current of gas becomes weaker.

Temperature has a great influence upon the durability of gas bags. I once had a large new bag of good quality rendered unserviceable through corrosion in six weeks. This occurred in the heat of summer, and the bag contained oxygen. A similar bag containing coal gas was sound at the end of that period. Hence gas bags are rarely used in hot countries, such as India, as the expense of maintenance is so great. During frosty weather the bags do not deteriorate at all, or else they do so very slowly indeed. If the question arises as to whether any surplus oxygen that may be left after an exhibition is to lie used or not the temperature should be taken into account. If the weather is frosty, keep the gas by all means; if it is warm, it will be better policy to empty the gas bag. It is a good rule to put bags containing oxygen in the coolest place available.

When the limelight first came into use it was customary to put both the oxygen and the hydrogen into one bag. This method certainly had the advantage of simplicity, as no adjusting of the jet taps was required; but the explosions that occurred induced those in charge, out of consideration for their personal safety, to adopt the system of separate bags for each gas. It is possible to work a mixed gas jet with one bag containing the two gases without rendering the services of the coroner necessary, the points to be observed being the use of a small aperture in the jet nozzle, and plenty of pressure on the gas bag; however, such a system is more dangerous than the use of ether tanks, which, as most lanternists are aware, are quite capable of being blown up.

I know a gentleman who some years ago gave a lantern exhibition to a crowded audience; he was using the mixed gas jets, and had his two bags of oxygen and coal gas in the pressure boards as usual. When half of the slides had been shown he was startled to find, after adjusting the two taps of a jet, that he had turned one completely off and the light was still burning brilliantly! It was plain, owing to some mistake made in the filling of his gas bags, that one of them contained a highly explosive compound. He was thus on the horns of a dilemma: either he must proceed with the exhibition at the risk of the lives of those near the apparatus, or else an awkward and undignified “five minutes’ interval” must be put into the exhibition. He chose the latter, and quickly turned the gas off at the jet; the audience sat in a state of surprise and total darkness for a few minutes, but, after a while, a change of bags was effected, and the exhibition was continued.

If any one who reads this article should find himself in a similar predicament, I would advise him to adopt the same plan unless he is alone, and has his life heavily insured, in which case prudence is, perhaps, not so essential.