CHAPTER IV
ACETYLENE
There is no doubt that at present acetylene holds second place to electric light in popularity for optical lantern work. The light is good; not, it is true, so good as limelight or the electric arc, but still sufficient for a picture up to 12 feet in diameter at a working distance from the screen of not more than 30 feet, and this suffices for the large majority of halls.
It has great advantages over limelight in convenience and cheapness, although on both these points it must yield place to the electric arc, always providing that current is available, and therefore it is chiefly used in country districts and in gas-lit halls in large towns.
Acetylene gas is formed, as is well known, by the action of water upon carbide of calcium, and the generators constructed for lantern work are essentially the same in construction as for other purposes.
The alterations introduced are chiefly directed towards obtaining a light as steady as possible from a comparatively small generator, and, secondly, towards the entire elimination of smell, which obviously is far more serious in a lecture hall than, for instance, on a motor car. The generators in most common use may be divided into two classes, i.e. those on the gasometer principle in which the carbide is gradually lowered into the water, and those in which the water is allowed slowly to gain access to the carbide. A good example of the former is perhaps that made by Messrs. Moss of Birmingham, though there are several others equally good, and clear and explicit directions for working should be supplied by the makers. The Moss Generator (Fig. 5) consists of a tall iron vessel A fitted with a gas tap at bottom, this communicating
with a vertical iron tube within the vessel. Into this container fits the inner bell or container B, divided internally into two concentric portions entirely separated from each other, but connected by the pipe P P and the tap T.
A guide inside the bell encircles the iron tube in the outer tank and prevents rotation. Into the inner portion fits again the carbide-container (shown separately on the left), which is locked when in place by giving it a half turn, when a hook inside the bell engages with the lower edge of the carbide container and prevents it from falling.
The carbide container is fitted with a series of shelves, and the contents of a 2 lb. tin of carbide should be roughly divided among them; there is no need to make any accurate division. The carbide used should be that known as ½ inch mesh, and should be pure. That described as 'chemically' treated is apt to give trouble by over-generation in these gasometers and should be scrupulously avoided.
The carbide having been placed in the receptacles, these should be closed by means of the loose flap and the whole pushed into the bell and secured.
Water should be poured into the outer vessel up to a mark on the iron tube, and the bell placed in position. The lower tap being then turned on and the upper one closed, air from the outer portion of the bell can gradually escape by means of the iron tube and lower tap, and the bell gradually sinks by its own weight until it is on the bottom, but still
no water can reach the carbide, the air imprisoned in the inner portion of the bell effectually excluding it.
The lower tap should now be connected by means of india-rubber or flexible metallic tubing to the burner in the lantern (of which more anon), and the upper tap on the generator turned on, the tap or taps on the burner being likewise opened. The air from the inner portion of the bell can now escape by the pipe P P into the outer part, and thence through the iron tube, and out through tubing and jet, and as it does so water will rise in the interior and attack the carbide.
In a few moments the burner can be lit; but the gas, being generated far in excess of requirements, and filling both the inner and outer portions of the bell faster than it can escape, lifts the latter until the carbide is entirely out of the water, when in a few minutes generation ceases.
If the jet is left burning the bell will gradually sink again as the gas is used up, and should thereafter maintain an automatic balance without attention.
It can be turned off at any moment by simply closing the taps at the jet or, better, the lower tap at the generator, when the bell rises sufficiently to take the carbide out of the water; but if it is required to leave the generator unlit for a considerable time, it is better to turn off the tap on the top first. This causes the inner portion of the bell to fill with gas which cannot escape, and as that in the outer part burns out, the bell sinks to the bottom and remains there, the gas itself imprisoned in the inner chamber excluding the water from the carbide. The exact arrangement varies in different patterns of generator, but the above may be taken as roughly indicating the action, and further information may always be obtained from the maker or dealer.
Emptying should always be done out of doors, as the odour of acetylene gas is most objectionable, and for the same reason rubber tubes, &c., should be securely tied on, so that the slightest escape may be avoided.
If the exhibition has been a short one it will often be found that the upper cells have not been affected by the water, in which case they may be put back in the tin and used again, but it is not generally advisable to put in less than the full charge to begin with as the weight of the carbide plays a definite part in securing the smooth action of the apparatus. The sludge should be thrown away (it forms a good manure for the garden) and the entire generator thoroughly dried, otherwise rust will quickly appear.
Theoretically one of these generators may be filled and left standing indefinitely, but in practice it is not advisable, as the damp in the atmosphere is apt to produce a very slow generation of gas, sufficient often to cause a decided smell.
Of generators which act by admitting water to the carbide perhaps the best known is the A.L. or 'Popular' Model (Fig. 6), this being, in fact, a pattern designed for motor-car head-lights, but which answers well for lantern work.
Its exact operation need hardly be described here in full detail. It will suffice to say that the water gains access to the carbide by 'creeping' up between two concentric copper cones, and in the event of over-generation the pressure of the gas automatically checks the flow.
This generator is smaller than the gasometer pattern, and hence can be recommended for portability; but in my experience the light is not quite so steady, and the control rather less delicate, thereby causing on occasions a perceptible smell, especially if left standing for a considerable time.
There are other types of generators, such as the 'Water
dropping' variety, in which the water drips on to the carbide, and the reverse, in which fine granulated carbide drops a little at a time into water; but these types are not very frequently met with and need hardly be described.
It should never be forgotten that acetylene is an explosive gas and should be treated as such. Searching for a leak with a lighted match, though perhaps permissible when the operator knows his business, may be a dangerous proceeding when the contrary is the case.
Acetylene burners are generally of the 'Batswing' type, and are as a rule four in number, mounted in a row with a reflector behind, each burner being separately controlled by its own tap (Fig. 7). An acetylene flame is very smoky, and care must be taken that the burners are not turned too high. A nipple cleaner, consisting of a fine wire in a short handle, can usually be obtained from any dealer, and is very handy.
Acetylene gas can also be used for lantern illumination in quite another way, viz. by a blast from a blowpipe, in combination with either air or oxygen, on to a special 'Pastille' provided for the purpose, or an ordinary limelight jet can be used. These methods entail the use of acetylene under pressure, and are so analogous to limelight that I shall for convenience deal with them in the chapter devoted to that illuminant.