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 {12} 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.
[Illustration: FIG. 5.--The Moss Generator.]
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 {13} 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.
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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.
[Illustration: FIG. 6.--The A.L. or 'Popular' Model.]
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 {15} 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.
[Illustration: FIG. 7.--Acetylene Jet.]
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.
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