LIMELIGHT AND THE ACETYLENE BLAST
The illumination possible with this light is almost unlimited, and for
really large halls it is, as remarked before, the _only_ substitute for the
electric arc. It consists essentially of a blowpipe flame, composed of
oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &c., or acetylene air
blast, heating to incandescence a block of lime, or other refractory
material, and the essential feature is that one at least of these gases
must be under _pressure_. Thirty years ago this was usually achieved by
storing the gas in rubber bags, and obtaining the requisite pressure by
means of heavy weights; but except in a very few outlying districts this
method has now been completely superseded by the use of compressed gas
cylinders. The earlier editions of this work contained very full directions
for manufacturing gas for storage in bags, but it is so exceptional now to
find an operator who uses this method that it seems hardly necessary to
devote much space to it, and the same may be said of automatic oxygen
'generators.' The present work will therefore deal chiefly with compressed
gas cylinders.
Most elaborate precautions are now enforced by the Board of Trade to ensure
the absolute safety of these, and any doubt existing from occasional
accidents of years ago may be promptly dismissed. Humanly speaking, an
accident nowadays _cannot_ happen, except by such wilful negligence on the
part of the maker or filler as would almost render the culprit subject to
criminal proceedings.
Compressed gas cylinders are painted a distinctive colour, oxygen for
example being black and coal gas or hydrogen red; the screw connections to
the pumps, and all nozzle {17} and regulator fittings, are made with a
totally different screw and therefore cannot be interchanged; the cylinders
themselves are bound by law to be reannealed and retested under hydraulic
pressure at regular intervals; the steel itself has to be of a guaranteed
quality; and, in fact, every possible risk is guarded against.
The most usual sizes of cylinders supplied for lantern exhibitions are
those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in
wooden or hemp cases.
[Illustration: FIG. 8.--Oxygen Cylinder in hemp cover.]
Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size
without case being 22 in. by 4 in. This size cylinder will supply an
average limelight jet for just over two hours. The extra powerful jets as
used for cinematograph work or for illuminating a very large screen take a
good deal more, but for the usual apparatus as supplied for ordinary
lantern purposes this is a pretty safe figure.
A 12-foot cylinder is therefore the favourite size for a lantern exhibition
lasting from an hour to one and a half hours, as it leaves a fair margin
for gas used in adjusting the instrument, &c., and a 20-foot cylinder will
usually suffice for _two_ such exhibitions.
The price of gas per cubic foot varies with the size of the cylinder, being
less for large cylinders than for small ones, and the cost of transit is
also less in proportion--hence it is frequently an economy to hire a large
cylinder and retain it for several exhibitions. On the other hand most
suppliers charge a small rent if a cylinder is retained beyond a definite
{18} time, so this is a question to be decided by each user on its own
merits.
Alternatively, of course, cylinders can be _purchased_, and the question of
rent does not then come in; also gas is supplied a little cheaper in a
customer's own cylinder than if sent on hire. If purchase is decided on it
is frequently an economy to buy _two_, or two of each gas, if coal gas
cylinders are required as well.
[Illustration: FIG. 9.--Double Lever Key.]
The whole contents of the cylinders can then be used up without waste, as
if a cylinder should become exhausted during the course of a lecture, it is
only a matter of a minute or two to change over to the spare one, whereas
the compressors are required by law to empty out every cylinder returned to
them for refilling, and any remaining gas is thereby wasted.
It is extremely tantalising, to say the least of it, to find the pressure
gauge indicating that there is, say, 8 feet of gas remaining in a cylinder,
and to be compelled to waste this or else risk running short for the next
exhibition, and duplicate cylinders are the only way of avoiding the loss.
The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb. per
square inch, and are closed by strong screw nozzles. The keys for opening
or closing these are of three types, viz. the 'T' pattern, 'Spanner'
pattern, and that known as the 'Double Lever' type. This latter is so made
that in closing the valve it shuts up to half its length and {19} opens out
to double the leverage when being used to _open_ the cylinder (Fig. 9). The
idea is to avoid the possibility, which has been known to occur, of the
cylinder valve being screwed down by a powerful wrist and defying the
efforts of the despairing lanternist to open it.
[Illustration: FIG. 10.--Fine Adjustment Valve.]
Cylinder nozzles are unfortunately not yet standardised, but those most
frequently met with in this country are those adopted by the British Oxygen
Company, both oxygen and coal gas cylinders being fitted with corresponding
_internal_ screws 7/8 inch diameter, those for oxygen being _right-handed_,
and those for coal gas _left-handed_, and in each case terminated at the
bottom by a hollow metal cone.
As such an internal screw cannot obviously be connected to a piece of
rubber tubing, some type of screw connector must be employed, and this may
take one of three forms: (1) A simple connecting nozzle, (2) a fine
adjustment valve, or (3) a regulator. The first is seldom used in practice
for lantern work, for the reason that the direct pressure of a full
cylinder (120 atmospheres) cannot be checked or controlled by a tap on the
jet, as the intervening rubber tubing would either burst or blow off, and
must therefore be regulated at the cylinder nozzle itself, and gradually
readjusted as the pressure diminishes.
To achieve this regulation with the ordinary cylinder key is difficult,
though possible to a careful operator, but for a slight extra expense a
combined nozzle and _fine adjustment valve_ (Fig. 10) can be obtained, and
regulation with this is {20} infinitely easier. The best plan of all,
however, is to use an automatic regulator, which not only reduces the
pressure so as to permit of the required adjustments being made at the
jet-taps, but also maintains a practically steady supply as the cylinder
empties, thereby obviating continual readjustments. Regulators are now so
inexpensive that they have come into almost universal use, and are
generally reckoned an indispensable part of a limelight lantern equipment.
The form of regulator in most common use is that usually known as
'Beard's,' having been originally designed and patented by Messrs. R. Beard
& Sons, though as the patent has now expired it is open to any firm to make
the same article if they desire.
[Illustration: FIG. 11.--Construction of Beard's Regulator.]
The construction of Beard's Regulator is shown in Fig. 11. The gas enters
from below into a rubber bag, C, from which it can emerge through the
nozzle.
Any accumulation of gas raises the bellows against the pressure of a spiral
spring pressing it down, and this brings into action an arrangement of
so-called 'Lazy Levers,' which in turn presses down a small conical valve
and closes the supply from the cylinder, this valve re-opening immediately
the pressure diminishes.
The outward form of this regulator is shown in Fig. 12, {21} which
incidentally also illustrates the usual form of connection to the cylinder,
referred to later on.
In Beard's Regulator the pressure at which the gas can be delivered is
determined by the strength of the spiral spring, and can only be altered by
changing this spring.
[Illustration: FIG. 12.--Beard's Regulator.]
In practice Beard's Regulators are supplied set to a low pressure for
ordinary mixed or 'blow-through' jets and for a higher pressure (14-16
inches) for 'injector' jets. At this latter pressure the rubber tubing used
must be fairly thick and strong and well tied on, and even so the taps of
the jet should not be turned entirely off unless the gas at the cylinder is
likewise turned off immediately afterwards. The British Oxygen Company make
a regulator which can be set to any desired pressure, but it is not quite
so delicate in its action as Beard's, and Messrs. Clarkson also make a
pattern regulator which is widely used and well spoken of. The attachment
of any of these fittings to the cylinder is a somewhat peculiar one, as
will be seen on reference to Fig. 10 or Fig. 12. The regulator or nozzle
ends at its lower extremity in a screw and cone, the latter being intended
to make a gas-tight connection with the internal cone on the cylinder, and
over this screws a loose wing piece with another outer screw, this latter
fitting the thread in the cylinder.
In making the connection care must be taken that the wing piece is not
screwed too far down the inner screw, or the cone will not reach down and
make a tight fit on its {22} seating; in its correct position the wing
piece when clamped down should leave a turn or two of its thread exposed,
in order to ensure that the cone does bed properly.
[Illustration: FIG. 13.--Regulator and Gauge.]
Care should be taken that the nozzle of the cylinder is free from dust
before attaching any of these fittings: the best plan is first to blow into
it, and finally wipe it round with the finger. Most professional operators
_hammer_ the wing piece home with a spanner or other convenient implement a
barbarous method and really unnecessary if the cones are in good condition,
but, nevertheless, almost always adopted in practice.
PRESSURE GAUGES.--These are useful in determining the amount of gas
remaining in a cylinder and are of a very usual type; they may either be
screwed on to the cylinder before commencing to work and taken off again to
screw on the regulator, or they can be supplied fitted to the regulator
itself, in which case they can be observed during the course of the
exhibition (Fig. 13). As the same gauge may be used for cylinders of
different sizes (though _never_ for those containing {23} different gases),
they simply register in atmospheres, and knowing that a full cylinder shows
a pressure of 120 atmospheres, the requisite calculation must be made to
determine how many cubic feet are unused.
In the case of oxygen cylinders an approximate idea of the amount of gas
remaining can be got by _weighing_ it carefully when known to be either
absolutely full or absolutely empty, and re-weighing it when information is
required. Oxygen weighs approximately 1.4 oz. per cubic foot, and this is
easily detected by an average scale. Coal gas is too light to be gauged in
this way.
GAS-BAGS AND GENERATORS.--It has already been remarked that there are two
alternative methods of obtaining gas under pressure for limelight purposes,
viz. gas-bags and generators (the latter for oxygen alone: there is no good
hydrogen generator that I know of). In both these cases the oxygen is
generated by heating a mixture of chlorate of potash and manganese black
oxide. In the case of gas-bags the gas is prepared beforehand and passed
through suitable purifiers into a rubber gas-bag. With a generator the
oxygen is evolved during the exhibition itself; but this method has never
come into very general use.
Coal gas or hydrogen is very seldom home generated; a gas-bag can, if
necessary, be filled a few miles away and brought full to the place of
exhibition, or filled on the spot if gas is laid on; or, failing this,
acetylene or ether, or even methylated spirit may be utilised instead.
The bags in use are placed between double pressure boards (if _both_ gases
are required under pressure) and weights sufficiently heavy placed on the
top (Fig. 14), or with a 'blow-through' jet only the oxygen need be stored
in a bag and the coal gas used from the supply main.
Cylinders have, however, so universally superseded these appliances, that
space is hardly warranted in fully describing them, especially as any
operator wishing to adopt {24} the process can obtain full directions from
any responsible dealer.
LIMELIGHT JETS.--These are of three general types, viz. the 'Blow-through,'
the 'Mixed,' and the 'Injector.'
Of these the 'Blow-through' is now very little made, having been largely
superseded by the 'Injector' pattern, but, as there are hundreds in common
use in this country, they cannot yet be regarded as a thing of the past.
[Illustration: FIG. 14.--Gas-bags.]
The exact design of this jet varies considerably, but all are alike in
this, that a jet of coal gas is burned at the orifice of a more or less
open nozzle, and a stream of oxygen _blown_ _through_ it on to a cylinder
of lime which it thereby renders incandescent. Fig. 15 represents the
various designs chiefly adopted for this jet, that marked A being perhaps
the most usual, though C is also frequently met with.
In light-giving power there is not much to choose between the various
types; probably D on the whole is the best in this respect, but so much
depends upon the exact position of the two nozzles, and the _smoothness_ or
otherwise of that {25} provided for the oxygen blast, that exact
comparisons are difficult.
[Illustration: FIG. 15.--'Blow-through' Nozzles.]
'Blow-through' jets are the weakest form of limelight as used at the
present day, and may be taken roughly as some 50 per cent. better than
acetylene, or in other words, sufficient to illuminate a 12-foot picture at
a distance of some 40 to 50 feet; but their advantage is, or was, that they
only required one gas (oxygen) under pressure, the coal gas supply being
obtained from the ordinary house main.
[Illustration: FIG. 16.--'Blow-through' Jet.]
This advantage is now shared by the more recently introduced 'Injector'
jets, which give a far better light, and have therefore rendered the
'Blow-through' type nearly extinct.
The general construction of a 'Blow-through' jet is shown in Fig. 16, and
it will be seen that a short vertical spindle is {26} provided to carry the
lime cylinder, and that this can be rotated from the back by means of
bevelled gear wheels, which at the same time screw the spindle up and down.
A lime cylinder of the usual pattern being placed on this spindle can be
rotated from time to time to expose a fresh surface, as that in use
gradually becomes 'pitted' by the blast, while the screw provides
sufficient vertical movement to ensure that a complete rotation does not
bring round the same position again.
Some arrangement is also generally provided by which the distance between
the lime spindle and the jet can be adjusted. The exact position of this
does not matter within a reasonable margin, but limes vary in size, and
'Pastilles,' and other substitutes for limes, which will be referred to
later, vary still more, at any rate as regards this adjustment. The average
distance which gives the best result is usually about half an inch, and
once set need not be altered with that particular jet unless a lime of
different size is employed; minor variations due to limes being drilled
slightly out of centre, &c., do not seriously matter.
There is no accepted rule for colouring jet-taps in accordance with the
cylinders, and although jets are sometimes met with painted in this way,
_i.e._ red for coal gas and black for oxygen, it is more usual to find coal
gas taps _black_ and oxygen _bright_, or sometimes both black or both
bright. Care must therefore be taken that the right cylinder is connected
to the right tap on the jet, but there should be no difficulty in telling
which is which, and fortunately any mistake, even if it be made, is quite
harmless.
THE MIXED-GAS OR DOUBLE-PRESSURE JET.--This jet is fundamentally different
from the 'blow-through' form, inasmuch as the two gases are combined in one
mixing chamber before combustion, and burn in their correct proportions at
one nipple.
It is usually stated that this jet necessitates both gases being under
equal or approximately equal pressure, but this {27} is not literally
accurate, and I have given many a lantern exhibition with one of these
jets, using coal gas from the ordinary supply, and oxygen from a cylinder.
To use a mixed jet in this way needs care, as a very slight excess of
oxygen puts the light out with a 'pop' which, although not dangerous, is
disconcerting, while the light obtained under these conditions is very
little better than with a 'blow-through' jet, and far inferior to the
'Injector' jets to be described next.
[Illustration: FIG. 17.--Mixed Jet.]
[Illustration: FIG. 18.--Mixed Jet, Gwyer pattern.]
The mixed-gas jet is intended then to be used with both gases under
pressure, and is the _only_ jet to be seriously {28} considered in cases
where a really powerful light is required. The power of this jet is indeed
almost unlimited, and those made with large bores, such for example as used
for cinematograph work, provide a light amounting often to some two or
three thousand candles, and consume an enormous amount of gas; but the
ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an
inch bore, and using some 5 feet of each gas per hour, or perhaps slightly
more for the coal gas, will suffice for all ordinary work.
[Illustration: FIG. 19.--Mixing Chamber of Jet.]
The mixed-gas jet, like the 'blow-through,' is made in many forms, but
these may be roughly divided into two main types, viz. those with small
mixing chambers immediately below the nipple (Fig. 17), and those with
larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern
(Fig. 18).
The construction of the mixing chamber itself varies also, but that
advocated by my father, the original author of this work, is generally
followed, the chamber being packed with alternate discs pierced as in Fig.
19, which ensures a thorough mixture of the gases. A layer or two of gauze
is often introduced as well by way of further improvement. The distance
between the lime and nipple is much less than with the 'blow-through' jet,
and the adjustment has to be more exactly made. About 1/8 inch is
approximately correct for a jet of moderate power, and rather more for a
bigger bore; also care must be taken to turn the lime frequently, as the
latter 'pits' pretty quickly with these jets, and if it is neglected the
jet may spurt back out of the hole, which is gradually formed, and crack
the condenser. {29}
There is still an erroneous opinion extant that these jets are dangerous,
and if the operator is working with the now obsolete gas-bags it is
certainly a fact that an accident in careless hands is _possible_; but with
cylinders there is, so far as I know, no possibility even of an accident
under ordinary conditions.
It is true that if too much oxygen is turned on the jet may suddenly go out
with a loud snap or pop, and this is in reality a miniature explosion in
the mixing chamber; but it can in any case hardly be serious enough to
matter, though I have found after such a snap that the gauze packing,
inside the chamber above referred to, has been pierced right through, and,
when first lit afterwards, the jet has for a few minutes burnt with a
characteristic green flame, denoting the presence in the gas of fine copper
or brass particles.
To obtain a good light with these jets, and in fact with _all_ jets, great
care must be taken that the nipple is absolutely smooth, otherwise the
flame is bound to hiss. The simplest plan is to slightly roughen a suitable
sized needle with emery paper and to burnish the inside of the nipple from
time to time with this. Especially if there has been one of the 'snaps'
referred to is it desirable to see that the inside of the nipple is
thoroughly smooth and polished.
MANIPULATION OF THE MIXED-GAS JET.--On this point there is not much to be
said. A good hard stone lime must be used--'soft' limes are useless for
this jet--and the coal gas flame should be lit first, and the lime
thoroughly heated with this before the oxygen is slowly turned on. As the
oxygen increases the flame will gradually disappear and the light increase,
until it is at a maximum for that particular amount of coal gas. This
latter can then be turned on a little more, and more oxygen passed to
balance it until the jet begins to 'roar,' when we are getting the maximum
light for that particular sized nipple. When the two gases are, however, in
the proper proportion to give the best light, there will always be a slight
excess of coal gas flame visible playing about the lime. {30}
THE INJECTOR JET.--This is essentially a mixed jet, and in outward
appearance differs but little from one of the ordinary type (Fig. 20), but
is so constructed that the pressure of oxygen 'sucks' coal gas into the
mixing chamber, and so obviates all necessity for the latter being under
pressure.
With this jet there is little or no danger of the jet 'snapping' out
through a surplus of oxygen, as the greater the flow of this gas, the
greater the suction on the coal gas side.
The light is not quite equal to a good mixed jet, but very nearly so, and
therefore this jet is deservedly gaining in favour every day.
[Illustration: FIG. 20.--'Injector' Jet.]
One point must be noted: the oxygen itself must be under greater pressure
than with the ordinary mixed jet if the best light is to be obtained, and
therefore a special regulator must be used, or one of ordinary type
modified (which can easily be done by the maker), and rubber connections
must be securely tied both on to jet and regulator, as the pressure
required to work this jet to advantage, while not enough to burst a rubber
tube, is enough to blow it off an easy fitting connection.
THE OXYETHER LIGHT.--This is practically similar to the oxyhydrogen, except
that ether vapour is used in place of the hydrogen or coal gas. The method
adopted consists essentially of passing a stream of oxygen through a vessel
packed with some porous material (such as cotton wool or cotton gauze)
which is saturated with ether. The oxygen {31} becomes saturated with ether
vapour, and the mixture is then used in place of the coal gas supply in a
double-pressure jet, an additional supply of free oxygen being still
required through the ordinary oxygen tap.
The arrangement is cheap, as it dispenses with the necessity for a coal gas
cylinder, and effective, as the light is little, if at all inferior to the
oxyhydrogen, but differs from the latter in this, that with careless
handling an accident is _possible_.
In competent hands there is no danger, and I have used ether saturators
myself scores of times without one single contretemps; but it should _not_
be entrusted to any chance amateur.
The use of the ether light has a curious history. In the earlier days
before the proper construction of ether saturators was understood, and
gas-bags were still in vogue, it was largely condemned on the score of
danger. Modern improvements in apparatus rendered it perfectly safe against
anything but gross carelessness or bungling, and the London County Council
and other similar bodies immediately supplied it broadcast to elementary
schools (in disregard of warnings offered by myself and others), where it
was often entrusted to incompetent operators or even senior boys. So far as
I know no serious accident ever resulted, a pretty conclusive proof that
the light is really safe, but in time the London County Council realised
that the universal adoption of this illuminant was not advisable, and I
believe _now_ prohibit it altogether in halls licensed by them for
entertainments.
In time, no doubt, they will learn to adopt a sane policy between the two
extremes, but at present the official attitude in many localities has
placed ether saturators out of the running, and before purchasing one the
would-be operator should ascertain that he will be allowed to use it.
Ether saturators as made at the present day may be divided into two
principal patterns, viz. those in which saturator and jet are combined in
one piece of apparatus {32} which fits bodily into the lantern, and
saturators which are used outside and connected by means of tubing to any
ordinary oxyhydrogen double-pressure jet.
Both forms have their advantages and disadvantages; the first pattern tends
to become too warm from its position in the lantern and generates ether
vapour too quickly, while the second has the fault of becoming too cold
(owing to evaporation of the ether) and therefore not vaporising quickly
_enough_.
[Illustration: FIG. 21.--'Gridiron' Saturator.]
Writing at the present date, when manufacturers are slowly beginning to
resume their normal occupations after the stress of war work, it is
impossible to say exactly what models will or will not be made, but I will
mention one typical example of each pattern as made in pre-war days.
The first of these is the 'Gridiron' (Fig. 21), adopted largely by the
London County Council in the days I have referred to, and certainly one of
the best designed saturators ever put on the market.
In the 'Gridiron' saturator there are three taps: two at the {33} rear and
one in front, between the saturator and the mixing chamber. Between the
rear taps is the inlet for the oxygen, which divides into two channels,
that on the left passing upwards through the U tube shown in the
illustration (the corresponding tube on the right is merely a dummy), and
thence through the saturator and out through the horizontal tube and tap
into the mixing chamber, whence the saturated stream of oxygen finally
passes to the nipple, and the combination burns with a whitish flame
closely resembling that produced by coal gas.
[Illustration: FIG. 22.--'Pendant' Saturator.]
The other channel for the oxygen is to the right, down the vertical tube
shown there (the lower vertical tube on the left is also a dummy),
underneath the saturator, and finally coming up into the mixing chamber
from below, transforming the white flame into an intensely hot blowpipe
exactly as it does with a coal gas jet. The front tap controls the supply
of saturated ether to the mixing chamber, and whereas at first a good
stream of oxygen is needed to pick up enough ether, by degrees as the
instrument warms in the lantern, the oxygen passing through the saturator
can be cut off entirely, and even then the front tap must be gradually
closed down to prevent the hot ether coming off too fast.
There is a disagreeable feeling of 'sitting on the safety-valve' in doing
this, but in reality the pressure is never likely to become great enough to
cause danger.
Of saturators for use outside the lantern the best-known is probably the
'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to
the inlet marked A; B goes {34} direct to the oxygen tap of any ordinary
mixed-gas jet; while C, from whence issues the saturated stream, is
connected to the coal gas tap of the jet. Whichever pattern is used, the
essential thing is to keep a good supply of oxygen well saturated. If the
lime becomes incandescent without any free oxygen, or it is found that this
requires gradually turning off, it indicates that the saturation is
becoming defective, and to continue is to risk the jet snapping out. In the
case of an outside saturator such as the 'Pendant,' this may even blow off
the connecting tubes with a loud report, though no worse accident is likely
to happen, and for this reason an outside saturator should be placed _as
close_ to the jet as possible, so that the rubber tube may be kept short,
and incidentally this keeps the saturator warm and accelerates
vaporisation.
As ether vapour usually contains a certain amount of moisture which does
not vaporise to any great extent, this gradually accumulates and the
capacity of the instrument becomes reduced. It is therefore usually
necessary to return a saturator to the makers every now and again for
repacking.
The only real danger with a modern saturator is not in using but in
_filling_. This should be done if possible in the open air, and at any rate
never near a light. Ordinary sulphuric ether of specific gravity 720-730 is
usually considered the best, and a quarter of a pint will keep an ordinary
small-bore jet going for nearly two hours.
More precise directions are usually sent out by the makers, and as the
various patterns of saturator in use are pretty numerous, it would be
useless here to attempt more detailed instructions for working.
OXY-ACETYLENE JETS.--Any good mixed gas jet may be used with acetylene
instead of coal gas, provided that it is under pressure more or less
corresponding to that from an oxygen cylinder, and at the present day there
is no difficulty in obtaining this, in civilised countries at all events,
by {35} means of compressed or, to speak more correctly, 'dissolved'
acetylene cylinders, referred to later on.
With an 'Injector' jet there is no need for the acetylene gas to be under
pressure at all, and a simple generator such as described on page 12 will
answer perfectly, though in practice very seldom used. With such a
generator the pressure is so low that in many cases the jet will not even
burn until _some_ oxygen is turned on; but this introduces no real
difficulty, as with a good 'Injector' a snap is practically impossible,
provided the generator is large enough to evolve sufficient acetylene. It
is far better in every way, however, to use the acetylene from a cylinder,
just as with coal gas. Only in this case the cylinder is completely filled
with a porous material, and this again filled with liquid acetone or other
suitable fluid, in which the acetylene is dissolved as rapidly as it is
pumped into the cylinder.
To compress acetylene in the ordinary way is neither safe nor practicable;
but these 'dissolved' cylinders are now used extensively for both
oxy-acetylene welding and motor car lighting, and may be entirely relied
upon.
The D.A. (Dissolved Acetylene) Company were the pioneers in this country of
the industry, and their methods of business are peculiar and ingenious. The
user is requested in the first place to purchase a cylinder, and he then
becomes the owner of _a_ cylinder, but not of one _particular_ cylinder. A
list is supplied to him of various depots in the country where the
Company's cylinders are stored, and when empty he can, on payment of a
fixed sum, exchange his empty cylinder for a full one, which then becomes
_his_ cylinder _pro tem_.
This saves the delay and expense of returning a cylinder to London, and
incidentally clears the customer of any question of deterioration, this
being obviously covered by degrees with each individual exchange. The
system was first introduced in connection with the lighting of cars and
only applies to the standard size for this purpose, viz. 20 cubic feet
capacity, {36} but as this is, on the whole, the most convenient size for
lantern work also, the limitation is not a disadvantage. The arrangement is
also in vogue to a less extent with cylinders of 6 feet capacity (a size
sometimes used for motor _cycles_), but the depots of exchange are at
present far fewer for this size.
The oxy-acetylene blast is much _hotter_ than the ordinary oxyhydrogen, and
therefore produces a more intense light. I have therefore used it with
success on occasions when even the ordinary limelight would fail, and the
choice has lain between an oxyhydrogen jet of enormous bore (and, of
course, corresponding consumption of gas), and the oxy-acetylene.
For this very reason great care must be taken only to use the hardest
limes, and even then to use the lime-turning movement frequently, or the
lime will pit or crack and a broken condenser follow.
THE FALLOT ACETYLENE LIGHT.--This light consists of a jet of acetylene
under pressure, without oxygen, but producing its own _air blast_ from the
atmosphere by suction, much as the 'Injector' jet does, but the reverse way
round.
The light is better than with an ordinary acetylene jet, though not quite
so good as with a 'blow-through' jet; but as it only requires a cylinder of
dissolved acetylene, or even a 'Pressure' generator, it is fast coming into
favour.
The peculiarity of the Fallot apparatus is that, instead of providing a
direct beam of light in the direction of the screen, it projects the beam
_backwards_ on to a concave mirror, and it is the reflected light from this
that is used (Fig. 23).
Instead of a lime is used a spherical 'Pastille' of peculiar composition,
and before use each pastille must be burnt off exactly like an incandescent
gas mantle, after which it is extremely fragile and difficult to handle.
To use this illuminant one lens of the condenser must be removed, the
curvature of the mirror taking its place, and it will be seen at once that
the pastille itself will get in its own {37} light and throw a shadow,
which actually happens, but it is hardly perceptible unless specially
looked for.
[Illustration: FIG. 23.--Fallot Air Blast.]
A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve,
pressure gauge and burner, with two spare pastilles, is shown in Fig. 24,
but if preferred a regulator, such as previously described for oxygen, can
be used instead of the fine adjustment valve.
[Illustration: FIG. 24.--Fallot Air Blast, and Cylinder.]
{38}
FALLOT OXY-ACETYLENE BLAST.--This is similar to the foregoing, utilising
oxygen from a cylinder instead of air, and the light is equal to a powerful
limelight, and may be considered as an efficient substitute, though for
_long range_ work the shadow before alluded to becomes more noticeable (for
optical reasons which need not be here discussed). The Fallot Company also
make a special 'Pressure Generator' which can be used instead of a D.A.
Cylinder; but my experience of this so far is that, although perfectly
safe, the blast from it is a little unsteady as compared with a cylinder.
LIMES AND ACCESSORIES.--Limes for Optical Lantern work are usually supplied
in the form of cylinders, the 'ordinary' size being 7/8 inch in diameter
and about 1½ inches long, with a hole drilled longitudinally to take the
lime pin. Extra large limes up to 2 inches in diameter are supplied for
more powerful jets.
So-called 'soft' limes used to be recommended for 'blow-through' jets as
giving a better light than 'hard' limes, but the advantage, if any, is very
little, and these limes are now very seldom heard of, possibly because
'blow-through' jets themselves are becoming less and less used, and 'soft'
limes will not stand the heat of a mixed or 'Injector' jet for long.
'Hard' limes are turned out of the hardest stone lime, and must be kept in
sealed tins until used, as they rapidly disintegrate when exposed to the
air. There are one or two quarries known to provide the best lime for
lantern purposes, and the various good brands on the market practically
have the same origin as regards raw material, though called by different
trade names; and the 'Hardazion' (hard as iron) limes, placed on the market
some years ago by a well-known wholesale firm, to be countered shortly
after by the 'Hardastil' (harder still) brand, are, I take it, legitimate
though amusing instances of phonetic advertisement.
Even the best of limes is liable to crack under the heat {39} of a powerful
jet, and so a pair of lime-tongs should always be provided, and there is
nothing better than the simple form shown in Fig. 25, and which is, or
should be, sold by all dealers.
[Illustration: FIG. 25.--Lime-tongs.]
SUBSTITUTES FOR LIMES.--A good substitute for lime, that will give the same
light, stand heat equally as well, and _not_ deteriorate if exposed to the
atmosphere, has long been sought for, and some of the more recently
discovered refractory materials are more or less satisfactory. 'Mabor'
limes, for example, belong to this class, and so do some of the
'pastilles,' which before the war came chiefly from France and to a less
extent from Germany.
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