SCREENING SMOKES
The intelligent use of screening smokes in modern infantry tactics
offers innumerable advantages through concealment and deception. It
confers upon daylight operations many of the advantages which were
gained by conducting operations at night with few of the disadvantages
of the latter.
Smoke screens have been frequently used by the Navy and by Merchantmen;
a common method of escape was to shut off the air from the fire with
consequent incomplete combustion of the fuel, thus causing a cloud of
dense black smoke. This is often mentioned in the blockade runners of
the days in the Civil War, where wood, high in pitch and rosin, was
freely introduced into the furnaces, in order that they might escape
under cover of this smoke.
Early in the present war it was found that black smoke had a low
obscuring power, showed frequent rents or holes and were difficult
to standardize. Their production also caused a considerable loss in
the speed of the vessel. They therefore fell into disuse except for
emergency purposes and today the standard smoke for screening purposes
of all kinds is, without exception, white.[33]
[Footnote 33: While it is a well known fact that black smoke is not as
efficient as white smoke for screening purposes, the reason for this
fact is not clear.]
PROPERTIES OF SMOKE CLOUD
The properties most desired in a screening smoke, apart from low cost,
are: (_a_) _Maximum screening power_, which refers to the question of
density, i.e., a relatively thin layer must completely obscure any
object behind it, and (_b_) _Stability_, which implies, among other
things, a low rate of settling or dissipation. There is little reason
to doubt that, within limits, the smaller the particles of a smoke
cloud, the more completely will the smoke possess these qualities.
The screening power of a smoke cloud depends very largely upon the
scattering of the light coming through it, and by analogy with those
peculiar solutions which we call colloidal, we should expect the
scattering to increase as the degree of subdivision increases, within
limits. The rate of settling is unquestionably an inverse function
of the size of the particles. The chief aim, therefore, in smoke
production is to attain as high a degree of subdivision as possible.
Methods may be classified as good or bad, in so far as they satisfy or
fail to satisfy this criterion.
RAW MATERIALS FOR SMOKE CLOUDS
It is obvious that only gases or substances capable of being brought
into the vapor state or into a very fine state of subdivision can be
used for producing smoke clouds. The reaction product, of which the
smoke particles consist, should preferably be:
(_a_) _Solid._ Otherwise the particles will tend to grow in size by
condensation of the liquid particles present in the cloud.
(_b_) _Non-volatile._ If volatile, the particles will disappear by
evaporation as the cloud is diluted by air currents. Larger particles
will also form at the expense of the smaller ones.
(_c_) _Non-deliquescent._ If the particles are deliquescent, they will
tend to grow by condensation of water vapor upon them.
(_d_) _Stable_ towards the usual components of the atmosphere,
especially moisture.
While it might seem that it would be difficult to fulfill these
conditions, there are several chemical compounds which have been
successfully used as smoke producers. This does not mean that they
fulfill all the conditions, but they represent a compromise between the
various requirements.
=Phosphorus.= One of the earliest materials to be used in smoke clouds
was phosphorus. This is prepared on a commercial scale by heating
phosphate rock (which contains calcium phosphate) with sand and coke
in an electric furnace. Phosphorus occurs in two forms, white and red.
_White phosphorus_, which is formed when the vapor of the substance
is quickly cooled, is, in the pure state, almost colorless, melts at
44° C., boils at 287° C., is readily soluble in various solvents, and
is luminous in the air, at the same time emitting fumes (the oxidation
product, phosphorus pentoxide). On gentle warming in the air, it takes
fire and burns with a brightly luminous flame. _Red phosphorus_ is
obtained by heating white phosphorus out of contact with the air, to a
temperature of 250° to 300° C. Red crusts then separate out from the
colorless liquid phosphorus, and almost the entire amount is gradually
converted into a red, solid mass. If this is freed by suitable solvents
from the small amounts of unchanged white phosphorus, a dark red powder
is obtained, which remains unchanged for a long time in the air, does
not appreciably dissolve in the solvents for white phosphorus, does
not become luminous, and can be heated to a fairly high temperature
without igniting. Further, red phosphorus is not poisonous, while white
phosphorus is highly so.
Either form burns to phosphorus pentoxide, which is converted by the
moisture of the air to phosphoric acid,
4P + 5O₂ = 2 P₂O₅
2P₂O₅ + 6H₂O = 4H₃PO₄
Since one pound of phosphorus takes up 1.33 pounds of oxygen and 0.9
pound of water, it is not surprising that phosphorus is one of the best
smoke producers per pound of material. Comparison of the value of the
two forms for shell purposes have invariably pointed to the superiority
of the white variety.
In addition to its use as a smoke producer, it is used in incendiary
shell and in tracer bullets. For incendiary purposes a mixture of red
and white phosphorus is superior.
=Chlorosulfonic Acid.= Chlorosulfonic acid, ClSO₂OH, was first
employed by the Germans to produce white clouds, both on land and on
sea. For this purpose, they sprayed or dropped it onto quicklime, the
reaction between it and the lime furnishing the heat necessary for
volatilization, though in this way about 30 per cent of the acid is
wasted.
Chlorosulfonic acid is obtained from sulfur trioxide and hydrogen
chloride, which combine when gently heated:
SO₃ + HCl = ClSO₂OH
[Illustration: FIG. 86.—75 mm. White Phosphorus Shell. 2 seconds after
bursting.]
On a commercial scale, hydrogen chloride is passed into 20 per cent
oleum, until saturation is reached. This is heated in a nitric acid
still, when the chlorosulfonic acid distills over between 150°-160° C.
With 30 per cent oleum, the conversion factor is about 42 per cent. The
residue in the still is about 98 per cent sulfuric acid.
It forms a colorless liquid, boiling at 152° C., and having a density
of 1.7.
Chlorosulfonic acid fumes in the air, because reaction with water forms
sulfuric acid and hydrochloric acid.
ClSO₂OH + H₂O = H₂SO₄ + HCl
This material was not used by the United States since oleum was found
superior.
=Oleum.= Oleum is a solution of 20 to 30 per cent sulfur trioxide
(SO₃) in concentrated sulfuric acid. It has been used by the Germans
to produce clouds on land and sea, by its contact with quicklime, and
by the Americans for screening tanks and aeroplanes. Sulfur trioxide
has been found to be superior as a shell filling. It is believed that
the smoke producing power of oleum is due solely to its sulfur trioxide
content, the sulfuric acid itself acting only as a solvent. The rather
high freezing point of the oleum containing high percentages of sulfur
trioxide is a disadvantage.
=Sulfur Trioxide.= Sulfur trioxide, SO₃, is a colorless mobile liquid,
which boils at 46° C. and solidifies to a transparent ice-like mass,
melting at 15° C. It is prepared by passing a mixture of sulfur dioxide
and oxygen over finely divided platinum or other catalysts at a
temperature between 400 and 450° C. Sulfur trioxide can only be used as
a filler for shell and bombs, and is probably the best substitute for
phosphorus.
=Tin Tetrachloride.= Tin tetrachloride, SnCl₄, is obtained by the
action of chlorine on metallic tin. It is a liquid, boiling at 114° C.,
and having a density of 2.2. It fumes in the air, because it hydrolyzes
to stannic hydroxide:
SnCl₄ + H₂O = Sn(OH)₄ + 4 HCl
It makes a better and more irritating smoke for shell and hand
grenades, than either silicon or titanium tetrachlorides. Since there
is practically no tin in this country, the other tetrachlorides were
developed as substitutes.
=Silicon Tetrachloride.= Silicon tetrachloride, SiCl₄, is prepared from
silicon or from impure silicon carbide by heating it with chlorine in
an electric furnace. The raw material (silicon carbide) is a by-product
in the manufacture of carborundum. It is a colorless liquid, boiling at
about 58° C., and fumes in moist air, owing to hydrolysis:
SiCl₄ + 4 H₂O = Si(OH)₄ + 4 HCl
It is not very valuable in shell, though it is more effective on moist,
cool days than on warm, dry ones. Its greatest use is found in the
smoke cylinder, combined with ammonia. By the action of the moisture of
the air, the following reaction takes place:
SiCl₄ + 4 NH₃ + 4 H₂O = Si(OH)₄ + 4NH₄Cl
The addition of a lachrymator gives a mixture which works well in hand
grenades for mopping up trenches.
=Titanium Tetrachloride.= Titanium tetrachloride, TiCl₄, is made from
rutile, TiO₂, by mixing with 30 per cent carbon and heating in an
electric furnace. A carbonitride is formed, which is said to have the
composition Ti₅C₄N₄, but the actual composition may vary from this to
the carbide TiC. This product is heated to 600-650° C., and chlorine
passed through, giving the tetrachloride. It is a colorless, highly
refractive liquid, which boils at about 136° C., is stable in dry air
and fumes in moist air. The best smoke is produced by using 5 parts of
water to one of the tetrachloride, instead of the theoretical 4 parts
[which would form Ti(OH)₄.] Since it is more expensive to manufacture
and not as effective as silicon or tin tetrachloride, it is used only
as an emergency material.
=Berger Mixture.= One of the most important smoke materials was the
zinc-containing mixture, which was used in the smoke box, the smoke
candle, certain of the smoke grenades and in various forms of colored
smokes. The basis of this was the _Berger Mixture_, which had the
composition:
Zinc 25
Carbon tetrachloride 50
Zinc oxide 20
Kieselguhr 5
This formula produced a light gray carbon smoke, with much carbon in
the residue. In this mixture the zinc and carbon tetrachloride react to
form zinc chloride and carbon; the kieselguhr keeps the mixture solid
by absorbing the tetrachloride, while the zinc oxide is practically
useless, as its absorbing power is small.
In order to accelerate the reaction and to oxidize the carbon, thereby
changing the color of the smoke from gray to white, an oxidizing
agent was added. Sodium chlorate was chosen for economic reasons. The
reaction now proved to be too violent, and the zinc oxide was replaced
by ammonium chloride. This cooled the smoke, retarded the rate of
burning and added to the density of the smoke, since the obscuring
power of the ammonium chloride is high. The kieselguhr was replaced by
precipitated magnesium carbonate, which is as good an absorbent, gives
a much smoother burning mixture, and also adds somewhat to the density
of the smoke by virtue of the magnesium mechanically expelled. The
mixture then had the composition:
Zinc 34.6
Carbon tetrachloride 40.8
Sodium chlorate 9.3
Ammonium chloride 7.0
Magnesium carbonate 8.3
SIZE OF SMOKE PARTICLES
In the problem of smoke production, the size of the particle is of
great importance. Being a physical quantity it can easily be correlated
with such physical properties as settling, diffusion, coagulation, and
evaporation. These factors are more important in connection with toxic
smokes, since there the penetration factor must be considered.
Smoke appears to consist of particles of all sizes from 10⁻³ cm.,
which may just be resolved by the unaided eye, to molecular
dimensions, 10⁻⁸ cm. The larger particles settle out most rapidly and
so do not remain long in suspension.
MEASUREMENT
Wells and Gerke have developed a form of ultra-microscope which is
well adapted to the measurement of the size of smoke particles. The
ultra-microscope is a low power microscope using intense dark ground
illumination for viewing particles which are too small to be seen
by transmitted light. They are rendered visible in this way, since
any object, no matter how small, which emits enough light to affect
the retina is visible, provided the background is sufficiently dark.
Thus stars are visible at night and dust particles are easily seen
in a sunbeam in a darkened room. The larger particles, viewed in
this way, do not appear larger but brighter. The apparent size of
the particles is determined by the diffraction pattern and is thus
dependent only on the optical system used to view them. The more
intense the incident light, the brighter the particles appear. In the
ultra-microscope described, the image of an intense source, such as a
concentrated filament lamp, or an arc, is focused upon the particles in
the microscopic field, but the axis of the illuminating beam, instead
of coinciding with the axis of the microscope, as ordinarily used,
is perpendicular to it. The beam itself, therefore, never enters the
microscope at all, but passes under the objective into a blackened
chamber where it is absorbed. The field of the microscope is made dark
by placing underneath the objective another “black hole” or blackened
chamber with an opening just a little larger than the field.[34]
[Footnote 34: This ultra-microscope is described in _J. Am. Chem. Soc._
=41=, 312 (1919).]
The method used for measuring the velocity consisted in causing the
particle to describe a definite stroke many times in succession in an
electric field. This was accomplished by reversing the direction of the
field with a rotating commutator. The convection due to the source of
light is perpendicular to this motion so that a zigzag line is obtained
(see Fig. 88). The amplitude of this oscillation is an accurate measure
of the distance traversed by the particle under the electric force for
a definite small interval of time. The speed of the rotating commutator
and the electric field are both susceptible of precise measurement, so
that the size of a single particle is precisely determined.
[Illustration: FIG. 87.—Ultramicroscope for Measuring Size of Smoke
Particles.]
[Illustration: FIG. 88.—Measurement of Smoke Particles by Use of
Ultramicroscope.]
When a sample of smoke is viewed in the ultra-microscope, it appears
like the starry heavens, except that the stars are dancing violently
about. At first little distinction is made between the particles, as
there seems to be no order in their motion, but soon it becomes evident
that the brighter particles are more sluggish than the dim ones. This
is due to the greater mass of the bright particles, for they are
larger. The particles are all moving slowly away from the source of
light and eventually diffuse to the walls of the cell.
When the electric field is turned on, about one-third of the particles
immediately migrate, about equally in both directions, to the two
electrodes. If the field is reversed, the direction of migration
is reversed and if the commutator is used the particles oscillate
regularly. Sometimes the particles may be seen to combine and become
neutral, in which case oscillation ceases.
CONCENTRATION OF SMOKE
In measuring the concentration of smokes, the following terms are
useful:
=Density.= The density of a smoke is defined as the reciprocal of the
thickness of the smoke layer in feet necessary to obscure a given
filament. Thus six inches of a smoke of density 2.0 is required to
obscure an electric light filament, whereas one requiring four feet
would have a density of 4. Another way to show the significance of this
definition is to point out that if a definite weight of a stable smoke
is diluted with air after it is formed, the product of the volume by
the density always remains constant. Any marked variation in this rule
may be taken as evidence that the particles of smoke are undergoing a
change, in most cases due to evaporation.
=Total Obscuring Power.= The volume of smoke produced per unit weight
of material used is the second factor in determining the value of a
smoke. The product of this volume per unit weight by the density of the
smoke is the real measure of effectiveness, and is called the total
obscuring power (T. O. P.) of the smoke. If the volume is expressed in
cubic feet per pound and the density in reciprocal feet, the unit of T.
O. P. is square feet per pound. That is, it expresses the square feet
of a smoke wall, thick enough to completely obscure a light filament
behind it, which could be produced from a pound of the reacting
substances. The total obscuring power of some typical smokes are as
follows:
Phosphorus 4600
NH₄Cl(NH₃ + HCl) 2500
SnCl₄ + NH₃ + H₂O 1590
Berger Mixture 1250
SnCl₄ + NH₃ 900
SO₂ + NH₃ 375
In all measurements of density, and therefore of T. O. P., the _rate
of burning_ must be considered. If a slow burning material be compared
with a rapid one, the former will not reach its true maximum density,
as a great deal of the smoke may settle out during the time of burning.
Comparisons of T. O. P. are significant only when made on smoke
mixtures of the same type and in about the same quantities.
MEASUREMENT
Two methods of measuring the effectiveness of a smoke cloud have
been devised, one, the smoke box, which measures the obscuring power
directly by observing at what distance a lamp filament is obscured by
intervening smoke, the other, the Tyndall meter, which measures the
intensity of the scattering of the light.
The earliest measurements of smoke intensity are perhaps those of
Ringelmann (_Revue Technique_, =19=, 286), who devised the well known
chart of that name, intended mainly for measuring intensities of black
smoke issuing from a chimney at a distance. The first measurements
for military purposes are probably due to Bertrand, who made numerous
comparative studies with his “salle opacimetrique.” This was a room
23 × 14 × 3.6 meters, with 7 windows. Two doors, one provided with
3 oculars 2 cm. in diameter, gave access to the room. On the other
door, opposite the first, were hung several black signs. Six pairs
of columns were spaced along the room at measured distances. When a
smoke is produced in the room, the black paper signs first become
invisible, then the door itself, and finally the columns, pair by
pair. They reappear in the reverse order, and as a measure of relative
opacity Bertrand took the time elapsing between the detonation and the
reappearance of the farther door.
[Illustration: FIG. 89.—Tyndall Meter.]
[Illustration: FIG. 90.—Cottrell Precipitation Tube.]
=Smoke Box.= The smoke box, used by the C. W. S., was constructed of
wood with tight joints, and had a moveable brass rod running through it
to which was attached a small size 25-Watt Mazda lamp. The density of
each smoke introduced in the box is determined by moving this lamp back
and forth until a point is reached when the pattern of the filament
can just be distinguished by the observer looking in at the glass
window, external light being excluded by a black cloth. The thickness
of the smoke layer between the glass window and the light is recorded
as the measure of the smoke density. For field tests, a larger box, 6
× 8 × 8 feet (288 cubic feet) was constructed. The observation light
was moveable in a line connecting the mid-points of opposite sides of
the box. To insure uniform distribution of smoke, a fan with 18-inch
blade revolved at any desired speed between 60 and 250 r.p.m. With
this, results are obtained indicating both the original density and its
stability.
=Tyndall Meter.= The Tyndall meter was first devised for studying
smokes and mists. Tolman and Vliet adapted it to Chemical Warfare
purposes, and used it in studying the properties of smokes.
The apparatus (Fig. 89) consists eventually of an electric light bulb,
a condensing lens giving a beam of parallel light which passes through
the diaphragm, and a Macbeth illuminometer for measuring the strength
of the Tyndall beam. In case the material is a liquid suspension or
solution, it is introduced into a cylindrical glass tube, while smokes
and mists are premixed directly through the apparatus. The long closed
tubes are provided, respectively, for absorbing the beam after it has
passed through the disperse system and for giving a dark background for
observing the Tyndall beam. Methods of standardization are given in the
_Journal of the American Chemical Society_, =41=, 299.
A third method for analyzing smokes consists in the use of an
electrical precipitator. This apparatus consists essentially of
a modified Cottrell Precipitator, with a central wire as cathode
surrounded by a cylindrical foil as anode (Fig. 90). The smoke to
be analyzed is drawn through the apparatus at a known rate, and the
particles of smoke precipitated on the foil by means of a high voltage,
direct current. The determination of concentration is made by weighing
the foil before and after precipitation.
APPARATUS FOR SMOKE PRODUCTION
SMOKE BOX
The smoke box was developed for the Navy for use when it was desirable
to have the smoke screen generated away from the ship. (The smoke
funnel, described later, was operated on board ship). The float
consists of an iron container (holding the smoke mixture) surrounded
by an iron float to support the apparatus when it is thrown into the
water (Fig. 91). The iron container consists of a cylinder 22 inches
high and 10 inches in diameter. One inch holes are bored 1½ inches
from the top of this cylinder, from which the smoke is emitted. The
iron float is about 2 feet in diameter and 8 inches deep. This box
holds approximately 100 pounds of smoke mixture, and is so constructed
that it will float one hour. When ignited, the mixture burns 9 to 9½
minutes. The smoke produced has a T. O. P. of about 1900. Fig. 92 shows
the Navy Smoke Box in action.
[Illustration: FIG. 91.—Navy Smoke Box.]
[Illustration: FIG. 92.—Navy Smoke Box in Action.]
SMOKE CANDLE
Smoke candles are used for producing a cloud of smoke for screening
purposes in or behind the lines. They are made by packing about
three pounds of the modified Berger Mixture in a container (Fig. 93)
(galvanized can 5¼ inches by 3½ inches) and are lighted by means of the
match head type of ignition. Smoke is given off at a uniform rate for
about 4 minutes, forming a dense, fog-like cloud which hangs low (Fig.
94). This smoke is absolutely harmless, and can be breathed without
discomfort. The obscuring power is high and, with a favorable wind, a
small number of the candles will produce a screen sufficiently dense to
allow operations to be carried out unseen by the enemy.
[Illustration: FIG. 93.—B. M. Smoke Candle.]
SMOKE GRENADE
The smoke grenade is also designed for use in trench and field warfare,
where it is desired to produce a dense smoke screen. It is made by
packing 340 grams of the standard smoke mixture in an ordinary light
metal gas grenade. Around the top of the grenade are vents closed by
a zinc strip. The ignition is caused by the standard bouchon when the
grenade is thrown. The heat of the reaction burns through the zinc
strip and a dense cloud of smoke is evolved for 45 seconds.
[Illustration: FIG. 94.—Smoke Cloud from B. M. Candle.]
Stannic chloride has also been used extensively in hand grenades, as
it gives a very disagreeable cloud of smoke upon detonation. Due to
the high prices and urgent need of tin for other purposes, silicon
tetrachloride was substituted for tin tetrachloride towards the close
of the war. A mixture of silicon tetrachloride and chloropicrin was
also used. This not only gives a very good smoke cloud, but combines
with it the toxic properties of the chloropicrin cloud.
The method of firing the smoke grenade is the same as that of any
grenade using the same type of bouchon. Usually the grenade is grasped
in the hand for throwing in such a manner that the handle of the
bouchon is under the fingers. The safety clip is pulled out with the
other hand and the grenade is thrown with an overhand motion. When the
grenade leaves the hand, the handle of the bouchon flies off, allowing
the trigger to hit the cap which ignites the fuse.
The white phosphorus combined hand and rifle grenade became the
standard smoke grenade by the end of the war. Stannic chloride was used
to clear out dugouts, but not as a smoke producer.
STOKES’ SMOKE SHELL
[Illustration: FIG. 95.—Stokes’ Smoke Shell.]
The Stokes’ smoke shell was perfected to furnish a means of maintaining
the best possible smoke screen at long ranges by means of an easily
portable gun. The 3-inch Stokes shell, as adapted for combustion
smokes, weighs about 13 pounds and contains about 4 pounds of standard
smoke mixture. This shell is designed to produce a continuous screen
over a period of 3 to 4 minutes.
LIVENS SMOKE DRUM
The Livens smoke drum was designed for use with the 8-inch Livens
projector, so as to produce a smoke screen of large volume and long
duration at long ranges. The drum, as adapted for combustion smokes,
weighs 17.5 pounds empty and 49 pounds loaded. The smoke-gas mixture
was specially adapted for use in the Livens drum.
[Illustration: FIG. 96.—Livens Smoke Bomb.]
Smoke mixtures in Livens were never used to any considerable extent
in the war and it is questionable if they ever will be. A Livens can
usually only be fired once before resetting, hence Stokes mortars are
used whenever possible.
SMOKE FUNNEL
The smoke funnel was developed for the production of a white smoke
cloud from the stern of a vessel. The smoke producing materials are
liquid ammonia and silicon tetrachloride, with carbon dioxide as a
compressing medium. This is the most satisfactory compressing medium,
because: (1) The silicon tetrachloride is forced out at nearly constant
pressure. (2) The carbon dioxide is easily compressed to a liquid and
can be handled in this form. Further, it has a vapor pressure of 800
pounds at 60° F., and a cylinder can be nearly emptied without loss
in efficiency. (3) Carbon dioxide is sufficiently soluble in silicon
tetrachloride to cause the latter to effervesce and thus materially aid
in its evaporation on spraying. (4) Liquid carbon dioxide, behaving in
a manner similar to liquid ammonia, affords a means for the silicon
tetrachloride to “keep pace” with the ammonia, under changes in
temperature, and thus ensures a more nearly neutral, and therefore the
most effective, smoke.
[Illustration: FIG. 97.—Navy Smoke Funnel.]
The smoke funnel proper consists of an open end cylinder, about 2 feet
in diameter and 7 feet long, mounted in a horizontal position on an
angle iron frame. At one end is an 18-inch fan securely fastened to the
cross supports. This fan is operated by hand, through gears giving a
ratio of about 30 to 1. The silicon tetrachloride enters the cylinder
through a pipe, which terminates in four spray nozzles, while the
ammonia enters through a single nozzle. The air forced into the funnel
serves to hydrolyze the silicon tetrachloride and mixes the vapors. The
resulting reaction evolves a dense white cloud of very large volume and
high obscuring power. One set of cylinders is capable of maintaining
this cloud for over 30 minutes. Under normal conditions the discharge
is at the rate of 2 pounds of silicon tetrachloride to 1 pound of
ammonia. To stop the smoke, the silicon tetrachloride is closed first,
the ammonia allowed to run about half a minute, and the fan is shut off
last.
[Illustration: FIG. 98.—Navy Smoke Funnel in Operation.]
SMOKE KNAPSACK
The smoke knapsack furnishes a portable apparatus for smoke production.
The gross weight is about 70 pounds; when in operation it gives a dense
white smoke for about 15 minutes. The operation may be intermittent
or continuous and the quantity of smoke is sufficient to completely
hide one platoon of men in skirmish formation with a 5-mile per hour
enfilade wind. The apparatus consists of two steel tanks about 26
inches in height and 6 inches in diameter. From the side of each tank,
but near the bottom, extends a short pipe on which is placed a suitable
valve. A flexible armored hose connects the valve to a short length of
pipe which is equipped with spray nozzle. The cylinders are charged
with silicon tetrachloride and ammonia under pressure. The valves may
be operated with the left hand, while the sprayer apparatus is held in
the right. The release buckles are within easy reach of both hands.
SHELL
While many special devices have been developed by means of which the
gas troops and infantry are able to set up smoke clouds on short
notice, the smoke shell, fired by the artillery, always played an
important part in this work. In the same way that a large number of the
poison gases were adapted to artillery use, so were most of the smoke
producing substances.
As a filler for smoke shell, phosphorus easily ranks first, and is
approached only by sulfur trioxide in very humid weather. A rough
approximation to the relative values of some of its rivals is given in
the following table:
White phosphorus 100
Sulfur trioxide 60-75
Stannic chloride 40
Titanium chloride 25-35
Arsenic chloride 10
Comparison of the value of different forms of phosphorus for shell
purposes has invariably pointed to the superiority of the white
variety. Mixtures of white and red (2 to 1) have also proved effective.
A complete barrage over a front of 200 yards can be established in from
40 seconds to 1 minute and maintained by firing a salvo followed by
battery fire of 3 seconds. Four 4.5-inch howitzers could maintain an
effective barrage over a front of 1000 yards. The influence of sunshine
is very marked, as in moist, cool weather one shell every 15 seconds is
sufficient.
[Illustration: FIG. 99.—Smoke Screen for Tanks.]
SCREENING TANKS
Tests have demonstrated (see Fig. 99) that successful smoke screens for
tanks may be produced by spraying oleum into the exhaust. On a 7-ton
tank of the Renault type (40 H. P.) 110 cc. per minute produced a large
volume of smoke, which had excellent covering power, and which could be
made intermittent or continuous at will.
The same method may be applied to aeroplanes, and to ships. It is
calculated that a cylinder containing 300 pounds of 20 per cent oleum
will maintain a smoke screen on a ship for a period of 15 minutes,
if oleum is used at the rate of 23.6 pounds per minute. Since the
cylinders may be arranged in batteries, the screen may be continued
for any period of time. The Tank Corps rather favor phosphorus rifle
grenades for producing a smoke screen at a distance from the tank.
PURPOSE OF SMOKE SCREEN
Smoke screens may be employed with one or more of the following objects
in view:
(1) To mask known enemy observation posts and machine gun nests; to
conceal the front and flanks of attacking troops, concentration of guns
and tanks, roads and concentration points; to blind the flashes of
batteries in action and to hamper aerial observations.
(2) As a feint to draw the enemy’s attention to a front on which no
attack is being made, so as to hold his troops to their trenches, or to
induce him to expend ammunition needlessly and to put down a barrage in
the wrong place.
(3) To simulate gas and force the enemy to wear his mask. Gas should
occasionally be mixed with smoke, to impress upon him the belief that
it is never safe to remain in a smoke cloud without wearing his mask.
(4) In rolling or mountainous country, to fill valleys with smoke and
thereby conceal the advance from all observation, including aerial.
(5) To cover the construction of bridges, trenches, etc., in the face
of the enemy.
THE TACTICAL VALUE OF SMOKE
The pall of smoke that hung over every battlefield of the Civil
War made a profound impression upon Fries when, as a boy, he first
read of those battles. However, practically every reference made
to smoke treated it as a nuisance. It obscured the field of vision
and interfered with troop movements as well as with the aiming and
firing of rifles and cannon, though due to their short range this
was not so serious as it would be nowadays. Nevertheless so deeply
was this interference appreciated that the most earnest efforts were
made to discover a smokeless powder. This, as the world well knows,
was developed with great efficiency during the latter part of the
nineteenth century. With the development of the smokeless powders
came also a better understanding of the action of powder, whereby the
velocity of projectiles, and consequently the range and accuracy, were
greatly increased. This increased range and accuracy of guns forced a
consideration of protection,—and concealment is one form of protection.
The Navy would appear to have been the first branch of the American
forces to realize how valuable a smoke screen may be. Thus Fries, in
August, 1913, had the interesting experience of witnessing a week’s
maneuvers at the eastern entrance to Long Island Sound between the
Navy and the Coast Artillery. During that week the Navy carried out
extensive experiments with smoke screens both by day and by night. The
smoke in all cases was generated by smothering the fires on destroyers
or other ships, thus causing dense clouds of black smoke to be given
out from the funnels.
After the World War had been in progress some time and particularly
about the time the United States entered it, a determined search was
begun for more efficient smokes and more efficient smoke producers.
In the Navy, smoke screens were expected to be established by small
craft behind which larger vessels could maneuver for position and
range. These screens were also established for the purpose of cutting
off the view of enemy submarines or other vessels, thus allowing
merchant ships or even warships when injured or outclassed to escape.
The Army was much slower to appreciate the value of smoke. In fact,
apparently no army really realized the value of a smoke screen until
after gas warfare became an accomplished fact. As is well known, the
evaporation of the large quantity of liquid used in wave attacks
caused a cloud of condensed moisture. This is what gave rise to the
designation “cloud attack.”
English regulations for defense against gas in the early days called
for every man and animal to stand fast upon the approach of a gas cloud
and remain quiet until the cloud had passed. Thus casualties were
reduced to a minimum and the English were fresh to receive the attack
that was frequently launched immediately after the cloud had passed.
The Germans finally thought of the plan of sending over a fake gas
attack. In that way they simply produced a smoke cloud that looked like
a gas attack. Naturally the English stood fast as before. The Germans
attacking in the fake cloud naturally caught the British at a complete
disadvantage with consequent disastrous results to the latter.
But that was a game at which two could play. About this time the
value of white phosphorus for producing a smoke screen was taken up
by the British and large numbers of 4-inch Stokes mortar shells were
filled for that purpose. All armies then began to experiment with
smoke producing materials. Most of these were liquid. Of them all, as
has been stated before, white phosphorus, a solid, proved the best.
Toward the close of the war these smoke screens began to be used to
a considerable extent for the purposes given above. No one who has
engaged in target practice and encountered a fog, or who has hunted
ducks and geese in a fog needs to be told of the difficulty of hitting
an object he cannot see.
The First Gas Regiment proved its worth and won everlasting glory by
using the Stokes’ mortars of the British with their phosphorus bombs
for attacking machine gun nests. The white phosphorus in that case had
a double effect. It made a perfect smoke screen, thereby making the
German machine gun shots simply shots in the dark, while at the same
time the burning phosphorus forced the gunners to abandon their guns
and surrender. Thus phosphorus played and will play in the future the
double rôle of forming a defensive screen and of viciously attacking
enemy troops. This phosphorus, which catches fire spontaneously, burns
wet or dry, total immersion in water alone sufficing to put it out.
This means of extinguishing the flames being almost totally absent on
the battlefield, it can be truthfully said that burning phosphorus is
unquenchable. The burns are severe and difficult to heal. For these
reasons white phosphorus will be used in enormous quantities in any
future war.
All armies have begun to realize this value of smoke. In the future it
will be the infantryman’s defense against all forms of weapons and it
will be used on every field of battle, by every arm of the service and
at all times, day or night. It is even more effective in shutting out
the light from searchlights, star bombs and similar illuminants for
use in night attacks than it is in daylight. With this straight use of
smoke for protection will go its use along with poisonous gases. Every
smoke cloud will be poisonous or non-poisonous at the will of the one
producing the cloud, and this will be true whether it is produced from
artillery shell, mortar bombs, hand grenades, smoke candles or other
apparatus. Thus smoke and gas together will afford a field for the
exercise of ingenuity greater than that of all other forms of warfare.
The only limitation to the use of smoke and gas will be the lack of
vision of commanders and the ignorance of armies.
Proper recognition and aid given to chemical warfare development
and instruction in peace are the only methods of overcoming these
limitations. In this, as in all other development work, the most
serious obstacle comes from the man who will not see, whether it be
from a lack of intelligence, laziness or inbred opposition to all forms
of advancement.