TOXIC SMOKES
The introduction of diphenylchloroarsine as a poison gas really
introduced the question of toxic smokes. This material, as has
already been pointed out, is a solid, melting at about 30°. In order
to secure efficient distribution, the material was mixed with a
considerable amount of high explosive. When the shell burst, the
diphenylchloroarsine was finely divided or atomized and produced a
cloud of toxic particles. Since smoke particles are only slightly
removed by the ordinary mask, this formed a very effective means of
chemical warfare.
An analogous result was obtained by the use of poison gases, such
as chloropicrin, in a smoke cloud produced from silicon or stannic
chloride. Here, however, the toxic material was a real gas, and so the
real result attained consisted in forcing the men to wear their masks
in all kinds of clouds. The true toxic smoke went further in that the
ordinary mask offered little protection and thus compelled the warring
nations to develop a special type of smoke filter.
These smoke clouds consist of very small particles, which may be
considered as a _dispersed_ phase, distributed in the air, which we may
call the _dispersing medium_. The dispersed phase may be produced by
_mechanical_, _thermal_, or _chemical_ methods.
_Mechanical dispersion_ consists in the tearing apart of the material
into a fine state of subdivision. It may be called a hammer and anvil
action. The more powerful the mechanical force, the smaller the
resulting particles. This may be accomplished by the use of a high
explosive, such as the Germans used in the case of diphenylchloroarsine.
The production of smoke by _thermal dispersion_ depends essentially
upon the fact that when a substance of sufficiently low vapor pressure
is volatilized, and the vapors are passed into the air, they
recondense on the nuclei of the air to form a smoke. Vaporization from
an open container, permitting the vapors to pass directly to the air
without being quickly carried away from the surface of evaporation,
produces smoke having larger particles, because each particle formed
remains for an appreciable period of time in contact with air saturated
with vapor, and hence grows very rapidly.
The easiest way to produce small smoke particles is to mix the toxic
material directly with some fuel which will produce a large amount of
heat and gas upon burning. When this mixture is enclosed in a container
having a small orifice, upon burning, the toxic vapor and gas will pass
through this orifice at high velocity; it has been demonstrated by Lord
Rayleigh that the size of the particles depends upon the velocity of
emission of the gas from a given orifice.
The product of _chemical combination_ may include a super-saturated
vapor, which condenses into small particles.
_Explosive dispersion_ is really a combination of _mechanical
dispersion_ followed by _thermal dispersion_.
PENETRATION
The fundamental idea underlying all the work on toxic smokes is to
obtain a smoke that has marked penetrating power. Screening power is
not important here. In addition to penetration, a smoke should be
highly toxic and have a slow rate of settling.
Penetration may be tested by the use of a standard filter; a suitable
filter for this purpose is one which does not remove the smoke to such
an extent that measurement of its concentration becomes difficult,
and one which does not become clogged quickly by the smoke. A filter
consisting of two pads of felt, placed side by side and arranged so
that the smoke first comes in contact with the thinner and less dense
pad has been found very satisfactory.
In testing penetration, smoke is produced by dispersing one gram of the
toxic substance in a sheet iron box of 1000 liters capacity. After 5
minutes a steady concentration is usually attained and the smoke is
then forced through a Tyndall meter, (see page 299) after dilution with
air, where the initial concentration is determined. It then passes
through the standard filter, and through a second Tyndall meter, where
the final concentration is measured. The difference of the two readings
gives the amount of smoke retained by the filter. The penetration is
ordinarily represented by a series of figures, which decrease from a
maximum value at the beginning of the test to a minimum at a point
where the filter permits the passage of so little smoke that it cannot
be measured. This decrease is due to decrease in penetrating power and
concentration of the smoke, and to increase in filtering power of the
filter as a result of plugging. Usually five degrees of penetration are
recognized, excellent, good, fair, poor and very poor.
[Illustration: FIG. 100.—Penetration Apparatus Used to Test Toxic
Smokes.]
A portable penetration apparatus is shown in Fig. 100. In using the
apparatus, the smoke producing material is so placed with reference to
the apparatus that the sample is taken about 20 feet down the wind, so
that the smoke is appreciably diluted. One man is stationed at each
Tyndall meter and takes readings as fast as his recorder can write
them, so that the smoke density, before and after the filter, can be
followed very closely.
PHYSIOLOGICAL ACTION
In addition to a high penetrating power a smoke should also possess
great toxic, irritant, sternutatory, or lachrymatory power. These
properties are tested by exposing mice to the smoke in the chamber.
They are placed in the chamber at the beginning of the run, and exposed
for 10 minutes to the smoke from 1 gram of the material. While these
tests are only qualitative in character, they give a fairly good notion
of the relative value of different materials.
QUANTITATIVE RELATIONSHIPS
It has been found that, if the optical readings from the Tyndall meter
are plotted as ordinates against the time _t_ (the time elapsed after
detonation) as abcissas, and that portion of the curve between _t_ =
0 and _t_ = 30 considered, the curve generally descends sharply at
first, from a high point representing the density immediately after the
production of the smoke, to a point in the neighborhood of _t_ = 8,
where it flattens out and descends much more slowly with a slope that
changes little. The area under the significant portion of the curve,
that is, the area circumscribed by the curve from the point _t_₃₀ to
_t_₀, the vertical axis from this point to the origin, the horizontal
axis from the origin to _t_₃₀ and the line perpendicular to this
axis, cutting the curve at _t_₃₀, is a rough measure of the relative
values of different smokes. This area is calculated as the sum of two
rectangles, from _t_₀ to _t_₈ and from _t_₈ to _t_₃₀.
Some results are as follows:
Area 30
Phenyldichloroarsine 181
Triphenyldichloroarsine 178
Diphenylcyanoarsine 137
Diphenylchloroarsine 101
Cyanogen bromide 94
Methyl dichloroarsine 70
Phenylimidophosgene 69
Mustard gas 38
The curves in Fig. 101 show the way in which the readings fall off with
time. Each substance of course has its characteristic curves.
[Illustration: FIG. 101.—Typical Curves Showing the Decrease in
Concentration of Smoke Cloud with Time.]
TOXIC MATERIALS
The selection of materials for the production of toxic smokes can only
be carried out experimentally. A number of very toxic substances have
been shown to be valueless as toxic smokes because of low penetration,
decomposition during the process of smoke production, or for other
reasons.
Arsenic compounds produce smokes distinctly better than the average.
Inorganic compounds which have high melting and boiling points are very
poor smoke producers. The only exception to this is magnesium arsenide,
which may suffer decomposition. Compounds like mercuric chloride
and arsenic tribromide, which boil or sublime at comparatively low
temperatures, produce good smokes. Most materials which boil below 130°
C. produce no smoke as they evaporate on dispersion. It is difficult
to set any upper limit for the boiling point beyond which materials do
not produce good smokes, but in all probability 500° C. is not far from
the maximum. Liquids and solids are, on the whole, almost equally good
as smoke producers. The physical condition of the material has no great
effect upon the amount of smoke which it will produce. This seems to
depend only upon the physical and chemical properties of the material.
TOXIC SMOKE APPARATUS
It has been mentioned above that the Germans used a shell, containing
solid diphenylchloroarsine and a high explosive. A 10.5 cm. shell
(Blue Cross) was about two-thirds filled with cast trinitrotoluene
and contained a glass bottle with 300-400 grams of toxic material.
Diphenylchloroarsine was also used in shell, in solution, a mixture of
phosgene and diphosgene (superpalite) being the ordinary solvent (Green
Cross). Mixtures of diphenylchloroarsine and phenyldichloroarsine were
also used.
In the case of high explosive shell, the use of a separate container
appears to be desirable, because a mixture with the explosive seriously
decreases its sensitiveness and even its destructive power. There is
also a question as to the stability of such a mixture. However, 75 mm.
shell containing 30 per cent diphenylchloroarsine mixed with T. N. T.
gave good clouds of toxic smoke.
TOXIC SMOKE CANDLE
Two toxic smoke candles were developed by the Chemical Warfare Service,
known as the B-M Toxic Smoke Candle, perfected by the Pyrotechnic
Section of the Research Division, and the Dispersoid Smoke Candle,
developed by the Dispersoid Section.
The B-M Toxic Smoke Candle consists of a bottle-shaped sheet steel
toxic container set into a can, containing smoke mixture. The heat from
the burning mixture causes the distillation of the toxic material.
The toxic vapor is discharged through a nipple, screwed into the neck
of the container and extending over the top of the smoke can. Steel
wool is used in the toxic container to reduce the violent boiling and
spattering of the material. A small amount of steel wool, held in place
by a wire screen, is also used in the nipple for the same purpose. The
toxic container is sealed by a fusible metal plug, melting at 90° C.,
cast into a retainer at the base of the nipple. The fusible plug melts
upon the first application of heat and allows free passage of the vapor
into the smoke cloud. The ignition of the apparatus is effected by
means of a simple match head and an accompanying scratcher.
[Illustration: FIG. 102.—Toxic Smoke Cloud from 500 D. M. Candles.]
The candles were placed in 5 parallel rows which were
2 yards apart, each row containing 100 candles on
a 100 yard front. The total time of active smoke
emission was 23 minutes.
The first evolution of smoke occurs about 10 seconds after the first
appearance of flame. About one minute after ignition the toxic
material will begin to distill into the smoke cloud and this will
continue for about four minutes. The burning of the candle should be
complete in about six minutes.
[Illustration: Dispersoid Candle British Candle
FIG. 103.—Comparison of Dispersoid and British D. M. Candles.]
The Dispersoid Toxic Smoke Candle differs from the B-M candle in that
the toxic container is not used. A mixture of smokeless powder and the
toxic material (diphenylchloroarsine or D. M., an arsenical obtained
from arsenic trichloride and diphenylamine) is filled directly into
the container, a cylindrical can 3.5 inches in diameter and 9 inches
high made from 27 gauge sheet metal, and packed under a total pressure
of 2,500 pounds. The top of the candle is a metal cover, containing
the match head scratcher, which is separated from the match head by a
Manila paper disc. These are the same as those used in the B-M candle.
The candle has a total weight of about 4.25 pounds, of which 3.6 pounds
are the smoke mixture, containing about 1.3 pounds of toxic material.
In operating the candles, the cover is removed and the match head
ignited by friction with the scratcher. The match head burns through
the cardboard and ignites the powder. The heat and gas produced by the
combustion of the powder vaporizes the particles of the toxic material
and carries the vapors out through the orifices at a high velocity
whereupon they recondense to form a smoke. The rapid emission of the
vapors through the orifice prevents any possibility of their ignition.
The time before good emission of smoke takes place after the ignition
of the match tip of a candle is 30 seconds. The average time of
vigorous smoke emission is from four to five minutes. The result
of a field test with the dispersoid candle is shown in Fig. 102. A
comparison of a British and a Dispersoid candle is shown in Fig. 103.
It should be stated that this may not have been a fair test as only one
British candle was available for the comparative test.