_Mechanical Mixtures._--In the preceding section, it was shown that an
explosion is simply the rapid oxidation of carbon and hydrogen. To form
an explosive agent, the problem is, how to bring together in a
convenient form the combustible, carbon or hydrogen, and the oxygen
required to oxidize it. Carbon may be obtained pure, or nearly pure, in
the solid form. As wood charcoal, for example, that substance may be
readily procured in any needful abundance; but pure oxygen does not
exist in that state, and it is hardly necessary to point out that only
the solid form is available in the composition of an explosive agent. In
nature, however, oxygen exists in the solid state in very great
abundance in combination with other substances. Silica, for example,
which is the chief rock constituent, is a compound of silicon and
oxygen, and the common ores of iron are made up chiefly of that metal
and oxygen. The elementary constituents of cellulose, or wood fibre, are
carbon, hydrogen, and oxygen; and the body known as saltpetre, or
nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But
though oxygen is thus found in combination with many different
substances, it has not the same affinity for all. When it is combined
with a substance for which its affinity is strong, as in the silica and
the iron oxide, it cannot be separated from that substance without
difficulty; but if the affinity be weak, dissociation may be more easily
effected. The former combination is said to be “stable,” and the latter
is, in contradistinction, described as “unstable.” It will be evident on
reflection that only those compounds in which the oxygen exists in
unstable combination can be made use of as a constituent part of an
explosive agent, since it is necessary that, when required, the oxygen
shall be readily given up. Moreover, it will also appear that when one
of these unstable oxygen compounds and carbon are brought together the
mixture will constitute an explosive agent, since the oxygen which is
liberated by the dissociation of the unstable compound will be taken up
by the carbon for which it has a stronger affinity. Saltpetre is one of
those compounds, and a mixture of this body with charcoal constitutes
gunpowder. The means employed to dissociate the elements of saltpetre is
heat. It is obvious that other compounds of oxygen might be substituted
for the saltpetre, but this body being easily procurable is always
employed. The chlorate of potash, for example, is less stable than the
nitrate, and therefore an explosive mixture containing the former
substance will be more violent than another containing the latter. For
the violence of an explosion is in a great measure determined by the
readiness with which the oxygen is given up to the combustible. But the
chlorate is much more costly than the nitrate. As, however, the force
developed is greater, the extra cost would perhaps be compensated by the
increased effect of the explosion. But the instability of the chlorate
is such that friction or a moderately light blow will produce explosion
in a mixture containing that substance, a circumstance that renders it
unfit to be the oxidizer in an explosive agent in common use. The
nitrate is therefore preferred on the ground of safety. Saltpetre, or
nitrate of potash, consists, as already pointed out, of the metal
potassium in combination with the substances nitrogen and oxygen. Of
these, the last only is directly concerned in the explosion; but the two
former, and especially the nitrogen, act indirectly to intensify its
effects in a manner that will be explained hereafter.
The chemical formula for nitrate of potash is KNO₃, which signifies that
three atoms of oxygen exist in this body in combination with one atom of
nitrogen and one atom of kalium or potassium. As the atomic weights of
these substances are 16, 14, and 39 respectively, the weight of the
molecule is 101, that is, in 101 lb. of nitrate of potash there are 39
lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen.
Hence the proportion of oxygen in nitrate of potash is by weight 47·5
per cent. It will be seen from this proportion that to obtain 1 lb. of
oxygen, 2·1 lb. of the nitrate must be decomposed.
The carbon of gunpowder is obtained from wood charcoal, the light woods,
such as alder, being preferred for that purpose. The composition of the
charcoal varies somewhat according to the degree to which the burning
has been carried, the effect of the burning being to drive out the
hydrogen and the oxygen. But, generally, the composition of gunpowder
charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per
cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the
charcoal, it is easy to calculate the proportion of saltpetre required
in the explosive mixture.
Thus far we have considered gunpowder as composed of charcoal and
saltpetre only. But in this compound, combustion proceeds too slowly to
give explosive effects. Were the chlorate of potash used instead of the
nitrate, the binary compound would be sufficient. The slowness of
combustion in the nitrate mixture is due to the comparatively stable
character of that body. To accelerate the breaking up of the nitrate, a
quantity of sulphur is mixed up with it in the compound. This substance
possesses the property of burning at a low temperature. The proportion
of sulphur added varies from 10 per cent. in powder used in fire-arms,
to 20 per cent. in that employed for blasting purposes. The larger the
proportion of sulphur, the more rapid, within certain limits, is the
combustion. Thus ordinary gunpowder is a ternary compound, consisting of
charcoal, saltpetre, and sulphur.
As the composition of charcoal varies, it is not practicable to
determine with rigorous accuracy the proportion of saltpetre required in
every case; a mean value is therefore assumed, the proportions adopted
being about--
Charcoal 15
Saltpetre 75
Sulphur 10
---
100
---
With these proportions, the carbon should be burned to carbonic acid,
and the sulphur should be all taken up by the potassium. Powder of this
composition is used for fire-arms. For blasting purposes, as before
remarked, the proportion of sulphur is increased at the expense of the
saltpetre, in order to quicken combustion and to lessen the cost, to 20
per cent. as a maximum. With such proportions, some of the carbon is
burned to carbonic oxide only, and some of the sulphur goes to form
sulphurous acid, gases that are particularly noisome to the miner.
It is essential to the regular burning of the mixture that the
ingredients be finely pulverized and intimately mixed. The manufacture
of gunpowder consists of operations for bringing about these results.
The several substances are broken up by mechanical means, and reduced to
an impalpable powder. These are then mixed in a revolving drum, and
afterwards kneaded into a paste by the addition of a small quantity of
water. This paste is subjected to pressure, dried, broken up, and
granulated; thus, the mixing being effected by mechanical means, the
compound is called a mechanical mixture. It will be observed that in a
mechanical mixture the several ingredients are merely in contact, and
are not chemically united. They may therefore be separated if need be,
or the proportions may be altered in any degree. Mechanical mixtures,
provided the bodies in contact have no chemical action one upon another,
are stable, that is, they are not liable, being made up of simple
bodies, to decompose spontaneously.
_Chemical Compounds._--In a mechanical mixture, as we have seen, the
elements which are to react one upon another are brought together in
separate bodies. In gunpowder, for example, the carbon is contained in
the charcoal, and the oxygen in the saltpetre. But in a chemical
compound, these elements are brought together in the same body. In a
mechanical mixture, we may put what proportion of oxygen we please. But
elements combine chemically only in certain definite proportions, so
that in the chemical compound we can introduce only a certain definite
proportion of oxygen. The oxygen in saltpetre is in chemical combination
with the potassium and the nitrogen, and, as we have already seen, these
three substances hold certain definite proportions one to another. That
is, to every atom of potassium, there are one atom of nitrogen and
three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of
saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and
0·475 lb. of oxygen. Moreover, these elements occupy definite relative
positions in the molecule of saltpetre. But in the mechanical mixture,
the molecules of which it is made up have no definite relative
positions. Even if the three substances--charcoal, saltpetre, and
sulphur--of which gunpowder is composed, could be so finely divided as
to be reduced to their constituent molecules, the relative position of
these would be determined by the mixing, and it would be impossible so
to distribute them that each should find itself in immediate proximity
to those with which it was to combine. But so far are we from being able
to divide substances into their constituent molecules, that when we have
reduced them to an impalpable powder, each particle of that powder
contains a large number of molecules. Thus, in a mechanical mixture, we
have groups of molecules of one substance mingled irregularly with
groups of molecules of another substance, so that the atoms which are to
combine are not in close proximity one to another, but, on the contrary,
are, many of them, separated by wide intervals. In the chemical
compound, however, the atoms are regularly distributed throughout the
whole mass of the substance, and are, relatively to one another, in the
most favourable position for combining. Viewed from this point, the
chemical compound may be regarded as a perfect mixture, the mechanical
mixture being a very imperfect one. This difference has an important
influence on the effect of an explosion. All the atoms in a chemical
compound enter at once into their proper combinations, and these
combinations take place in an inconceivably short space of time, while,
in a mechanical mixture, the combinations are less direct, and are much
less rapidly effected. This is the reason why the former is more violent
in its action than the latter. The one is crushing and shattering in its
effects, the other rending and projecting. The compound gives a sudden
blow; the mixture applies a gradually increasing pressure. It is this
sudden action of the compound that allows it to be used effectively
without tamping. The air, which rests upon the charge, and which offers
an enormous resistance to motion at such inconceivably high velocities,
serves as a sufficient tamping.
Gun-cotton may be taken as an example of a chemical compound. The woody
or fibrous part of plants is called “cellulose.” Its chemical formula is
C₆H₁₀O₅, that is, the molecule of cellulose consists of six atoms of
carbon in combination with ten atoms of hydrogen and five atoms of
oxygen. If this substance be dipped into concentrated nitric acid, some
of the hydrogen is displaced and peroxide of nitrogen is substituted for
it. The product is nitro-cellulose, the formula of which is
C₆H₇(NO₂)₃O₅. If this formula be compared with the last, it will be
seen that three atoms of hydrogen have been eliminated and their place
taken by three molecules of the peroxide of nitrogen NO₂; so that we now
have a compound molecule, which is naturally unstable. The molecules of
the peroxide of nitrogen are introduced into the molecule of cellulose
for the purpose of supplying the oxygen needed for the combustion of the
carbon and the hydrogen, just as the groups of molecules of saltpetre
were introduced into the charcoal of the gunpowder for the combustion of
the carbon and the hydrogen of that substance. Only, in the former case,
the molecules of the peroxide are in chemical combination, not merely
mixed by mechanical means as in the latter. The compound molecule of
nitro-cellulose may be written C₆H₇N₃O₁₁, that is, in 297 lb. of the
substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of
hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen;
or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent.
nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by
the action of heat, the oxygen combines with the carbon and the
hydrogen, and sets the nitrogen free. But it will be observed that the
quantity of oxygen present is insufficient to completely oxidize the
carbon and the hydrogen. This defect, though it does not much affect the
volume of gas generated, renders the heat developed, as shown in a
former section, considerably less than it would be were the combustion
complete, and gives rise to the noxious gas carbonic oxide.
Cotton is one of the purest forms of cellulose, and, as it may be
obtained at a cheap rate, it has been adopted for the manufacture of
explosives. This variety of nitro-cellulose is known as “gun-cotton.”
The raw cotton made use of is waste from the cotton mills, which waste,
after being used for cleaning the machinery, is swept from the floors
and sent to the bleachers to be cleaned. This is done by boiling in
strong alkali and lime. After being picked over by hand to remove all
foreign substances, it is torn to pieces in a “teasing” machine, cut up
into short lengths, and dried in an atmosphere of 190° F. It is then
dipped into a mixture of one part of strong nitric acid and three parts
of strong sulphuric acid. The use of the sulphuric acid is, first, to
abstract water from the nitric acid, and so to make it stronger; and,
second, to take up the water which is formed during the reaction. After
the dipping, it is placed in earthenware pots to digest for twenty-four
hours, in order to ensure the conversion of the whole of the cotton into
gun-cotton. To remove the acid, the gun-cotton is passed through a
centrifugal machine, and subsequently washed and boiled. It is then
pulped, and again washed with water containing ammonia to neutralize
any remaining trace of acid. When rendered perfectly pure, it is
compressed into discs and slabs of convenient dimensions for use.
Another important chemical compound is nitro-glycerine. Glycerine is a
well-known, sweet, viscous liquid that is separated from oils and fats
in the processes of candle-making. Its chemical formula is: C₃H₈O₃; that
is, the molecule is composed of three atoms of carbon, in combination
with eight atoms of hydrogen, and three atoms of oxygen. In other words,
glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and
oxygen 52·2 per cent. When this substance is treated, like cellulose,
with strong nitric acid, a portion of the hydrogen is displaced, and
peroxide of nitrogen is substituted for it; thus the product is:
C₃H₅(NO₂)₃O₃, similar, it will be observed, to nitro-cellulose. This
product is known as nitro-glycerine. The formula may be written
C₃H₅N₃O₉. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36
lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen;
and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per
cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When
the molecule is broken up by the action of heat, the oxygen combines
with the carbon and the hydrogen, and sets the nitrogen free. And it
will be seen that the quantity of oxygen present is more than sufficient
to completely oxidize the carbon and the hydrogen. In this, the
nitro-glycerine is superior to the nitro-cotton. In both of these
compounds, the products of combustion are wholly gaseous, that is, they
give off no smoke, and leave no solid residue.
In the manufacture of nitro-glycerine, the acids, consisting of one part
of strong nitric acid and two parts of strong sulphuric acid, are mixed
together in an earthenware vessel. When quite cold, the glycerine is run
slowly into this mixture, which, during the process, is kept in a state
of agitation, as heat is developed in the process; and, as the
temperature must not rise above 48° F., the vessels are surrounded with
iced water, which is kept in circulation. When a sufficient quantity of
glycerine has been run into the mixture, the latter is poured into a tub
of water. The nitro-glycerine being much heavier than the dilute acid
mixture, sinks to the bottom; the acid liquid is then poured off, and
more water added, this process being repeated until the nitro-glycerine
is quite free from acid.
Nitro-glycerine is, at ordinary temperatures, a clear, nearly
colourless, oily liquid, having a specific gravity of about 1·6. It has
a sweet, pungent taste, and if placed upon the tongue, or even if
allowed to touch the skin in any part, it causes a violent headache.
Below 40° F. it solidifies in crystals.
Dynamite is nitro-glycerine absorbed in a silicious earth called
kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine
and 25 per cent. kieselguhr. The use of the absorbent is to remove the
difficulties and dangers attending the handling of a liquid. Dynamite is
a pasty substance of the consistence of putty, and is, for that reason,
very safe to handle. It is made up into cartridges, and supplied for use
always in that form.