purposes. The one is for carbonizing steel; experiments in Germany
show that iron or soft steel takes up carbon more readily when it is
heated with carbide than when it is heated with coal dust or charcoal.
Some steel works are now using carbide for this purpose. The other use
of carbide is more important. It is found to be a valuable germicide.
It is said to be the most effectual preventive of black rot, and to
destroy the _Phylloxera_, the two worst enemies of the grape. The
action of the carbide as a germicide depends on its decomposition by
the moisture of the soil, forming acetylene, which kills the
_Phylloxera_. If the use of carbide on a large scale substantiates the
claims made for it, this is a discovery of vast importance. The
ravages caused by the _Phylloxera_ in the vineyards of southern
Europe, of Africa, and Australia must be ranked as great national
calamities.
A temperature ranging from 2000° to 2500° C. (3600° to 4500°
Fahrenheit) is required to make carbide. It is probable that this
temperature can be economically attained only by the electric furnace
using water power as the source of the electric current, and this is
the only method used for making carbide, with the exception of the
Walther process, which does not use electricity but depends on the
intense heat generated by burning acetylene under pressure. In
electric furnaces the formation of carbide depends simply on the heat
of the arc, which fuses the mixture of lime and coke. The latest
improvements on the first very simple forms of furnace have secured
continuity of work and economy of electric energy. In the United
States carbide is made exclusively in the Horry furnace. This furnace
consists of a huge short cylinder or hollow wheel, mounted to revolve
slowly on a horizontal shaft. The periphery of the cylinder is closed
by removable cast-iron slats. As the cylinder is partly revolved on
its axis from time to time, the slats are taken off from one side and
replaced on the other, thus leaving the top always open. The cylinder
is filled on one side with the powdered mixture of coke and lime. Into
the mixture two vertical carbon electrodes project downward through
the open top of the cylinder. As the carbide is formed, the cylinder
is revolved, lowering the mass from the electrodes. The fused carbide
cools, hardens, and is broken off and removed as it rises on the other
side of the slowly revolving cylinder; new material is constantly fed
in to maintain the level around the electrodes. The process in the
Horry furnace is continuous; the furnace can be run without arresting
the current until repairs are necessary. It is said to combine the
different theoretical improvements referred to, and to reduce the cost
of production. The Horry furnace is in use at Niagara Falls and at
Sault Ste. Marie. At St. Catherine's, Canada, Willson is using his own
furnace. Abroad, the older types of furnace, the Willson, Bullier, and
Héroult, are those chiefly in use.
The actual ingot of good commercial carbide is nearly pure--ninety-six
to ninety-nine per cent--but the ingot is surrounded by a crust of
carbide mixed with unchanged material, containing forty to seventy
per cent of carbide. Foreign makers break and blend ingot and crust to
standard size, the best makers guaranteeing their carbide ninety per
cent pure, giving five cubic feet of acetylene per pound (pure carbide
gives 5.89 cubic feet). Eight to nine pounds of carbide per horse
power in twenty-four hours, averaging five cubic feet of acetylene, is
considered satisfactory work. The Union Carbide Company, which
controls the sale of carbide in the United States, is selling graded
carbides under guarantee, the first grade being the nearly pure ingot,
the lower grade the crust.
[Illustration: HORRY FURNACE, SHOWING ELECTRODES.[5]]
[Illustration: HORRY FURNACE, SHOWING GEARING.[5]]
[Illustration: HORRY FURNACE, SHOWING CARBIDE JUST REMOVED.[5]]
[Footnote 5: NOTE.--We are indebted to the courtesy of the
Electrical World and Engineer for cuts showing the Horry furnace.]
As the moisture of the air decomposes the carbide, it must be broken
up as soon as made, and packed in air-tight tin cans, varying in size
from one to four hundred pounds.
The present price of carbide abroad averages $96.80 in large lots, and
$7.26 per hundredweight in small lots, packing included; in the United
States, $70 per ton in large lots, and $4.50 per hundredweight in
small lots, packing included. In 1898, 4,650 tons are said to have
been made in the United States and Canada, and a much larger amount
abroad. The output for 1899 is estimated at 12,000 tons for the United
States, with a capacity in the new works in erection at Sault Ste.
Marie and at Niagara Falls of 41,000 tons. The new works building in
Europe, to be finished in 1899-1900, have a capacity for making
80,000 metric tons. These figures will justify the statement made at
the beginning of this article, that the new industry has found ample
capital.
The statement is still current that acetylene attacks copper and
brass, forming an explosive compound. This is not true. Exhaustive
experiments by Moissan and by Gerdes, keeping these and other metals
in contact with acetylene for months at a time, have shown that the
metals were not affected. The conditions under which the explosive
copper acetylide is made in laboratories can not well occur in
generators or gas holders. It has been said that acetylene is very
poisonous; the experiments of many observers, and especially those of
Gréhant, do not confirm this statement. Gréhant experimented on dogs,
causing them to breathe mixtures of acetylene, air, and oxygen, which
always contained 20.8 per cent of oxygen, this being the percentage of
oxygen in pure air. By this device he was able to discriminate between
the poisoning caused by acetylene and suffocation caused by
insufficient oxygen. A mixture containing twenty per cent acetylene
inhaled for thirty-five minutes did not seem to trouble the animal. A
sample of the dog's arterial blood contained ten per cent of
acetylene. A dog which inhaled a mixture containing forty per cent of
acetylene died suddenly after fifty-one minutes, having inhaled one
hundred and twelve litres of the mixture; the arterial blood contained
twenty per cent acetylene. Gréhant proved that acetylene simply
dissolves in the blood plasma, while carbon monoxide forms a compound
with the hæmoglobin of the blood. A dog breathing a similar mixture of
air, oxygen, and illuminating gas containing only one per cent of
carbon monoxide quickly showed convulsive movements, and died after
ten minutes; its blood contained twenty-four per cent of carbon
monoxide. Thus acetylene, while slightly poisonous, is less poisonous
than coal gas, and vastly less than water gas, which contains a high
percentage of carbon monoxide.
A pressure of thirty-nine atmospheres and three quarters at 20° C.
converts acetylene into a liquid weighing one third as much as the
same volume of water, while one cubic foot of the liquid when released
from pressure gives five hundred cubic feet of gas.
Hitherto acetylene is used only as a source of heat or as a source of
light; yet with very cheap carbide it would prove useful in many ways
in chemical industry, and its use would have the most wide-spread
effect on industry and agriculture. For instance, a method of making
alcohol from acetylene is patented abroad, and by another patented
process it is proposed to make sugar from acetylene. With the present
prices of alcohol, sugar, and carbide, these processes have no
commercial value.
Acetylene may be made from the carbide in gas works and delivered to
the consumer through mains like ordinary illuminating gas; or it may
be liquefied at a gas works and delivered to the consumer in the
liquid form under pressure; or the consumer may purchase carbide and
generate acetylene for his own consumption. All three of these methods
are in use.
To understand the attitude of insurance companies and of consumers
toward liquid acetylene it will be well to examine its record for the
last few years. Those interested in methods for liquefying acetylene,
and for reducing the pressure of the liquid at the place of
consumption so that the consumer actually uses it as a gas under a
water pressure of six inches or less, may find processes described in
detail in the Progressive Age, and in other technical journals.
Suffice it to say that the methods in use in this country and abroad
are simple and effective. The purified acetylene is delivered in
strong steel cylinders, which may be placed in a special building or
case and need not be handled by the consumer. It has been proved by
the exhaustive experiments of the eminent French chemist Berthelot
that liquefied acetylene in cylinders can not be exploded by blows or
shocks to the closed cylinder. If it is exploded, however, by causing
a spark within the cylinder, the explosive force is very great, being
about equal to that of gun cotton.
The use of the liquefied acetylene is so simple and clean that the
attention of inventors was first turned to this mode of supply. It may
in future come again into prominence despite the present strong
feeling against it, its use in many cities being prohibited. This
feeling was caused by a number of explosions, accompanied by loss of
life. Three of these explosions occurred in factories for liquefying
acetylene; one in a factory where liquid acetylene regulators were
made; several in buildings of consumers. In October, 1896, Pictet's
works in Paris were wrecked by the explosion of a cylinder filled with
liquid acetylene; evidence proved that the cylinder was held in a
vise, and that the two workmen killed were at the ends of a wrench,
closing or opening the valve, supposing the cylinder to be empty. The
explosion was caused either by a spark from friction in turning the
screw, or by the too sudden opening of the valve and releasing the
pressure, causing a shock sufficient to decompose the liquid. In
December, 1896, the works of G. Isaac, in Berlin, were destroyed by an
explosion in the condenser where the cooled acetylene was liquefied by
pressure; Isaac and three workmen were killed. Evidence showed that
through carelessness warm water instead of cold water was in contact
with the condenser, thus warming the liquid and increasing the
pressure to a point which burst the condenser. In December, 1897, the
works of the Dickerson & Suckert Acetylene Gas Liquefying and
Distributing Company in Jersey City were destroyed by fire caused by
the explosion of a cylinder filled through carelessness of workmen
with a mixture of air and liquid acetylene--i. e., with an explosive
mixture--killing the superintendent and a workman. In the explosion at
the regulator factory at New Haven, January, 1895, the valve of the
cylinder, on which one of two workmen killed was working, broke; a
large volume of acetylene escaped and ignited from a lighted candle.
In all four cases the explosions were caused by ignorance or
carelessness incident to the beginnings of a new industry, and could
be avoided by experience and skill.
It should be stated that in the explosion at Paris all of the full
acetylene cylinders were dug out of the ruins unhurt. The same was
true at Berlin, where five full cylinders were blown against the wall
of the building by the explosion of the condenser, but did not
explode. At Jersey City sixty filled cylinders were exposed to the
heat of the fire following the explosion; they were fitted with safety
diaphragms of fusible metal; forty-eight remained intact, the
acetylene burning off quietly as it escaped through the fused
diaphragm, and twelve exploded, either on account of imperfection of
the diaphragms or stoppage of the air passage leading from the
diaphragm. The explosions of liquid acetylene in buildings of
consumers have been due in every case to gross carelessness and
ignorance on the part of the consumer.
Although one of the chief points in favor of the liquid acetylene is
its portability, yet it can be shown that it is still easier to carry
carbide to the consumer. One cubic metre of acetylene is compressed to
two litres in liquid form; two litres of carbide weigh 4.44
kilogrammes, which will produce a cubic metre and a third of
acetylene, reckoning three hundred litres to the kilogramme, which is
the average guaranteed yield of carbide. The light tin carbide cans
occupy less space and weigh less than the heavy steel cylinders, while
the generation of the gas is simple and, with proper generators,
perfectly safe. On the other hand, the generators must be cared for,
must often be filled with fresh carbide, and from time to time must be
cleaned. With the generator system acetylene is as safe as or safer
than illuminating gas. Berthelot has shown that at pressures below two
atmospheres a vessel filled with acetylene can not be exploded by the
explosion of a cap of fulminating mercury within the vessel, nor by
heating a wire which extends into the vessel to a white heat by an
electric current. The reason is that the acetylene can not explode
unless it is decomposed into its elements, carbon and hydrogen; to
decompose it requires a certain amount of energy. While the energy of
the glowing wire or of the exploding cap causes a local decomposition
at the point of contact, it is not sufficient to spread the
decomposition further. Acetylene forms an explosive mixture with air;
so does illuminating gas. The odor of acetylene is unpleasant; so is
the odor of the water gas used generally in the United States, and the
acetylene can be cheaply deodorized.
As the generator system, then, is the general one, the most important
question to the consumer is what generator to buy, and it is a
perplexing question. The carbide manufacture is so organized that it
is everywhere under the control of powerful and responsible companies
which sell a guaranteed product. The burners now in use are nearly all
good. With generators it is different; the market is flooded with them
at all prices, ranging in value from worse than useless to very good,
as regards safety, economy, and quality of light. As the generator
question is by far the most important and the least understood in the
whole acetylene industry, it will be well to give a full account of
the results of the experiments which have been made within the last
two years on this question. The most exhaustive experiments are those
of the English expert, Professor Lewes, and his results agree with
those of other observers.
Lewes first determined the amount of heat developed by the
decomposition of carbide by water, and the conditions which tend to
lessen or increase the intensity of the reaction. The average result
of the experiments as to the amount of heat was 446.6 calories for
pure carbide, and a little less for commercial carbide (to state this
differently, one pound of carbide, when decomposed by water, gives off
heat enough to raise the temperature of 446.6 pounds of water 1° C.,
or to raise the temperature of one pound of water 446.6° C.). As the
intensity of the heat developed determines the highest temperature
attained during the decomposition, and is a function of the time
needed to complete the action, and as the decomposition of carbide in
contact with water is extremely rapid, it is evident that the
temperature developed may be so high as to cause disaster. All the
generators at present before the public may be classified under three
heads: 1. Those in which water is allowed to drip or flow slowly on a
mass of carbide, the evolution of the gas being regulated by the
stopping of the water. 2. Those in which water in considerable volume
is allowed to rise in contact with carbide, the evolution of the gas
being regulated by the driving back of the water by the increase of
pressure in the generating chamber. 3. Those in which the carbide is
dropped or plunged into an excess of water.
The conclusions deduced from a large number of experiments were that
when, as in type 1, water is allowed to drip or flow in a fine stream
upon a mass of carbide, the temperature rapidly rises until after
eighteen to twenty-five minutes the maximum is reached, which varies
from 400° to 700° C. (720° to 1120° Fahrenheit), and it is probable
that in some of the mass the higher limit is always reached, as
traces of tar are usually found in the residual lime, in some cases in
sufficient quantity to make the lime yellow and pasty, while vapors of
benzene and other polymerization products pass off with the gas.
Leaving the question of temperature in this type of generator, another
important question is the length of time during which the generation
of gas continues after the water supply is automatically cut off. It
is found that gas is evolved with increasing slowness sometimes for an
hour and three quarters after the water supply has ceased, the total
volume of gas so evolved being large.
The experiments showed that in any automatic generator of this type
the cut-off should be so arranged that one quarter of the total
capacity of the gas holder is still available to store the slowly
generating gas.
The second class of generators bring about contact either by water
rising from below to the carbide suspended in the cage (II, _A_), or
by a cage of carbide suspended in a movable bell which, as it falls,
dips the carbide into water, withdrawing the carbide from the water as
the excessive generation of gas lifts the bell (II, _B_). Lewes found
that under certain conditions generators of the type II, _B_ were far
worse than those of type I.
The trials were made with a movable glass bell, with counterweights,
containing a half-pound of carbide. The maximum temperatures reached
in four trials were 703°, 734°, 754°, and 807° C. Excessive heating
took place in every case; in the last mentioned the temperature was
far above the point at which acetylene is decomposed into carbon and
hydrogen, a thin black smoke being formed immediately around the
carbide while tar vapor poured out. On removing the residue after
cooling it was found to be coated with soot and loaded with tar. On
several occasions the charge was removed from the generator just after
the maximum temperature was reached, and was found to be at a bright
red heat.
These experiments are of the greatest practical importance. At 600°
acetylene begins to polymerize--i. e., to form more complex
hydrocarbons, which are liquid, or solid, at ordinary temperatures.
Probably in the generator acetylene is first given off so rapidly that
the heat does not act on it, but as decomposition advances into the
center of the mass of carbide, the acetylene generated has to pass
through the external layers, which, as shown, may be at high
temperatures, above that at which acetylene decomposes; thus a
considerable amount of gas is lost, and the tar formed may distill
into the generator and tubes, clogging the tubes. A more serious evil
is the deterioration in the illuminating quality of the gas. Samples
of the gas were taken as the maximum temperature was approached, and
analyzed with this average result: Acetylene, seventy per cent; other
hydrocarbons, eleven per cent; hydrogen, nineteen per cent. This
reduces the illuminating value from two hundred and forty to one
hundred and twenty-six candles. The hydrocarbons consist largely of
benzene, which requires three times as much air for complete
combustion as acetylene does. The best possible acetylene burner
smokes when the acetylene contains benzene.
[Illustration: Type I.
TYPE OF GENERATOR.[6]]
[Illustration: Type II, _A_. Type II, _B_. Type III.
TYPES OF GENERATORS.[6]]
[Footnote 6: NOTE.--We are indebted to the courtesy of the
Progressive Age for cuts showing types of generators.]
At first sight these experiments would seem absolutely to condemn
generators of class II, yet the fact remains that some excellent
generators are of this type. Under certain conditions excessive
overheating may be avoided. The rising bell shown in II, _B_ should be
discarded. Generators in which the water rises from below, and slowly
attacks the carbide, can be made safe if the water is never driven
back from the carbide, and the carbide is in separated layers as in
II, _A_. Under these conditions the water is always in excess at the
point where it attacks the carbide, so that the evaporation, by
rendering heat latent, keeps the temperature down, the temperature of
the melting point of tin, 228° C., being rarely reached in good
generators where these conditions are met.
Undoubtedly the best generators, and the only ones which from a
scientific point of view should be employed, are those of class III,
in which carbide falls into an excess of water. In such generators it
is impossible to get a temperature higher than the boiling point of
water, 100° C., while with a properly arranged tank the temperature
never exceeds that of the air by more than a few degrees. Under these
conditions the absence of polymerization and the washing of the
nascent and finely divided bubbles of gas by the limewater in the
generator yield acetylene of a degree of purity unapproached by any
other form of generator.
When acetylene is burned in air under such conditions that the flame
does not smoke, it has been proved by Gréhant that there is no carbon
monoxide among the combustion products; the acetylene combines with
the oxygen of the air to form carbon dioxide and water (C_{2}H_{2} +
5O = 2CO_{2} + H_{2}O). One cubic foot of acetylene requires two and a
half cubic feet of oxygen. Supposing a room to have an illumination
equal to sixty-four standard candles; this amount of light from
candles would use up 38.5 cubic feet of oxygen from the air, and would
give off forty-three cubic feet of carbon dioxide; petroleum requires,
in cubic feet, twenty-five of oxygen, and gives off forty of carbon
dioxide; gas burned with a flat flame requires about twenty-five
oxygen and gives nineteen carbon dioxide--with an Argand flame a
little less, while with the Welsbach burner gas requires only three
oxygen, and gives off 1.8 carbon dioxide; acetylene requires five
oxygen and yields four carbon dioxide. So that, light for light,
acetylene fouls the air less than any ordinary illuminant excepting
the Welsbach gas burner. (With incandescent electric light there is no
combustion and no fouling of the air.)
Under the best conditions five cubic feet of acetylene give a light of
two hundred and forty candles for one hour, or we may speak of
acetylene as a two-hundred-and-forty-candle gas. Yet this statement,
though strictly true, may be misleading. When ordinary illuminating
gas is tested with the photometer, it is burned from a standard
flat-flame burner, burning five cubic feet per hour. Now the amount of
light given by such a gas flame is no greater than is pleasant to the
eye; it is true that if we burn five cubic feet of acetylene from a
suitable flat-flame burner, a light of two hundred and forty candles
is given, but it is unfair to take this ratio as representing the
actual relative illuminating value of the two lights, because we
neither need a light of two hundred and forty candles, nor is such an
amount of light issuing from one burner endurable to the eye. One-foot
or one-half foot acetylene burners are used for domestic lighting;
light from the best one-foot burners averages thirty-two to
thirty-five candles per cubic foot. With acetylene, as with every
other illuminating gas, the smaller the burner and consumption, the
less light per cubic foot of gas is obtained. Another important point
is that while these figures represent the best practical illumination
obtained from acetylene by the burners hitherto in use, the standard
flat-flame burner does not give the best gaslight; with a good
Welsbach burner a cubic foot of illuminating gas will give a
seventeen-candle light as an average. The comparison, to be fair,
should be between acetylene and the Welsbach light.