Pistola di Volta
Recreating Volta’s dramatic 19th-century displays of energy released
from methane provides insights into anaerobic microbial metabolism
Ralph S. Wolfe
he Italian physicist Alessandro
Volta, who lived 1745–1827 and after whom the volt is named, was
dedicated to attracting people to science, a crucial step toward educating
successive generations of scientists. To this end
he employed a number of eye-catching demonstrations with electrical equipment to impress
visitors to his laboratory.
One of Volta’s early favorites was a combustion chamber made of glass and shaped like a
pistol (Fig. 1). This 225-year-old glass combustion chamber is preserved at the History Museum of the University of Pavia, Italy (http://
ppp.unipv.it/volta/ or Google: Pistola di Volta).
On the underside of the chamber are two ports,
each of which is equipped with an external brass
knob. Wires run from these sealed knobs to gold
foil that is rigged to produce an electric spark
within the chamber when the knobs are connected to an electrostatic source.
T
Hydrogen or Methane Gases Explosively
Ignited in Pistol-Shaped Chambers
Over his lifetime, Volta appeared to have great
fun designing increasingly sophisticated versions of this combustion chamber. Some were
round glass or metal vessels, while others made
in the 1800s were shaped like typical pistols of
that time, with a wooden handle and metal
barrel. For the demonstration, he collected the
“air” produced when acid comes in contact with
iron filings. This “air,” later named hydrogen, is
highly flammable, and he conducted studies
adding portions of hydrogen to ordinary air to
find out which mixtures yield the loudest bang.
Later, when Volta discovered that combustible “air” also is produced in sediments and
marshes, he conducted experiments to deter-
mine what proportions of sediment “air” and
ordinary air could produce the loudest bang
when they are ignited. His surprising results are
conveyed in an excerpt from his first letter of
1777 to Father Carlo Campi of Milano on this
subject:
No, sir, no air is more combustible than
the air from marshy soil. In the first place,
we can deduce this from the extraordinary
number of small explosions we can get
from it. But a surer indication is that it
transmits the property of flammability to
the ordinary air with which it comes into
contact, and in this respect it far surpasses
other combustible air. The strongest of all
these, obtained by dissolving iron filings in
vitriolic [sulfuric] acid, makes the loudest
explosions when combined with a volume
of ordinary air twice its own. The air of
swamps, on the other hand, ignites and
explodes most loudly, if to one part of it,
we add 8 or 10 parts of ordinary air.
(Translated from Lettera Prima.)
Volta concluded correctly that sediment “air”
from marshy soil contains more energy than
does hydrogen. Indeed, the methane from
marshy soils carries even more energy than he
thought. Thus, he was not aware that methane
accounts for only about 65% of such sediment
“air”—the remainder consisting of carbon dioxide, which did not contribute any noise or energy
to the explosive discharges that he observed.
Microbial Methane Production Occurs on
a Massive Scale
Microbes produce methane on a massive scale,
yielding about 109 metric tons per year on a
Ralph S. Wolfe is
Professor Emeritus
in the Department
of Microbiology,
University of Illinois, Urbana-Champaign.
Volume 70, Number 1, 2004 / ASM News Y 15
FIGURE 1
Volta’s original, 225-year-old glass pistol. Reproduced with permission. The pistol
is preserved in the History Museum of the University of Pavia, Italy.
global basis. This methane output stems from
anaerobic degradation of plant material in bogs,
marshes, and sediments as well as in the rumen,
cecum, and intestines of animals.
Why do anaerobes discard so much chemically entrapped energy? The main reason is that
anaerobic oxidation-reduction reactions are favorable only under particular conditions. For
example, fermentative anaerobes carry out only
limited oxidations, passing electrons to a more
oxidized fraction of the original substrate molecule or to protons in water to form molecular
hydrogen. The major reduced end-products of
fermentative biodegradations are acetate, propionate, butyrate, and hydrogen. These products
represent a dead end for fermentative anaerobes, nature’s first trophic level of microbes, in
anaerobic biodegradation (Fig. 2).
The second trophic level, involving the unfavorable oxidation of propionate and butyrate by
microorganisms such as Syntrophomonas and
Syntrophobacter, employs protons in water as
electron acceptors, producing molecular hydrogen. This second trophic level succeeds only
because microbes in the third trophic level carry
out anaerobic respiration, thereby removing hydrogen and making the oxidation of propionate
and butyrate thermodynamically favorable. In
freshwater environments, methanogens repre-
16 Y ASM News / Volume 70, Number 1, 2004
sent the major organisms in the third
trophic level. Methane, the final reduced product, serves as an ideal waste
product for such microorganisms because it is poorly soluble, not in equilibrium with the methylreductase that produces it, and largely inert under
anaerobic conditions.
Organisms in the three trophic levels
share available energy. Consider glucose as an example of a monomer from
a plant polymer that represents the total
energy available to organisms of the
three trophic levels (Fig. 2, reaction 1).
Note the small amount of energy available. Similarly, most of the energy from
anerobic degradations remains in methane, making it available for aerobic oxidation by methanotrophs or for chemical oxidation in Volta’s pistol (Fig. 2,
reaction 2). Moreover, when one compares the energy produced from the
partial oxidation of glucose (reaction 1)
to the complete oxidation of glucose
under aerobic conditions (Fig. 2, reaction 3), the limitations of life under anaerobic
conditions become striking. Thus, only -418 kJ
mol-1 of glucose are shared by the three trophic
levels, and about 85% of the energy from glucose remains in methane!
Recreating Volta’s Early Pistol-Shaped
Combustion Chamber
To stimulate interest among contemporary microbiologists by recreating a pistola di Volta
replica, or at least a facsimile, I asked a glassblower to construct a version based on Volta’s
original design (Fig. 3), but with modernadapted specifications (Fig. 4). To ensure safety
features, the body of this replica was blown
from heavy borosilicate glass tubing with walls
at least 3 mm thick. It also was fashioned with
standard crimp-and-seal flanges 13 x 20 mm
that are used for the ports.
As a substitute for the electrostatic generator
used by Volta to deliver sparks, we devised a
system (Fig. 4) that is simple, works well, and
preserves the spirit of the original device used by
Volta, who also invented the battery. Our modern, spark-generating device consists of a 12-V
battery from a portable hand drill (which has
sufficient amperage to produce a hot spark es-
sential for ignition of flammable gases),
FIGURE 2
and two brass rods that are 3 mm in
diameter, one about 7 cm long and the
other about 12 cm, each of which has
been threaded at one end with a No.
6 – 40 threader to receive the flint of a
gas lighter (Fisher Scientific, http://www.fishersci.com/, Cat. No. 12– 007).
Each rod is inserted through a Balch
stopper (Bellco, Vineland, New Jersey,
Cat. No. 2048 –11800) or a no. 0 rubber stopper cut to a length of 15 mm. A
3-mm drill is used to produce a hole in
the stopper through which the nonthreaded end of a brass rod can be inserted easily, forming an airtight seal.
Each rubber stopper with its brass
rod is seated firmly in a port, with the
flints being about 4 mm apart (Fig. 4).
Insulated wires with electrical clips are
connected to the brass rods and battery
terminals. A thin serum-type rubber
stopper (13 x 20 mm) is inserted into
the port at the end of the combustion
chamber and crimped into place with
an aluminum seal (Bellco, Cat. No.
2048 –11020). This type of rubber
stopper is easily penetrated by a hypodermic needle, which is used to inject
gas into the chamber.
Volta developed an ingenious method
for producing different mixtures of
gases. He simply added a particular volume of millet seed to the pistola that
Simplified scheme of anaerobic biodegradation and equations, showing the high ineffiwas equal to the volume of flammable
ciency of the process.
gas he wanted to displace. He then inverted the pistola over a second vessel
that contained the flammable gas,
bringing the opening of each vessel into close
proves convenient for collecting methane from
contact. As the millet seed descended into the
the shore of a marsh or other wet sites. At the
lower vessel, an equal volume of flammable gas
source of collection, submerge the funnel on its
was displaced upward into the pistola.
side in water above the sediment, then place the
Use of a syringe provides a convenient alterstem of the funnel in an upright position, keepnative means for injecting particular mixtures of
ing the rim under water so as not to allow any
gases into a modernized version of the pistola.
air to enter. Disturb the sediment, and collect
Meanwhile, a convenient way for collecting sedthe gas. Because 40 ml of gas is needed to yield
iment gas is to use a small plastic funnel—for
one part of gas to 10 parts of air (400 ml) in the
example, one with a rim about 10 cm in diamecombustion chamber, plastic syringes (20 ml or
ter and a volume of about 200 ml. The stem of
60 ml, equipped with 20-gauge needles) are
the funnel is cut at an appropriate place so that it
convenient to use.
may be snuggly inserted into the bottom of a
Transferring the gas from the funnel into a
rubber serum stopper. One practical advantage
syringe is a critical step. If air is allowed to enter
of using a small funnel is that this approach
the funnel from its rim or stopper as the syringe
Volume 70, Number 1, 2004 / ASM News Y 17
FIGURE 3
A modified version of Volta’s original pistol with a septum seal added to the closed end
through which a measured amount of sediment gas may be injected by syringe.
FIGURE 4
is being filled, efforts to ignite the contents of the pistola may not succeed.
After collecting gas from the funnel,
insert each syringe needle deeply in a
rubber stopper for storage. Several syringes may be prepared and stored for a
week or so before use. Insert a cork or
rubber stopper (not too firmly) into the
open end of the chamber; then inject 40
ml of sediment gas and pump the piston
of the syringe back and forth a few
times to mix the gases.
Meanwhile, connect the wires from
the pistola to the terminals of a fullycharged battery. To prevent a short circuit, one wire clip at the battery should
be covered with a thin plastic tube. Pull
the longer brass rod as a trigger to contact the flints quickly, producing only a
spark to ignite the gas mixture. For an
immediate second ignition to be successful, carbon dioxide in the chamber
should be displaced by injecting into the
barrel of the pistol several 60 ml volumes of fresh air from a syringe
equipped with a blunt-end, 15-cm, 12gauge needle. Should the flints become
corroded, they may be reactivated by
using a small file to brighten them at
their points of contact.
Conducting the Volta experiment
provides an impressive way to attract
the attention and interest of observers,
particularly students in a seminar or
classroom, or before a more generalinterest audience. The experiment
should be conducted by individuals
who are instructed in safety before they
perform it.
ACKNOWLEDGMENTS
I thank Dr. Paolo Boccazzi for introducing me to
the Volta museum on the web and for his kind
assistance in contacting Dr. Lucio Fregonese of
the University of Pavia, Italy, who graciously
granted permission to reproduce the photograph
of Volta’s original pistola, which is stored in the
Museo per la Storia dell Università di Pavia, Italy.
SUGGESTED READING
Specifications of the modified Pistola di Volta described in the text.
18 Y ASM News / Volume 70, Number 1, 2004
Ferry, J. G. (ed.). 1993. Methanogenesis. Chapman and Hall, New York.
Wolfe, R. S. 1996. 1776 –1996: Alessandro Volta’s combustible air. ASM News 62:529 –534.