The natural flames near Pietramala, on the Apennine route between Bologna and Florence, had attracted explanations ranging from underground vapours to petroleum and volcanic activity. Alessandro Volta visited in mid-September 1780 to investigate a more specific possibility: combustible gas was escaping through the ground and feeding the fire. His surviving account describes observations and tests, rather than simply repeating a traveller’s marvel.
This is an explained natural-phenomenon case. Volta’s experiments supported gas combustion as the immediate cause of the flames he examined. That result needs to be distinguished from his more speculative account of how the gas formed underground. It also supplies no promise that flames are still burning there today.
From analogy to a field question
Volta began his memoir by connecting the subject with his earlier work on inflammable air from stagnant water. He had observed that gas associated with decomposing organic material could burn. Reports of burning ground and springs suggested that similar gas might collect beneath the surface and escape through cracks or porous soil.
Pietramala was a natural place to test the idea because its fires were already well known and described by earlier travellers. Volta discusses Ferber’s observations and Dietrich’s notes. He reports having spoken with Dietrich in 1777 and having found support for the gas interpretation. Agreement between naturalists encouraged his hypothesis, but Volta wanted direct evidence at the site.
That distinction is explicit in his memoir. He worried that enthusiasm for his own discovery might make him too ready to explain every flame with the same cause. He therefore set out to examine whether combustible gas was actually present and continually emerging from the burning ground in sufficient quantity.
What he saw in 1780
Volta places the burning patch on a slope below the small village of Pietramala. He travelled with the Marquis Torelli and Giuseppe Re, assisted locally by a guide. The account describes scattered separate areas of flame rather than one neatly bounded volcanic crater.
Local inhabitants told him the flames looked blue and waved above the ground at night. That night-time description was reported to him. His own visit took place in bright sunlight, when the flames were hard to see and their heat could be more obvious than their outline. The distinction matters: the text combines his observations with attributed local knowledge, and should not turn both into one personal night-time inspection.
He describes dry, comparatively loose, stony ground, with individual flames changing in volume and sometimes joining or separating. The report is about a real physical fire. It does not require an invented apparition or a legend to make it a mystery worth investigating.
Evidence of an escaping fuel
Volta varied conditions around the burning patches and observed changes in where the flames emerged. In his interpretation, restricting one route caused more gas to escape elsewhere. Those changes made sense if a combustible supply lay beneath a permeable surface, rather than if isolated solid stones were themselves burning.
He also observed movement of light surface material where gas seemed to emerge. Such observations helped connect the visible fire with an escaping substance. They were not yet a complete identification of that substance’s composition. The term inflammable air belongs to the scientific vocabulary of the account and should not be treated as if it were a modern laboratory certificate.
A nearby water source supplied another comparison. Bubbles rose through the water, while earlier travellers had described a cold pool that appeared to boil. Volta interpreted that apparent boiling as gas escaping, not necessarily as water heated to its boiling point. The presence of bubbles and flame therefore raised related but distinguishable questions about the escaping fuel and the water’s temperature.
The decisive collection and test
The most direct passage occurs on page 116 of the collected edition reproduced above. Volta describes collecting gas from the ground and taking enough back to his lodging at Pietramala to test. The same companions who had helped with the fieldwork were present. He reports that the collected gas burned with a blue, low flame similar to the material from marshes and the nearby spring.
This is stronger than an explanation based only on resemblance. The sample taken from the site itself was combustible. It supported the proposed immediate connection between an underground gas supply and the flames. The article recounts the historical result without turning the old experiments into instructions for visitors to repeat around a gas seep.
The observation did not establish every part of the underground system. It showed a burning fuel could be collected from the site. The depth, size and renewal of a reservoir remained different questions, requiring evidence beyond the simple fact that the collected gas burned.
Petroleum, bitumen and volcanic appearances
Volta argued against a need to invoke petroleum or bitumen at the surface. He described the ground he examined and discussed Dietrich’s tests, which had not supplied convincing bituminous material. He also rejected treating the site as a conventional volcano simply because flame emerged from the earth.
These are arguments within a historical scientific debate. They should not be enlarged into a modern geological survey excluding every hydrocarbon association beneath the district. Establishing gaseous fuel at a burning patch is a narrower result than identifying all source rocks, migration routes or deep chemical processes.
The same care applies to the word methane. Later accounts commonly connect Pietramala with natural gas, but the original memoir’s tests should be described at the level they establish. This article relies on Volta’s demonstrated combustible gas, without inventing a modern measured percentage or purity for his sample.
Where observation became hypothesis
Volta considered buried organic deposits and the movement of water into underground spaces as possible ways to explain the gas supply. He discussed local reports that rain strengthened the flames and proposed displacement of gas as an explanation. These are reasoned interpretations in his text, not a measured map of an underground chamber.
He also described a model apparatus intended to imitate the behaviour. An apparatus can demonstrate that a proposed mechanism is physically possible. It cannot, by resemblance alone, establish that the natural site has exactly the same hidden structure. Keeping this distinction preserves the interest of the investigation without making eighteenth-century reasoning more conclusive than it was.
Dates, place and assessment
The observations belong to 1780. The collected edition identifies the memoir as written in 1782 and first published in 1784. The page shown here comes from volume 7 of the 1929 national edition of Volta’s works. Those publication dates are not new flame incidents. The incident-year entry is 1780 because it records the specifically dated firsthand investigation, not a claimed beginning of the long-known fires.
Approximate map area: 44.16275, 11.33551, the Pietramala locality reference in a Regione Toscana settlement table. The marker is not a plotted vent or Volta’s inn. This reference page establishes no present-day flame condition, visitor route or permission to enter land.
The explained outcome concerns the immediate fuel of the flames Volta investigated: escaping combustible gas. His underground-origin proposals and the site’s present condition remain outside that demonstrated result. That is a useful historical resolution, rather than an invitation to regard every unfamiliar flame as already understood.
Primary source
Read Alessandro Volta, Le opere, volume 7, memoir CXXII, pages 109–119, particularly the field observations and sample test on pages 113–116. The edition’s source note separates the 1780 observations, 1782 writing and 1784 publication. The Regione Toscana locality table supplies the village coordinate. The image is a faithful rendering of public-domain original text in an expired typographical edition. Sources checked 13 November 2026.