together with his fellow acadamician Pierre Laplace, carried out a proper scientific experiment, based on quantitative principles, to test the hypothesis.
Their experiment involved a guinea pig, which was placed in a container within another container, itself insulated from the outside world, with the gap between the two containers filled with snow at 0 ºC. Under these conditions, the animal was quiet and did not move about much. They waited for ten hours, and collected and measured the water that had melted from the snow as a result of the warmth of the guinea pig’s body. It came to 13 ounces (369 grams). Then, in a separate series of experiments Lavoisier and Laplace measured how much fixed air the animal breathed out in ten hours while it was resting. Finally, they compared their guinea-pig measurements with the amount of snow that could be melted by burning enough charcoal to make the same amount of fixed air. This was slightly less than 13 ounces, but the agreement was close enough to convince Lavoisier and a wider circle of scientists that animals keep warm by combining the substance we now call carbon, obtained from their food, with something from the air (oxygen) to make fixed air (carbon dioxide). This was a key step in seeing animals, including human beings, as systems obeying the same laws as burning candles or falling stones.
© Science Photo Library
Antoine Lavoisier (1743–1794) in his laboratory with his wife and his assistants. His wife (Marie-Anne Pierrette Paulze, 1758–1836) is taking notes at far right.
It was Lavoisier who gave oxygen its name, and who established that burning does indeed involve oxygen from the air combining with the burning substance. This replaced the old idea, still adhered to even by people such as Priestley, that a substance called ‘phlogiston’ is escaping from the substance as it burns. Lavoisier published his definitive demolition of the phlogiston model in the Mémoires of the French Academy in 1786, using the term ‘air’ where we would say ‘gas’:
1 There is true combustion, evolution of flame and light, only in so far as the combustible body is surrounded by and in contact with oxygen; combustion cannot take place in any other kind of air or in a vacuum, and burning bodies plunged into either of these are extinguished as if they had been plunged into water.
2 In every combustion there is an absorption of the air in which the combustion takes place; if this air is pure oxygen, it can be completely absorbed, if proper precautions are taken.
3 In every combustion there is an increase in weight in the body that is being burnt, and this increase is exactly equal to the weight of the air that has been absorbed.
4 In every combustion there is an evolution of heat and light.9
Lavoisier also gave their modern names to many other substances, and produced the first list of 33 chemical elements, as well as introducing a system of symbols to represent the elements, although not all of them turned out to be elements as we know them today. The key point, though, is that he discarded the old idea of four mystical ‘elements’ (Earth, Air, Fire, and Water) and replaced it with the idea of an element as a substance that could not be broken down into any simpler substances using chemical processes, while more complex substances were made by combining elements. Indeed, Lavoiser’s definition still stands: ‘We must admit, as elements, all the substances into which we are capable, by any [chemical] means, to reduce bodies by decomposition.’ His naming system used logical rules based on this idea, so that, for example, ‘vitriol of Venus’ became ‘copper sulphate’.
© Gregory Tobias/Chemical Heritage Foundation/Science Photo Library
Nineteenth-century artwork of the ice calorimeter developed in the period 1782 to 1784 by the French scientists Antoine Lavoisier (1743–1794) and Pierre-Simon Laplace (1749–1827). The central space (centre right) would contain burning oil (upper right), or an animal such as a guinea pig; the surrounding chamber would contain ice; the outer, melted snow. The lid would be added and the amount of heat produced would be measured in terms of the volume of meltwater from the ice (lower left).
His book Traité Éleméntaire de Chimie (Elementary Treatise on Chemistry) was published in 1789, and laid the foundations of chemistry as a proper scientific subject. It is seen by chemists as their equivalent to Isaac Newton’s Principia. Lavoisier also spelled out clearly what we now call the law of conservation of mass, which states that matter is neither created nor destroyed in chemical reactions, but only converted from one form into another. In the same year, he also founded a journal, Annales de Chimie, which carried research reports about the new science.
As far as such things can be pinned down to a specific year or a specific event, the publication of Lavoisier’s book marks the moment when chemistry shed the last traces of alchemy and magic, and became a proper scientific discipline.
| No. 20 | TWITCHING FROGS AND ELECTRIC PILES |
During the 1790s, a series of experiments led to two major discoveries: that electricity can flow from one place to another, and that electricity is important in operating the muscles of living animals. The second discovery came first, when the Italian physician Luigi Galvani was dissecting a frog. Galvani was also interested in the nature of electricity, and had in his laboratory a hand-cranked machine that could generate electric sparks by the friction of two surfaces rubbing together. This kind of ‘static’ electricity had been known about since the time of the Ancient Greeks. While Galvani was dissecting a pair of frog’s legs, a metal scalpel that had been in contact with the machine and had become electrified touched the sciatic nerve of one of the legs. The leg kicked as if it were still alive.
Galvani carried out many experiments to investigate the phenomenon. He found that legs from a dead frog would twitch if they were connected directly to the electric machine, or if they were laid out on a metal surface during a thunderstorm. But his most important discovery was a result of an observation, rather than a planned experiment.
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Luigi Galvani’s 1791 experiment on the legs of a frog. The upper diagram shows a silver rod (left) and a brass rod (right) being placed in contact with a foot and the spine of the frog. Bringing the two rods together resulted in the leg twitching as the muscles contracted. The lower diagram shows the metal rod connecting foils of two different metals, with the same result.
When preparing frogs’ legs for study, he would hang them up on brass hooks to dry out. When the hooks came into contact with an iron fence, the legs twitched. In case this was due to some influence from electricity in the air, Galvani took the legs and hooks indoors, away from any source of electricity (including his electrostatic generator) and brought the hooks into contact with iron again. Again, the legs twitched. He concluded that electricity must be manufactured in the body, and stored in the muscles of the frog. He called this ‘animal electricity’, and proposed that a fluid manufactured in the brain carries this electricity through the nerves of the body to its muscles. But he believed that this animal electricity was something different from the natural electricity of lightning, or the electricity produced artificially through friction.
Most of Galvani’s colleagues went along with this idea, which reinforced the idea of a special ‘life force’, or spirit, which distinguished living things from the non-living world. But one person in particular strongly disagreed. He was another Italian, a physicist called Alessandro Volta. Volta said that electricity was indeed the cause of the twitching of the legs of the dead frogs, but that it had not been stored in the muscles, and that there was no difference between animal electricity and natural electricity. Instead, he suggested that it was being generated from an outside source,