Antoine Lavoisier

Lavoisier's central instrument was the balance and his central discipline was to weigh everything, including the air. Chemistry before him worked with a substance called phlogiston, supposed to be released when things burn, and the awkwardness that metals gain weight when they calcine had been accommodated by increasingly strained explanations. By conducting combustion in sealed vessels and weighing the whole system before and after, Lavoisier showed that the gain in the metal matched a loss from the air, identified the portion of air responsible, named it oxygen, and disposed of phlogiston entirely. The same accounting gave him the principle that the total mass of matter is unchanged by chemical reaction.

For an atlas concerned with the alchemical tradition, this is the moment the two paths finally separate. Alchemy had always been a quantitative craft in practice, and the assaying techniques it developed were precise, but its theory permitted matter to be transmuted between the elements. Lavoisier's table of simple substances that resist further decomposition made transmutation not merely unachieved but incompatible with the framework, and the practical craft passed to chemistry while the philosophy went elsewhere.

Lavoisier sat on the 1784 commission on animal magnetism alongside Benjamin Franklin and the astronomer Jean-Sylvain Bailly, and the experimental logic of that inquiry is recognisably his. The question was posed as one about an alleged material agent, the tests were arranged so that the agent could be present without the subject knowing and absent while the subject believed it present, and the outcome was read from the difference. It is the same move as weighing a sealed vessel: isolate the variable, then measure.

Bailly wrote the principal report and Lavoisier is generally credited with much of the experimental design. His end came from the other half of his life. He had been a member of the Ferme generale, the private consortium that collected taxes for the crown, an association that funded his laboratory and made him wealthy and that the Revolution treated as a capital offence. He was tried and guillotined on the same day in May 1794, at fifty. The remark attributed to Lagrange, that a moment sufficed to strike off that head and a hundred years might not produce its like, is reported at second hand and is probably too neat, but it has stuck because it is not far wrong.