Among Lavoisier’s papers, published in the third volume of his collected works in 1865, is a bundle on animal magnetism that the editor says Lavoisier had gathered “with the intention of publishing them”. It holds the minister’s letter of appointment, two summaries of the doctrine as Mesmer’s school and d’Eslon taught it, the declarations of two men who had taken Mesmer’s course and seen nothing, Lavoisier’s own remarks on how the commission should reason, a plan for one day’s experiments at Franklin’s house, and a draft summary of the Report. The remarks and the plan are translated here in full, because together they are the design of the first experiment that kept its subjects from knowing what was being done to them, written down before it was run. The translation is mine.

The remarks

After summarising d’Eslon’s doctrine, Lavoisier wrote:

“This simple exposition, concise as it is, shows with what art and what assurance animal magnetism is presented. Some truths of fact and observation have been mixed with the pretended results of a purely hypothetical principle, and the result has been a body of doctrine that imposes even on enlightened physicians.

“The art of the commissioners consists in following the chain of reasoning, in recognising the places where it is broken, in setting down facts before reasoning. Good logic does not permit the admission of new principles to explain facts when they can be explained by other principles already known. We shall therefore admit animal magnetism only in so far as it presents effects which cannot be referred to any other cause. We shall examine whether the imagination alone, without magnetism, could not produce similar effects, and we shall accordingly apply ourselves to making a series of experiments on magnetism separated from imagination and on imagination separated from magnetism. These reflections have suggested to me the plan which follows.

“The commissioners named by the king for the examination of M. Deslon’s method have already seen enough to be entitled to suspect that all the effects he produces can be explained without introducing into physics and into medicine an animal magnetic fluid whose existence no positive fact demonstrates, which enjoys none of the properties of the other known fluids, and to which are gratuitously attributed properties incompatible with one another and with everything that is known.

“The means M. Deslon employs reduce principally to two: touch, and the pretended emission of a fluid which is conducted and condensed either with the finger or with a small iron rod. We have seen only one effect produced by these two means: it consists in making some persons fall into convulsive states, but without any alteration in the pulse or derangement of health resulting.

“But it is known that the imagination alone, struck or prepossessed to a certain point, suffices to produce these effects, that there exist a host of examples of imitative convulsions; all the more can touch act when its effects are united with those of the imagination.

“The art of concluding from experiments and observations consists in evaluating probabilities, and in estimating whether they are great enough or numerous enough to constitute proofs. This kind of calculation is more complicated and more difficult than one thinks; it demands great sagacity and it is in general beyond the powers of the common run of men. It is on their errors in this kind of calculation that the success of charlatans, of sorcerers, of alchemists is founded; that the success of magicians, of enchanters and of all those in general who deceive themselves or who seek to abuse public credulity was founded in the past.

“It is above all in medicine that the difficulty of evaluating probabilities is greatest. Since the principle of life is in animals a force always acting which tends continually to overcome obstacles, since nature, left to her own forces, cures a great number of diseases, when remedies are employed it is infinitely difficult to determine what belongs to nature and what belongs to the remedy. Thus, while the multitude regards the cure of a disease as a proof of the efficacy of the remedy, there results from it, in the eyes of a wise man, only a greater or lesser degree of probability, and this probability can be converted into certainty only by a great number of facts of the same kind.

“These reflections struck the commissioners on magnetism, and they recognised that, to prove methodically the existence of magnetism by the cure of diseases, it would perhaps take the lives of several men. They therefore rejected a kind of proof which might lead them to admit an agent that has no reality; they thought it better to go back to the source and prove that magnetism did not exist, in order to be entitled to conclude that the cures attributed to it were not due to it.

“This kind of experiment once excluded, the course the commissioners have to follow is simple and presents itself. The manner of operating of the partisans of magnetism consists in two things: first, in touches; second, in different procedures for passing the magnetic fluid from one body into another at some distance; and the result in both cases is to make the patient operated on fall into the state called crisis, that is to say to give him convulsions. But to conclude that the convulsions which very sensitive persons undergo are due to a particular agent, to a magnetic fluid, it would be necessary that the convulsions they undergo could be attributed to that cause alone. For, in good logic, when an effect may depend on several causes, it is not permitted to conclude that it belongs to one rather than to another. Now it is known that the least contradiction, the least constraint, an exalted imagination, suffices to give convulsions to very sensitive persons, and that there exist besides imitative convulsions, like yawning, which pass from one person to another. If therefore the effects of magnetism, and the convulsions which are sometimes their consequence, can equally be explained by the effect of the imagination struck or exalted, all the attention of the commissioners must be given to distinguishing in magnetism what belongs to physical causes from what belongs to moral causes, the effects of a real agent from those due to the imagination. There is only one way to reach this end: it is to magnetise very sensitive persons without their knowledge, and to persuade them that they are being magnetised while they are not. By uniting these two kinds of experiment, one will obtain separately the effects of magnetism and those of imagination, and one will be able to conclude from them what ought to be attributed to the one and to the other.

“Every experiment that departs from this plan will obscure the matter instead of clearing it; for in the sciences and in the arts, a single conclusive experiment is more precious than a great number of others which do not go directly to the end.”

The plan of experiments

“One will go to Passy, to M. Franklin’s, at noon precisely.

“All the commissioners will gather with M. Deslon and the subjects who are to be magnetised in M. Franklin’s bedroom; it will be explained there in a positive manner that M. Deslon will not utter a single word, nor the other commissioners either, with the exception of one alone, who will question the patient.

“Employ M. Deslon in magnetising Mme de Roumagné; M. Franklin with M. Majault, M. Darcet, Mme Moré in the salon; commissioners M. Guillotin, M. Le Roy, M. de Bory, directly in the other room; M. Bailly, M. Lavoisier, M. Sallin, through the door.

“Then, on the pretext of agreeing on the experiments to be made, the patients will be led to a part of the house where they will be kept in sight, each in separate rooms; one may be placed in the salon.

“As the object of the first two experiments is to try the effect of the imagination on persons who are not magnetised but who believe they are, several may be made at once. Thus the experiment of the basin and that of direct magnetism may be made.

“To this end two of the commissioners, MM. . . , will go and take one of the patients, blindfold him and lead him to the basin, where a sufficient number of chairs will have been prepared. A third commissioner will arrive a few moments later quietly, and so as to make sure the patient is persuaded that it is M. Deslon. One of the commissioners, M. . . , will be charged with feeling the patient’s pulse from time to time, with questioning him on what he feels, observing that the object is to persuade him that he is magnetised, that he must feel effects, and with giving the questions a captious form which confirms him in that idea. One of the other commissioners, M. . . , will write down faithfully the questions, the answers and all the circumstances. It is important that only one of the commissioners speaks.

“If the patient falls into crisis, the commissioners will have no other task to fulfil than to give him help, to observe and to write.

“One of them may however detach himself to go and fetch M. Deslon, as shall be judged necessary. But M. Deslon will be held to observe the law of silence, even towards the patient in crisis and even on the supposition that he should appear to be unconscious.

“If on the contrary, at the end of thirty minutes, he feels no effect, one of them, M. . . , will go and fetch M. Deslon and bring him near gently, while the commissioner charged with questioning distracts the patient’s attention with questions. Besides, as it is not the patient himself that M. Deslon will magnetise, but the water of the basin, the distance may be great enough for the patient not to perceive his presence.

“During the same time three other commissioners will be charged with making the experiment of direct magnetism on another patient in the salon. M. . . will question, M. . . will write and M. . . will be charged with imitating M. Deslon magnetising, without affectation. In this experiment everything will be done that is thought most fit to deceive the patient’s imagination, so that he cannot doubt that M. Deslon is present. The one charged with questioning, M. . . , will put the suitable art into his questions; he will even appear at moments to address words in a low voice to M. Deslon magnetising, who will always be supposed to be executing without answering, in conformity with the conventions.

“A few minutes before the end of the agreed time, the patient will be told: ‘The experiment is about to end and you will be taken back to the same place where your eyes were bandaged; but one more thing is asked of your kindness; it is to remain another quarter of an hour with your eyes bandaged, so as to give us an account of the sensations you feel without being magnetised, and to compare them with those you have just felt.’

“Then the patient will be led into a room to which M. Deslon will have been taken, and he will be given the sign to magnetise; but it is probable that it will not be necessary to come to this, and that the patient will have fallen into crisis before the thirty minutes, with M. Deslon absent.

“For exact execution, each of the commissioners will take an extract of what he has to execute.”

What the two documents show

The remarks state the question as a question of logic before it is a question of fact: a new cause may be admitted only for effects no known cause can produce, and the known cause here, imagination, had to be given its own trial. The plan turns that into procedure. One person speaks. One person writes. The subject is deceived in a way agreed beforehand and recorded. The practitioner is silenced, kept away, and brought in only on a signal, “gently”, from a distance, so that the real agent can be applied without the subject’s knowledge. Each outcome has its next step written down, including the one the planner expected and the one he did not. The last line, which asks every commissioner to carry an extract of his own part, is the earliest written protocol I know of in which the roles of questioner, recorder and decoy are separated so that none of them can shape the result alone.

The day described is recognisably the one the Report tells, when two women were separated at Passy while d’Eslon magnetised Franklin in another room, one of them blindfolded and told he was present, the other seated before a shut door; the Report gives three commissioners to the first woman and two to the second, where the plan had three and three, and it does not say whether the basin was used that day. The plan also expected the subjects to be men; the Report’s subjects were women. Plans are plans. What matters is that it exists, in the hand of the chemist who three years later would publish the method of chemical nomenclature, and that it is dated by its own contents to before the experiments it describes.

Sources

• Œuvres de Lavoisier, tome III, Mémoires et rapports sur divers sujets de chimie et de physique pures, Paris: Imprimerie impériale, 1865, “Sur le magnétisme animal”, pages 499 to 527: the baron de Breteuil’s letter, the summaries of Mesmer’s and d’Eslon’s doctrines, the declarations of Lafayette and Berthollet, the “Remarques de Lavoisier”, the “Plan d’expériences” and the “Résumé du rapport”. Translated here by the author of this entry. archive.org