The Controversy Over Distillation
Distillation — the process whereby plants were placed in a receptacle and heated to obtain liquid and solid products — was the obvious choice for analyzing plants. It was also known to be flawed. Academicians had to justify their choice in the face of well-aired criticisms, but the shortcomings of distillation were most forcefully presented to them by their own research. Its defenders argued not only against contemporary chemical literature but also against objections raised within the Academy itself.
Duclos was the first academician to explain how to distill plants. He described in detail how to extract the chemical constituents of plants, that is, "their distilled waters, their acrid, sulphurous, acidic, and mercurial spirits, their oils, and their fixed or volatile salts." In explaining how distillation worked, Duclos used the kind of old-fashioned teleological language that Perrault and Mariotte sometimes ridiculed: the heat of the fire, he argued, made an impression on the plant and then rarefied it; rarefied matter would rise, but some matter was more disposed to rise than others. Duclos's subsequent dismissal as director of the project, however, meant that others had to defend the method he had chosen, and they did so in very different language. Dodart argued the case in his Mémoires des plantes, Mariotte used the results in his Végétation des plantes, Homberg tried to exonerate Bourdelin, Tournefort studied Bourdelin's method and findings, and Fontenelle summarized some of the arguments for distillation when he wrote the Histoire .
Two analogies seemed to warrant the distillation of plants. Distillation could be considered the equivalent of dissection (with the fire serving as the knife) or as the counterpart of digestion (with the still replacing the stomach). These were variations of ideas that had been current since Paracelsus. Nicolas Lémery, John Ray, and Nicaise Le Febvre, among others, had advocated "anatomizing" plants by distilling them; Le Febvre also emphasized that distillation would show how the heat of the stomach acted on the food it digested. But the analogy with the stomach also drew attention to some shortcomings of distillation: a fire could not transform plants the way the stomach could, did not extract the same nourishing substances, and required higher temperatures. These analogies reveal not only academicians' assumptions but also their motives. They wanted to
anatomize plants in order to understand the secrets of their structure and to know what caused their effects on humans.
Even more revealing than such justifications are the Academy's debates about the shortcomings of distillation. One difficulty was that the products were not necessarily extracted in their purest forms. Because chemists could not always tell when the distillants changed in nature from one substance to another, in any single distillation they would get mixed substances as well as relatively pure ones. Dodart expected that this problem was not serious, because even mixtures would reveal information about the composition of a plant. Obtaining pure distillants was not the original goal: the Academy wanted to discover the constitution of plants.
Another problem was that distillation was destructive. Analyzing plants destroyed the very components that produced the effects — nutritive, gustatory, poisonous, or medicinal — academicians sought to understand. Dodart responded that such effects did not necessarily result from "the union of all the principles [that is, chemical constituents]," and, anyway, that effects "which depend on several of these principles joined together often depend on the dominant principle." He did not deny that the fire itself might pass through the apparatus and mix with the plants, but he replied that even so distillants differed from one another. So long as the fire and the vessels were the same, any variations must derive from the plants and not from the distillation.
The most common objection was that, since all plants released the same constituents, these could not account for diversity among plants. Dodart, in reply, pointed out subtle differences in the proportions and strengths of the constituents. He hoped that some of the "more ordinary effects" of plants might thus be explained and that, with more experiments, unusual effects might become understandable. Mariotte, in contrast, granted the premise on which the objection rested but was more interested in why all plants had the same basic constituents. He concluded — using a thought-experiment that resembled an actual experiment described by Helmont and Boyle — that all plants had the same "gross and sensible" constituents because they received their nourishment from the same sources, earth and water.
Where academicians sought diversity — in the products of distillation — they found uniformity. Where uniformity was essential — in the replicability of experiments — they found diversity. They recognized the importance of being able to reproduce experimental results. Bourdelin weighed the plants he distilled and the products he extracted from them; he recorded the exact conditions of each distillation, including, as best he could, the temperature
of the fire. But even with all his precautions, the results of iterated experiments might vary, in some cases dramatically. Dodart tried to minimize this discrepancy by saying that chemists were entitled to ignore small variations and should take only major ones as significant.
Unexpected variations in the results of similar experiments aggravated still another problem — the unmanageability of the data. Even by 1676, when Bourdelin had been distilling plants for only eight years, academicians found the amount of information compiled from his distillations so vast as to defy their analytical skills. The responsibility of interpreting Bourdelin's data fell on Dodart, who did his best to extrapolate a few generalizations from them.
The most threatening objection, however, was that the fire created new substances instead of merely separating substances that already existed in the plant. This view was widely accepted and had been asserted by numerous English scientists throughout the century. Some academicians feared that this was indeed happening, and Dodart had to acknowledge certain disadvantages of distillation in refuting this view. As Fontenelle later pointed out, something as violent as fire must alter the constituents of a plant, especially the fixed salts, which were obtained by lessives only after calcination. Mariotte suspected that distillation might fix volatile salts and make fixed ones volatile; the fire could even create a poisonous substance from a nutritious plant or form new chemical unions from the plant's constituents. On the whole, however, Mariotte believed that fire did not produce the constituents found in plants, because all of them could also be obtained naturally without recourse to fire.
This problem worried academicians, who searched the data for reassurance. Even Duclos changed his mind about the effects of distillation. In 1668 he had believed that the fire assembled similar elements and separated dissimilar ones when heat excited motion in the substance being distilled. By 1676 he came to believe that fire changed a plant's material virtues without making its formal and specific virtues better known. Homberg later wrote that fire united some parts of a plant to form oil. Against such views, Dodart argued that a fire did not often create new products, although he admitted that it might change the structure of the basic particles that compose plants and that some elements might escape through the vessels. Dodart tried to define the nature and limits of any changes that fire could produce and asserted that any loss from the vessels was inconsequential with respect to both weight and character.
Academicians criticized the procedure they had selected, and they disagreed about continuing to use it. Whatever doubts existed when the
project started were not assuaged as it progressed; rather, Bourdelin's research brought to light still more problems. As a result, his colleagues considered abandoning or refining the method, sought a more effective one, and in the meantime changed Bourdelin's procedures.