A.-L. Lavoisier’s ideas on matter, and more particularly his operational definition of the chemical element, gave a satisfactory solution to all or most of the problems posed by the composition of mineral bodies. Thus, chemical analysis not only indicated the elements composing a mineral body, but also provided the key to the understanding of their properties.

In the case of organised bodies, however, chemical analysis could only show they were composed by a small number of elements. In Traité elémentaire de chimie, Lavoisier explains that as chemical analysis showed plants to be mostly composed of hydrogen, oxygen, and charcoal, the various properties exhibited by the bodies from the plant kingdom and used, e.g., as medicines, were determined by double and triple affinities.

Long known as “medicinal virtues”, such properties manifested by means of so-called “medicinal principles” – or “active principles” – whose presence in a given material lent it its characteristic identity, and distinguished it from all others. One of the major problems as how to isolate such principles, since distillation – then a common procedure to decompose the organised bodies – destroyed them, or gave rise to substances non-existing in the source material.

Within that context, in the section devoted to “plant chemistry” of Système des connaissances chimiques (1801), A.-F. Fourcroy suggests “means to decompose the products of plants, and discover the order of their composition”. He explains that four different kinds of analysis - immediate or proximate, mediate or remote, simple or true, and complicated or false analysis - originate also different ‘products’. Only “true analysis” allows isolating the ‘compounds’ related with the medicinal action of plants, and thus, a thorough knowledge of that process would not only make the preparation of medicines easier, but would also allow clustering together the plants exhibiting one same effect. Still more important, “true analysis” could contribute to the understanding of the action of medicines in the human body (or “animal economy”, according to the expression used at that time).

In the present study, we address the debate among Fourcroy and other contemporary scholars on the possibilities to isolate and recognise the “medicinal” or “active principles” at a time when elementary composition was considered as the key to the understanding of the nature of substances. (Funded by FAPESP – Proc. 2011/14040-9 and CNPq – Proc. 309691/2011-7)