But what Lavoisier was seeking to understand after 1791 was the general cycle of living matter.Prior to his work, it was thought that the living world consisted of only two kingdoms: the vegetable, destined to develop nutriments, and the animal,which drew its required energy from the nourishment provided by the former."I consider nature a vast chemical laboratory in which all kinds of compositions and decompositions are formed.Vegetation is the basic instrument the Creator uses to set all of nature in motion. Vegetables are organized bodies that grow on the dry areas of the globe and within its waters.Their function is to combine immediately the four elements (water, earth, air and fire) and to serve as food for animals.Nature uses both kingdoms to form all existing combustible matter."(A. Baumé, Chymie expérimentale et raisonnée, Paris, Didot, 1774, vol. I, p. xi.)
Later, Bernard would criticize this way of dividing up Nature's kingdoms: "In living beings, the phenomena of destruction or vital combustion were absolutely separated from the phenomena of reduction or organic synthesis.The role of vital creation had been assigned to the vegetable kingdom, whereas organic destruction had been reserved for the animal one.The animal organism was incapable of forming any of the principles of which it is made up - fat, albumin, fibrin, starch, sugar; they were all provided by the vegetable kingdom and animal nutrition was hardly more than the placing inthem of materials uniquely developed by plants."(C. Bernard, Leçons sur les phénomènes de la vie communs aux animaux et aux végétaux, Paris, Baillière, 1878, vol. I, p. 136.)
But Lavoisier's vision was much more modern.In the Traité élémentaire de chimie in 1789, he explained how he had been led to invent the technique for analyzing organic bodies by combustion as early as 1774.
The chemical analysis of organic matter by fire allowed him to show that oak, for example, was a combination of water, acids and oils of which the residue was charcoal, itself decomposable into earth and fixed alkali.But Lavoisier felt that the analysis needed to be pushed further: "We still do not know," he wrote in his third laboratorynotebook, "1) what is the quality of that immense amount of air that is released during distillation: it is probably a combination of fixed and inflammable air; 2) the nature of oil: it appears that it can be reduced to air and water by combustion, but we know nothing more; 3) what charcoal is; we know, of course, that when it burns, it converts the surrounding air into fixed air; but we do not know if it itself gives fixed air; nor do we know what is released during combustion orthe relationship of what remains with the original weight of charcoal." (Quoted byM. Berthelot in Larévolution chimique Lavoisier, Paris, Félix Alcan, 1902, p. 260.)
But the inconvenience ofthetechnique of anlysis by fire was that instead of separating the components into vegetable matter, it destroyed them completely.In July 1777, learning that Torbern Bergman (1735-1784) had obtained a saccharic acid (oxalic) throughdistillation, by having nitric acid react on sugar,Lavoisierhadtried to repeat the experiment.Butinstead of oxalic, he had obtained nitric acid, water anda charcoal-like residue.In February 1799, heating sugar more slowly, he had been able to synthesize oxalic acid.He understood that in the previous experiment the intense heat had decomposed the sugar into hydrogen and carbon.In combining with the oxygen of the nitric acid, the carbon formedcarbonic gas and hydrogen formed water.That meant that by measuring the quantities of carbonic gas and water produced by the decompositon of sugar, one could deduce the quantities of carbon and hydrogen contained in the sugar.
This success had allowed Lavoisier to apply to organic acids his general theory on the formation of acids.But , above all, it had revealed the principle oforganic chemical analysis : it was possible to determine the quantity of carbon and hydrogen contained in any organic matter by measuring the quantities of carbonic gas and water produced by its combustion. Using this method to analyze spirits of wine, olive oil and wax, he had seen that vegetable as well as animal matter results from the combination of oxygen, hydrogen and carbon.(Lavoisier, Oeuvres, vol. II, p. 588.)"The least degree of heat, as long as it is slightly higher than boiling water, suffices for uniting oxygen and hydrogen, and hydrogen and carbon, and for forming oil and water.But also the phenomena become more complicated, because there exists, as M. Berthollet has shown, a fourth principle in animal matter, nitrogen, which, with hydrogen, forms volatile alkali or ammonia." (Lavoisier, Oeuvres, vol. II, pp. 664-66.)
Lavoisier had established the essential relationships.From the point of view of interest, the theoretical approach prevailed over the experimental part, and when he sought to demonstrate the equation that linked these diverse elements by calculating their weight, all the figures were inaccurate, but his general conclusion was correct.He had understood that no separation exists between the chemistry of minerals, vegetables and animals.He had undertaken the exploration of life by the analysis of its physio-chemical mechanisms, thus opening the way to modern biochemistry.He would have neither the time nor the means to go further; ambition, his responsibility for State finances and, very soon, his personal concerns were going to distance him from research.