Antoine Laurent Lavoisier

Lavoisier, the father of modern chemistry, went to the guillotine, crucifix in hand.

That's quite an introduction, isn't it? Here's where it comes from:

Lavoisier, the father of modern chemistry, went to the guillotine, crucifix in hand, a fervent child of the Church, at the bidding of ... the friends of "Reason" who declared, "Nous n'avons plus besoin de chimistes."
[Windle, The Catholic Church and Its Reactions With Science]

That French means "We no longer need chemists."

Oh, you say. That was during the French Revolution, when everyone went crazy. But they were going around saying they were for "Reason" - indeed, for Science!

But who is Lavoisier? Why is he called the Father of Modern Chemistry? You need to know - yes, even if you prefer history to chemistry, and religion to physics, you need to know at least a little about these things. It would be as pointless to talk about Lavoisier and not mention phlogiston as it would be to talk about St. Athanasius and not mention "homoousion".

Though Father Kneller limited his study to the 19th century, he found it necessary to mention Lavoisier in several places:

The modern science of Chemistry is, comparatively speaking, of very recent growth. It begins with Lavoisier who was the first to lay proper stress on accurate measuring and weighing, and whose analysis of the process of combustion gave the first insight into the nature of chemical union. A theory of fundamental importance is that which conceives the elements as composed of atoms. ... Antoine Laurent Lavoisier does not fall within the scope of this book, inasmuch as he died under the guillotine of the Revolution before the beginning of the 19th century. We content ourselves with remarking that he died in the Christian faith. ... The science of Physiology properly speaking came into existence, according to Bernard, with the discovery "that life is conserved by physico-chemical processes, which, so far as their immediate cause and character are concerned, do not differ from those which take place in non-living matter". It was Lavoisier who first made this proposition the starting point of scientific Physiology. He treated respiration as a process of oxidisation pure and simple. The heat of the body he showed, in conjunction with Laplace, to be due to a process of combustion, identical in essence with that which takes place in our ovens.
[Kneller, Christianity and the Leaders of Modern Science]

So Lavoisier has connections to physiology and biochemistry, to physical biology and other subdivisions of science! But it is as a chemist he is remembered, and (as Jaki might point out) as a careful student of science writ large.

But let us deal with this odd word "phogiston".

Long ago there was an old belief - yes, belief - that there was a substance in certain things that made them flammable - they called it "phlogiston". It came out as the thing burned, and filled the air. (Oh yes. If you put a candle in a jar(they said) the air got so full of phlogiston the candle would go out!) Once the phlogiston was gone from the thing, as when a wooden stick is burnt to ashes, it could not be burned any more.

By careful study and experiment - by precise weighing of things before and after burning - Lavoisier showed that this was nonsense. There is no phlogiston.

But there is something called "oxygen" in the air. If that is used up, or absent, things cannot burn - which is what happens to candles in jars. (Incidentally, this fact is of great importance to firefighting - computer centers often have fire-suppressing systems which rely on this technique!)

Lavoisier did not "discover" the element oxygen; that is credited to Joseph Priestley and Carl Wilhelm Scheele. But he did give it its name, "the acid-former", mistakenly thinking it was a component of acids. (It isn't.) But chemist John Emsley points out that "What Lavoisier did was, perhaps, more important than claiming priority for the discovery [of oxygen]." (Nature's Building Blocks, 300)

Oh my, yes. Father Jaki's vast study casts out that "perhaps". Indeed, Lavoisier's work is of critical importance to both history and to science, so I shall quote at length:

It was Becher's terra pinguis that was renamed phlogiston by Stahl in 1703, or motion of heat and fire, which according to Stahl formed the metals when mixed with calx.

Farfetched as this generalization [about the nature of matter - the "elements"] was, it presented before very long a clear-cut challenge to quantitative verification, which in turn led to a revolution in chemistry, a revolution that also meant a parting with views claiming too much authority in the scientific investigation of matter. The lion's share of the credit for ushering in this revolution and a better understanding of the elements should no doubt go to Lavoisier. He towered above his fellow chemists not only in seeing more deeply in the jumble of accumulated data but also in having a keener appreciation of the extent of what still might remain unknown in man's understanding of matter. Lack of caution was indeed the shortcoming for which he took his predecessors to task. to task. Such criticism was valid not only of the predecessors and followers of Stahl but also of those who, like Peter Shaw, John Friend, Boerhaave, and others, were prompted to dogmatic statements by their hopes that the chemistry of their day could be made an exact science along the lines of Newtonian physics. To both groups applied the remark that Lavoisier made in the preface to his Elements of Chemistry (1789): "All these chemists were carried along by the influence of the genius of the age in which they lived, which contented itself with assertion without proofs; or, at least, often admitted as proofs the slightest degrees of probability, unsupported by that strictly rigorous analysis required by modern philosophy." The scope of these harsh words was, however, not so much a self-righteous indictment of the failure of his forbears in chemistry, as a warning of the debilitating influence that the state of mind of individuals or the "genius" of an age might have on scientific research. To advance science therefore was to break with inherited ways of thought, a break with blatantly careless reasonings, "scientific" prejudices, and self-flattery, or, in short, to initiate a revolution. To this he referred as early as 1773 in his laboratory notebook, where he described his program as one that "seemed destined to bring about a revolution in physics and chemistry."

Still, the prospect of revolutionizing a fundamental branch of science did not go to his head. He spoke of the safeguards with which he intended to repeat experiments to establish the real import of hundreds of experiments performed before him, and he never lost sight of the most important of his goals, which he stated in 1777 as follows: "It is time to bring chemistry to a more rigorous way of reasoning." This rigor he achieved in a measure far surpassing any of the chemists before him. But the price of rigor was a cautious, noncommittal attitude to be taken at junctures where almost anyone else would have been carried away into making "definitive" statements. The temptation of doing so must have been high in view of the exciting vista that opened before him once he recognized the role of the oxygenic principle and turned his back on phlogiston. As he put it in the same Mémoire: "Once this principle is admitted, the chief difficulties of chemistry seem to dissipate themselves and to vanish, and all the phenomena may be explained in astonishing simplicity."

Yet, when chemistry came to be laid on firm ground for the first time in his Elements of Chemistry, Lavoisier's tone could not have been more soberingly objective. It might have reminded the reader of Newton, correlating the most disparate phenomena through a single mathematical relation, without committing himself ever so slightly to the nature or cause of gravitation. Similar was the manner in which Lavoisier presented his views on the elements. About their nature and number he wrote that it "can be speculated upon in a thousand different ways, not one of which, in all probability, is consistent with nature." Consequently he contented himself with saying that his definition of an element was a provisional one and depended on the actually available chemical means of decomposing substances. Anything that could not be further reduced was therefore for the time being to be considered an element; or to quote him, an element "is the last point that analysis is capable of reaching." None of the thirty-three elements he listed did he want to endow with an aura of absolute finality, although twenty-three of them took their places in Mendeleev's table. Although he listed the caloric as an element, he added that one is "not obliged to suppose this to be a real substance." In the same vein he explicitly indicated that what he called the "earths" might soon cease to be considered simple bodies. This was a conjecture, however, and Lavoisier felt a duty to advise his reader: "I trust the reader will take care not to confound what I have related as truths, fixed on the firm basis of observation and experiment, with mere hypothetical conjectures."

To emphasize the wide difference between conjectures and experimental evidence was not to be construed as an intent to depreciate theory. Dangerous as the "spirit of systems" proved for science, no less to be feared, according to Lavoisier, was the inordinate accumulation of facts. Long and painstaking efforts deserved, in his view, more than being left in disorder and confusion. Theory, Lavoisier argued, had to have rather a liberating effect on scientific investigation: it had to show the road to clarification without curtailing the freedom of the investigator to follow a new lead, as fresh data came to light. This was a timely reflection, for the process of conquering unknown areas began to accelerate more rapidly than ever in the study of matter. "Chemistry advances towards perfection," wrote Lavoisier, "by dividing and subdividing," and of this process he found it impossible to say "where it is to end." But he too had his moments of weakness as a scientific prognosticator. Contrasting the chasms of the chemistry of yesterday with the vision of a great synthesis suddenly looming ahead, he could not resist the lure of sanguine expectations: "We have ground to hope, even in our own times, to see it [chemistry] approach near to the highest state of perfection of which it is susceptible." His days, however, were not the ones destined to see the completion of chemistry, even if the Revolution had not extinguished in a second the brilliance of a genius, which a hundred years won't suffice, as Lagrange remarked, to reproduce. The extent of the secrets of matter was not to be measured in the small units of complacent hopes.

It was the precision of Lavoisier's balance that led to the abandonment of the concept of phlogiston and made possible the reorganization of the study of matter on a basis that was designed to emulate the clarity of the Newtonian system. As the younger Herschel put it, the mistakes and confusion of Stahlian chemistry "dissipated like a morning mist as soon as precision came to be regarded as essential." Phlogiston theory was only one of the various non-mechanical theories that came to be abandoned during the eighteenth and nineteenth centuries, chiefly under the impact of increased precision in measurement.
[Jaki, The Relevance of Physics 150-3, 249

Perhaps you find this interesting - perhaps even stimulating. Good. I have some news for you, but first let me set the stage:

All copies of the few editions and translations of Lavoisier's Traité élémentaire de chimie (1789) are on the rare books list.
[Jaki, The Only Chaos (1990)]

I am happy to inform you that, like phlogiston, Jaki's statement has been superseded! The wonderful publishers known as Dover (which also reprint a number of GKC's books) have reprinted a translation, and you can now acquire your own copy of Lavoisier's Elements of Chemistry.

See here for more.