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Heroes of Science: Chemists. M. M. Pattison Muir
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isbn 4064066188115
Автор произведения M. M. Pattison Muir
Жанр Документальная литература
Издательство Bookwire
John Mayow (1645–1679), a physician of Oxford, experimented on the basis of facts established by Hooke. He showed that when a substance, e.g. a candle, burns in air, the volume of air is thereby lessened. To that portion of the air which had dissolved the burned substance he gave the name of nitre-air, and he argued that this air exists in condensed form in nitre, because sulphur burns when heated with nitre in absence of common air. Mayow added the most important fact—a fact which was forgotten by many later experimenters—that the solid substance obtained by burning a metal in air weighs more than the metal itself did before burning. He explained this increase in weight by saying that the burning metal absorbs particles of "nitre-air" from the atmosphere. Thus Hooke and Mayow had really established the fact that common air consists of more than one definite kind of matter—in other words, that common air is not an element; but until recent times the term "element" or "elementary principle" was used without any definite meaning. When we say that the ancients and the alchemists recognized four elements—earth, air, fire, and water—we do not attach to the word "element" the same definite meaning as when we now say, "Iron is an element."
From earth, air, fire and water other substances were obtained; or it might be possible to resolve other substances into one or more of these four. But even to such a word as "substance" or "matter" no very definite meaning could be attached. Although, therefore, the facts set forth by Hooke and Mayow might now justify the assertion that air is not an element, they did not, in the year 1670, necessarily convey this meaning to men's minds. The distinction between element and compound was much more clearly laid down by the Hon. Robert Boyle (1627–1691), whose chemical work was wonderfully accurate and thorough, and whose writings are characterized by acute scientific reasoning. We shall again return to these terms "element" and "compound."
But the visible and striking phenomenon in most processes of burning is the production of light and sometimes of flame. The importance of the fact that the burned substance (when a solid) weighs more than the unburned substance was overshadowed by the apparent importance of the outward part of the process, which could scarcely be passed over by any observer. There appears to be an outrush of something from the burning substance. There is an outrush of something, said Becher and Stahl, and this something is the "principle of fire." The principle of fire, they said, is of a very subtle nature; its particles, which are always in very rapid motion, can penetrate any substance, however dense. When metals burn—the argument continued—they lose this principle of fire; when the burned metal—or calx as it was usually called—is heated with charcoal it regains this "principle," and so the metal is re-formed from the calx.
Thus arose the famous theory of phlogiston (from Greek, = "burned"), which served as a central nucleus round which all chemical facts were grouped for nearly a hundred years.
John Joachim Becher was born at Speyer in 1635, and died in 1682; in his chemical works, the most important of which is the "Physica Subterranea," he retained the alchemical notion that the metals are composed of three "principles"—the nitrifiable, the combustible, and the mercurial—and taught that during calcination the combustible and mercurial principles are expelled, while the nitrifiable remains in the calx.
George Ernest Stahl—born at Anspach in 1660, and died at Berlin in 1734—had regard chiefly to the principles which escape during the calcination of metals, and simplifying, and at the same rendering more definite the idea of Becher, he conceived and enunciated the theory of phlogiston.
But if something (name it "phlogiston" or call it by any other name you please) is lost by a metal when the metal is burned, how is it that the loss of this thing is attended with an increase in the weight of the matter which loses it? Either the theory of phlogiston must be abandoned, or the properties of the thing called phlogiston must be very different from those of any known kind of matter.
Stahl replied, phlogiston is a "principle of levity;" the presence of phlogiston in a substance causes that substance to weigh less than it did before it received this phlogiston.
In criticizing this strange statement, we must remember that in the middle of the seventeenth century philosophers in general were not firmly convinced of the truth that the essential character of matter is that it possesses weight, nor of the truth that it is impossible to destroy or to create any quantity of matter however small. It was not until the experimental work of Lavoisier became generally known that chemists were convinced of these truths. Nevertheless, the opponents of the Stahlian doctrine were justified in asking for further explanations—in demanding that some other facts analogous to this supposed fact, viz. that a substance can weigh less than nothing, should be experimentally established.
The phlogistic theory however maintained its ground; we shall find that it had a distinct element of truth in it, but we shall also find that it did harm to scientific advance. This theory was a wide and sweeping generalization from a few facts; it certainly gave a central idea around which some facts might be grouped, and it was not very difficult, by slightly cutting down here and slightly adding there, to bring many new discoveries within the general theory.
We now know that in order to explain the process of combustion much more accurate knowledge was required than the chemists of the seventeenth century possessed; but we ought to be thankful to these chemists, and notably to Stahl, that they did not hesitate to found a generalization on the knowledge they had. Almost everything propounded in natural science has been modified as man's knowledge of nature has become wider and more accurate; but it is because the scientific student of nature uses the generalizations of to-day as stepping-stones to the better theories of to-morrow, that science grows "from more to more."
Looking at the state of chemistry about the middle of the eighteenth century, we find that the experiments, and especially the measurements, of Hooke and Mayow had laid a firm basis of fact concerning the process of combustion, but that the phlogistic theory, which appeared to contradict these facts, was supreme; that the existence of airs, or gases, different from common air was established, but that the properties of these airs were very slightly and very inaccurately known; that Boyle had distinguished element from compound and had given definite meanings to these terms, but that nevertheless the older and vaguer expression, "elementary principle," was generally used; and lastly, that very few measurements of the masses of the different kinds of matter taking part in chemical changes had yet been made.
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