The Story of Alchemy and the Beginnings of Chemistry Part 9

The apparatus used by Priestley, in his experiments on different kinds of air, is represented in Fig. XVI., which is reduced from an ill.u.s.tration in Priestley's book on _Airs_.

Priestley had made a discovery which was destined to change Alchemy into Chemistry. But he did not know what his discovery meant. It was reserved for the greatest of all chemists, Antoine Lavoisier, to use the fact stumbled on by Priestley.

After some months Priestley began to think it possible that the new "air" he had obtained from calcined mercury might be fit for respiration. To his surprise he found that a mouse lived in this air much longer than in common air; the new air was evidently better, or purer, than ordinary air. Priestley measured what he called the "goodness" of the new air, by a process of his own devising, and concluded that it was "between four and five times as good as common air."

Priestley was a thorough-going phlogistean. He seems to have been able to describe the results of his experiments only in the language of the phlogistic theory; just as the results of most of the experiments made to-day on the changes of compounds of the element carbon cannot be described by chemists except by making use of the conceptions and the language of the atomic and molecular theory.[6]

[6] I have given numerous ill.u.s.trations of the truth of this statement in the book, in this series, ent.i.tled _The Story of the Wanderings of Atoms_.

The upholder of the phlogistic theory could not think of burning as possible unless there was a suitable receptacle for the phlogiston of the burning substance: when burning occurred in the air, the part played by the air, according to the phlogistic chemist, was to receive the expelled phlogiston; in this sense the air acted as the _pabulum_, or nourishment, of the burning substance. Inasmuch as substances burned more vigorously and brilliantly in the new air than in common air, Priestley argued that the new air was more ready, more eager, than ordinary air, to receive phlogiston; and, therefore, that the new air contained less phlogiston than ordinary air, or, perhaps, no phlogiston. Arguing thus, Priestley, of course, named the new aeriform substance _dephlogisticated air_, and thought of it as ordinary air deprived of some, or it might be all, of its phlogiston.

The breathing of animals and the burning of substances were supposed to load the atmosphere with phlogiston. Priestley spoke of the atmosphere as being constantly "vitiated," "rendered noxious,"

"depraved," or "corrupted" by processes of respiration and combustion; he called those processes whereby the atmosphere is restored to its original condition (or "depurated," as he said), "dephlogisticating processes." As he had obtained his _dephlogisticated air_ by heating the calx of mercury, that is the powder produced by calcining mercury in the air, Priestley was forced to suppose that the calcination of mercury in the air must be a more complex occurrence than merely the expulsion of phlogiston from the mercury: for, if the process consisted only in the expulsion of phlogiston, how could heating what remained produce exceedingly pure ordinary air? It seemed necessary to suppose that not only was phlogiston expelled from mercury during calcination, but that the mercury also imbibed some portion, and that the purest portion, of the surrounding air. Priestley did not, however, go so far as this; he was content to suppose that in some way, which he did not explain, the process of calcination resulted in the loss of phlogiston by the mercury, and the gain, by the dephlogisticated mercury, of the property of yielding exceedingly pure or dephlogisticated air when it was heated very strongly.

Priestley thought of properties in much the same way as the alchemists thought of them, as wrappings, or coverings of an essential something, from which they can be removed and around which they can again be placed. The protean principle of phlogiston was always at hand, and, by skilful management, was ready to adapt itself to any facts. Before the phenomena of combustion could be described accurately, it was necessary to do two things; to ignore the theory of phlogiston, and to weigh and measure all the substances which take part in some selected processes of burning.

Looking back at the attempts made in the past to describe natural events, we are often inclined to exclaim, "Why did investigators bind themselves with the cords of absurd theories; why did they always wear blinkers; why did they look at nature through the distorting mists rising from their own imaginations?" We are too ready to forget the tremendous difficulties which beset the path of him who is seeking accurate knowledge.

"To climb steep hills requires slow pace at first."

Forgetting that the statements wherein the men of science of our own time describe the relations between natural events are, and must be, expressed in terms of some general conception, some theory, of these relations; forgetting that the simplest natural occurrence is so complicated that our powers of description are incapable of expressing it completely and accurately; forgetting the uselessness of disconnected facts; we are inclined to overestimate the importance of our own views of nature's ways, and to underestimate the usefulness of the views of our predecessors. Moreover, as naturalists have not been obliged, in recent times, to make a complete renunciation of any comprehensive theory wherein they had lived and moved for many years, we forget the difficulties of breaking loose from a way of looking at natural events which has become almost as real as the events themselves, of abandoning a language which has expressed the most vividly realised conceptions of generations of investigators, of forming a completely new mental picture of natural occurrences, and developing a completely new language for the expression of those conceptions and these occurrences.

The younger students of natural science of to-day are beginning to forget what their fathers told them of the fierce battle which had to be fought, before the upholders of the Darwinian theory of the origin of species were able to convince those for whom the older view, that species are, and always have been, absolutely distinct, had become a matter of supreme scientific, and even ethical, importance.

A theory which has prevailed for generations in natural science, and has been accepted and used by everyone, can be replaced by a more accurate description of the relations between natural facts, only by the determination, labour, and genius of a man of supreme power. Such a service to science, and humanity, was rendered by Darwin; a like service was done, more than three-quarters of a century before Darwin, by Lavoisier.

Antoine Laurent Lavoisier was born in Paris in 1743. His father, who was a merchant in a good position, gave his son the best education which was then possible, in physical, astronomical, botanical, and chemical science. At the age of twenty-one, Lavoisier gained the prize offered by the Government for devising an effective and economical method of lighting the public streets. From that time until, on the 8th of May 1794, the Government of the Revolution declared, "The Republic has no need of men of science," and the guillotine ended his life, Lavoisier continued his researches in chemistry, geology, physics, and other branches of natural science, and his investigations into the most suitable methods of using the knowledge gained by naturalists for advancing the welfare of the community.

In Chapter VI., I said that when an alchemist boiled water in an open vessel, and obtained a white earthy solid, in place of the water which disappeared, he was producing some sort of experimental proof of the justness of his a.s.sertion that water can be changed into earth.

Lavoisier began his work on the transformations of matter by demonstrating that this alleged trans.m.u.tation does not happen; and he did this by weighing the water, the vessel, and the earthy solid.

Lavoisier had constructed for him a pelican of white gla.s.s (see Fig.

XI., p. 88), with a stopper of gla.s.s. He cleaned, dried, and weighed this vessel; then he put into it rain-water which he had distilled eight times; he heated the vessel, removing the stopper from time to time to allow the expanding air to escape, then put in the stopper, allowed the vessel to cool, and weighed very carefully. The difference between the second and the first weighing was the weight of water in the vessel. He then fastened the stopper securely with cement, and kept the apparatus at a temperature about 30 or 40 below that of boiling water, for a hundred and one days. At the end of that time a fine white solid had collected on the bottom of the vessel. Lavoisier removed the cement from the stopper, and weighed the apparatus; the weight was the same as it had been before the heating began. He removed the stopper; air rushed in, with a hissing noise. Lavoisier concluded that air had not penetrated through the apparatus during the process of heating. He then poured out the water, and the solid which had formed in the vessel, set them aside, dried, and weighed the pelican; it had lost 17-4/10 grains. Lavoisier concluded that the solid which had formed in the water was produced by the solvent action of the water on the gla.s.s vessel. He argued that if this conclusion was correct, the weight of the solid must be equal to the loss of weight suffered by the vessel; he therefore separated the solid from the water in which it was suspended, dried, and weighed it. The solid weighed 4-9/10 grains. Lavoisier's conclusion seemed to be incorrect; the weight of the solid, which was supposed to be produced by the action of the water on the vessel, was 12-1/2 grains less than the weight of the material removed from the vessel. But some of the material which was removed from the vessel might have remained dissolved in the water: Lavoisier distilled the water, which he had separated from the solid, in a gla.s.s vessel, until only a very little remained in the distilling apparatus; he poured this small quant.i.ty into a gla.s.s basin, and boiled until the whole of the water had disappeared as steam. There remained a white, earthy solid, the weight of which was 15-1/2 grains. Lavoisier had obtained 4-9/10 + 15-1/2 = 20-2/5 grains of solid; the pelican had lost 17-2/5 grains. The difference between these weights, namely, 3 grains, was accounted for by Lavoisier as due to the solvent action of the water on the gla.s.s apparatus wherein it had been distilled, and on the gla.s.s basin wherein it had been evaporated to dryness.

Lavoisier's experiments proved that when distilled water is heated in a gla.s.s vessel, it dissolves some of the material of the vessel, and the white, earthy solid which is obtained by boiling down the water is merely the material which has been removed from the gla.s.s vessel. His experiments also proved that the water does not undergo any change during the process; that at the end of the operation it is what it was at the beginning--water, and nothing but water.

By this investigation Lavoisier destroyed part of the experimental basis of alchemy, and established the one and only method by which chemical changes can be investigated; the method wherein constant use is made of the balance.

Lavoisier now turned his attention to the calcination of metals, and particularly the calcination of tin. Boyle supposed that the increase in weight which accompanies the calcination of a metal is due to the fixation of "matter of fire" by the calcining metal; Rey regarded the increase in weight as the result of the combination of the air with the metal; Mayow thought that the atmosphere contains two different kinds of "airs," and one of these unites with the heated metal.

Lavoisier proposed to test these suppositions by calcining a weighed quant.i.ty of tin in a closed gla.s.s vessel, which had been weighed before, and should be weighed after, the calcination. If Boyle's view was correct, the weight of the vessel and the tin would be greater at the end than it was at the beginning of the operation; for "matter of fire" would pa.s.s through the vessel and unite with the metal. If there was no change in the total weight of the apparatus and its contents, and if air rushed in when the vessel was opened after the calcination, and the total weight was then greater than at the beginning of the process, it would be necessary to adopt either the supposition of Rey or that of Mayow.

Lavoisier made a series of experiments. The results were these: there was no change in the total weight of the apparatus and its contents; when the vessel was opened after the calcination was finished, air rushed in, and the whole apparatus now weighed more than it did before the vessel was opened; the weight of the air which rushed in was exactly equal to the increase in the weight of the tin produced by the calcination, in other words, the weight of the inrushing air was exactly equal to the difference between the weights of the tin and the calx formed by calcining the tin. Lavoisier concluded that to calcine tin is to cause it to combine with a portion of the air wherein it is calcined. The weighings he made showed that about one-fifth of the whole weight of air in the closed flask wherein he calcined tin had disappeared during the operation.

Other experiments led Lavoisier to suspect that the portion of the air which had united with the tin was different from the portion which had not combined with that metal. He, therefore, set himself to discover whether there are different kinds of "airs" in the atmosphere, and, if there is more than one kind of "air," what is the nature of that "air"

which combines with a metal in the process of calcination. He proposed to cause a metallic calx (that is, the substance formed by calcining a metal in the air) to give up the "air" which had been absorbed in its formation, and to compare this "air" with atmospheric air.

About this time Priestley visited Paris, saw Lavoisier, and told him of the new "air" he had obtained by heating calcined mercury.

Lavoisier saw the great importance of Priestley's discovery; he repeated Priestley's experiment, and concluded that the air, or gas, which he refers to in his Laboratory Journal as "l'air dephlogistique de M. Priestley" was nothing else than the purest portion of the air we breathe. He prepared this "air" and burned various substances in it. Finding that very many of the products of these combustions had the properties of acids, he gave to the new "air" the name _oxygen_, which means _the acid-producer_.

At a later time, Lavoisier devised and conducted an experiment which laid bare the change of composition that happens when mercury is calcined in the air. He calcined a weighed quant.i.ty of mercury for many days in a measured volume of air, in an apparatus arranged so that he was able to determine how much of the air disappeared during the process; he collected and weighed the red solid which formed on the surface of the heated mercury; finally he heated this red solid to a high temperature, collected and measured the gas which was given off, and weighed the mercury which was produced. The sum of the weights of the mercury and the gas which were produced by heating the calcined mercury was equal to the weight of the calcined mercury; and the weight of the gas produced by heating the calcined mercury was equal to the weight of the portion of the air which had disappeared during the formation of the calcined mercury. This experiment proved that the calcination of mercury in the air consists in the combination of a const.i.tuent of the air with the mercury. Fig. XVII. (reduced from an ill.u.s.tration in Lavoisier's Memoir) represents the apparatus used by Lavoisier. Mayow's supposition was confirmed.

[Ill.u.s.tration: FIG. XVII.]

Lavoisier made many more experiments on combustion, and proved that in every case the component of the atmosphere which he had named oxygen combined with the substance, or with some part of the substance, which was burned. By these experiments the theory of Phlogiston was destroyed; and with its destruction, the whole alchemical apparatus of Principles and Elements, Essences and Qualities, Souls and Spirits, disappeared.

CHAPTER XII.

THE RECOGNITION OF CHEMICAL CHANGES AS THE INTERACTIONS OF DEFINITE SUBSTANCES.

The experimental study of combustion made by Lavoisier proved the correctness of that part of Stahl's phlogistic theory which a.s.serted that all processes of combustion are very similar, but also proved that this likeness consists in the combination of a distinct gaseous substance with the material undergoing combustion, and not in the escape therefrom of the _Principle of fire_, as a.s.serted by the theory of Stahl. After about the year 1790, it was necessary to think of combustions in the air as combinations of a particular gas, or _air_, with the burning substances, or some portions of them.

This description of processes of burning necessarily led to a comparison of the gaseous const.i.tuent of the atmosphere which played so important a part in these processes, with the substances which were burned; it led to the examination of the compositions of many substances, and made it necessary to devise a language whereby these compositions could be stated clearly and consistently.

We have seen, in former chapters, the extreme haziness of the alchemical views of composition, and the connexions between composition and properties. Although Boyle[7] had stated very lucidly what he meant by the composition of a definite substance, about a century before Lavoisier's work on combustion, nevertheless the views of chemists concerning composition remained very vague and incapable of definite expression, until the experimental investigations of Lavoisier enabled him to form a clear mental picture of chemical changes as interactions between definite quant.i.ties of distinct substances.

[7] Boyle said, in 1689, "I mean by elements ... certain primitive and simple, or perfectly unmixed bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved."

Let us consider some of the work of Lavoisier in this direction. I select his experimental examination of the interactions of metals and acids.

Many experimenters had noticed that gases (or airs, as they were called up till near the end of the 18th century) are generally produced when metals are dissolving in acids. Most of those who noticed this said that the gases came from the dissolving metals; Lavoisier said they were produced by the decomposition of the acids.

In order to study the interaction of nitric acid and mercury, Lavoisier caused a weighed quant.i.ty of the metal to react with a weighed quant.i.ty of the acid, and collected the gas which was produced; when all the metal had dissolved, he evaporated the liquid until a white solid was obtained; he heated this solid until it was changed to the red substance called, at that time, _red precipitate_, and collected the gas produced. Finally, Lavoisier strongly heated the red precipitate; it changed to a gas, which he collected, and mercury, which he weighed.

The weight of the mercury obtained by Lavoisier at the end of this series of changes was the same, less a few grains, as the weight of the mercury which he had caused to react with the nitric acid. The gas obtained during the solution of the metal in the acid, and during the decomposition of the white solid by heat, was the same as a gas which had been prepared by Priestley and called by him _nitrous air_; and the gas obtained by heating the red precipitate was found to be oxygen. Lavoisier then mixed measured volumes of oxygen and "nitrous air," standing over water; a red gas was formed, and dissolved in the water, and Lavoisier proved that the water now contained nitric acid.

The conclusions regarding the composition of nitric acid drawn by Lavoisier from these experiments was, that "nitric acid is nothing else than _nitrous air_, combined with almost its own volume of the purest part of atmospheric air, and a considerable quant.i.ty of water."

Lavoisier supposed that the stages in the complete reaction between mercury and nitric acid were these: the withdrawal of oxygen from the acid by the mercury, and the union of the compound of mercury and oxygen thus formed with the const.i.tuents of the acid which remained when part of its oxygen was taken away. The removal of oxygen from nitric acid by the mercury produced _nitrous air_; when the product of the union of the oxide of mercury and the nitric acid deprived of part of its oxygen was heated, more nitrous air was given off, and oxide of mercury remained, and was decomposed, at a higher temperature, into mercury and oxygen.

Lavoisier thought of these reactions as the tearing asunder, by mercury, of nitric acid into definite quant.i.ties of its three components, themselves distinct substances, nitrous air, water, and oxygen; and the combination of the mercury with a certain measurable quant.i.ty of one of these components, namely, oxygen, followed by the union of this compound of mercury and oxygen with what remained of the components of nitric acid.

Lavoisier had formed a clear, consistent, and suggestive mental picture of chemical changes. He thought of a chemical reaction as always the same under the same conditions, as an action between a fixed and measurable quant.i.ty of one substance, having definite and definable properties, with fixed and measurable quant.i.ties of other substances, the properties of each of which were definite and definable.

Lavoisier also recognised that certain definite substances could be divided into things simpler than themselves, but that other substances refused to undergo simplification by division into two or more unlike portions. He spoke of the object of chemistry as follows:--[8] "In submitting to experiments the different substances found in nature, chemistry seeks to decompose these substances, and to get them into such conditions that their various components may be examined separately. Chemistry advances to its end by dividing, sub-dividing, and again sub-dividing, and we do not know what will be the limits of such operations. We cannot be certain that what we regard as simple to-day is indeed simple; all we can say is, that such a substance is the actual term whereat chemical a.n.a.lysis has arrived, and that with our present knowledge we cannot sub-divide it."

[8] I have given a free rendering of Lavoisier's words.

In these words Lavoisier defines the chemical conception of _elements_; since his time an element is "the actual term whereat chemical a.n.a.lysis has arrived," it is that which "with our present knowledge we cannot sub-divide"; and, as a working hypothesis, the notion of _element_ has no wider meaning than this. I have already quoted Boyle's statement that by _elements_ he meant "certain primitive and simple bodies ... not made of any other bodies, or of one another." Boyle was still slightly restrained by the alchemical atmosphere around him; he was still inclined to say, "this _must_ be the way nature works, she _must_ begin with certain substances which are absolutely simple." Lavoisier had thrown off all the trammels which hindered the alchemists from making rigorous experimental investigations. If one may judge from his writings, he had not struggled to free himself from these trammels, he had not slowly emerged from the quagmires of alchemy, and painfully gained firmer ground; the extraordinary clearness and directness of his mental vision had led him straight to the very heart of the problems of chemistry, and enabled him not only calmly to ignore all the machinery of Elements, Principles, Essences, and the like, which the alchemists had constructed so laboriously, but also to construct, in place of that mechanism which hindered inquiry, genuine scientific hypotheses which directed inquiry, and were themselves altered by the results of the experiments they had suggested.

Lavoisier made these great advances by applying himself to the minute and exhaustive examination of a few cases of chemical change, and endeavouring to account for everything which took part in the processes he studied, by weighing or measuring each distinct substance which was present when the change began, and each which was present when the change was finished. He did not make haphazard experiments; he had a method, a system; he used hypotheses, and he used them rightly. "Systems in physics," Lavoisier writes, "are but the proper instruments for helping the feebleness of our senses. Properly speaking, they are methods of approximation which put us on the track of solving problems; they are the hypotheses which, successively modified, corrected, and changed, by experience, ought to conduct us, some day, by the method of exclusions and eliminations, to the knowledge of the true laws of nature."

In a memoir wherein he is considering the production of carbonic acid and alcohol by the fermentation of fruit-juice, Lavoisier says, "It is evident that we must know the nature and composition of the substances which can be fermented and the products of fermentation; for nothing is created, either in the operations of art or in those of nature; and it may be laid down that the quant.i.ty of material present at the beginning of every operation is the same as the quant.i.ty present at the end, that the quality and quant.i.ty of the principles[9]

are the same, and that nothing happens save certain changes, certain modifications. On this principle is based the whole art of experimenting in chemistry; in all chemical experiments we must suppose that there is a true equality between the principles[10] of the substances which are examined and those which are obtained from them by a.n.a.lysis."

[9, 10] Lavoisier uses the word _principle_, here and elsewhere, to mean a definite h.o.m.ogeneous substance; he uses it as synonymous with the more modern terms element and compound.