The Story of Alchemy and the Beginnings of Chemistry Part 8

CHAPTER X.

SUMMARY OF THE ALCHEMICAL DOCTRINE.--THE REPLACEMENT OF THE THREE PRINCIPLES OF THE ALCHEMISTS BY THE SINGLE PRINCIPLE OF PHLOGISTON.

The _Sacred Art_, which had its origin and home in Egypt, was very definitely a.s.sociated with the religious rites, and the theological teaching, recognised by the state. The Egyptian priests were initiated into the mysteries of the divine art: and as the initiated claimed to imitate the work of the deity, the priest was regarded by the ordinary people as something more than a representative, as a mirror, of the divinity. The sacred art of Egypt was trans.m.u.ted into alchemy by contact with European thought and handicrafts, and the tenets and mysticism of the Catholic Church; and the conception of nature, which was the result of this blending, prevailed from about the 9th until towards the end of the 18th century.

Like its predecessor, alchemy postulated an orderly universe; but alchemy was richer in fantastic details, more picturesquely embroidered, more prodigal of strange fancies, than the sacred art of Egypt.

The alchemist constructed his ordered scheme of nature on the basis of the supposed universality of life. For him, everything lived, and the life of things was threefold. The alchemist thought he recognised the manifestation of life in the form, or body, of a thing, in its soul, and in its spirit. Things might differ much in appearance, in size, taste, smell, and other outward properties, and yet be intimately related, because, according to the alchemist, they were produced from the same principles, they were animated by the same soul. Things might resemble one another closely in their outward properties and yet differ widely in essential features, because, according to the alchemist, they were formed from different elements, in their spiritual properties they were unlike. The alchemists taught that the true transformation, in alchemical language the trans.m.u.tation, of one thing into another could be effected only by spiritual means acting on the spirit of the thing, because the trans.m.u.tation consisted essentially in raising the substance to the highest perfection whereof it was capable; the result of this spiritual action might become apparent in the material form of the substance. In attempting to apply such vague conceptions as these, alchemy was obliged to use the language which had been developed for the expression of human emotions and desires, not only for the explanation of the facts it observed, but also for the bare recital of these facts.

The outlook of alchemy on the world outside human beings was essentially anthropomorphic. In the image of man, the alchemist created his universe.

In the times when alchemy was dominant, the divine scheme of creation, and the place given to man in that scheme, were supposed to be thoroughly understood. Everything had its place, designed for it from the beginning, and in that place it remained unless it were forced from it by violent means. A great part of the business of experimental alchemy was to discover the natural position, or condition, of each substance; and the discovery was to be made by interpreting the facts brought to light by observation and experiment by the aid of hypotheses deduced from the general scheme of things which had been formed independently of observation or experiment. Alchemy was a part of magic; for magic interprets and corrects the knowledge gained by the senses by the touchstone of generalisations which have been supplied, partly by the emotions, and partly by extra-human authority, and accepted as necessarily true.

The conception of natural order which regulates the life of the savage is closely related to that which guided the alchemists. The essential features of both are the notion that everything is alive, and the persuasion that things can be radically acted on only by using life as a factor. There is also an intimate connexion between alchemy and witchcraft. Witches were people who were supposed to make an unlawful use of the powers of life; alchemists were often thought to pa.s.s beyond what is permitted to the creature, and to encroach on the prerogative of the Creator.

The long duration of alchemy shows that it appealed to some deep-seated want of human beings. Was not that want the necessity for the realisation of order in the universe? Men were unwilling to wait until patient examination of the facts of their own nature, and the facts of nature outside themselves, might lead them to the realisation of the interdependence of all things. They found it easier to evolve a scheme of things from a superficial glance at themselves and their surroundings; naturally they adopted the easier plan. Alchemy was a part of the plan of nature produced by this method. The extraordinary dominancy of such a scheme is testified to by the continued belief in alchemy, although the one experiment, which seems to us to be the crucial experiment of the system, was never accomplished. But it is also to be remembered that the alchemists were acquainted with, and practised, many processes which we should now describe as operations of manufacturing and technical chemistry; and the practical usefulness of these processes bore testimony, of the kind which convinces the plain man, to the justness of their theories.

I have always regarded two facts as most interesting and instructive: that the doctrine of the essential unity of all things, and the simplicity of natural order, was accepted for centuries by many, I think one may say, by most men, as undoubtedly a true presentation of the divine scheme of things; and, secondly, that in more recent times people were quite as certain of the necessary truth of the doctrine, the exact opposite of the alchemical, that the Creator had divided his creation into portions each of which was independent of all the others. Both of these schemes were formed by the same method, by introspection preceding observation; both were overthrown by the same method, by observation and experiment proceeding hand in hand with reasoning. In each case, the humility of science vanquished the conceit of ignorance.

The change from alchemy to chemistry is an admirable example of the change from a theory formed by looking inwards, and then projected on to external facts, to a theory formed by studying facts, and then thinking about them. This change proceeded slowly; it is not possible to name a time when it may be said, here alchemy finishes and chemistry begins. To adapt a saying of one of the alchemists, quoted in a former chapter; alchemy would not easily give up its nature, and fought for its life; but an agent was found strong enough to overcome and kill it, and then that agent also had the power to change the lifeless remains into a new and pure body. The agent was the accurate and imaginative investigation of facts.

The first great step taken in the path which led from alchemy to chemistry was the subst.i.tution of one Principle, the Principle of Phlogiston, for the three Principles of salt, sulphur, and mercury.

This step was taken by concentrating attention and investigation, by replacing the superficial examination of many diverse phenomena by the more searching study of one cla.s.s of occurrences. That the field of study should be widened, it was necessary that it should first be narrowed.

Lead, tin, iron, or copper is calcined. The prominent and striking feature of these events is the disappearance of the metal, and the formation of something very unlike it. But the original metal is restored by a second process, which is like the first because it also is a calcination, but seems to differ from the first operation in that the burnt metal is calcined with another substance, with grains of wheat or powdered charcoal. Led thereto by their theory that destruction must precede re-vivification, death must come before resurrection, the alchemists confined their attention to one feature common to all calcinations of metals, and gave a superficial description of these occurrences by cla.s.sing them together as processes of mortification. Sulphur, wood, wax, oil, and many other things are easily burned: the alchemists said, these things also undergo mortification, they too are killed; but, as "man can restore that which man has destroyed," it must be possible to restore to life the thing which has been mortified. The burnt sulphur, wood, wax, or oil, is not really dead, the alchemists argued; to use the allegory of Paracelsus, they are like young lions which are born dead, and are brought to life by the roaring of their parents: if we make a sufficiently loud noise, if we use the proper means, we shall bring life into what seems to be dead material. As it is the roaring of the parents of the young lions which alone can cause the still-born cubs to live, so it is only by the spiritual agency of life, proceeded the alchemical argument, that life can be brought into the mortified sulphur, wood, wax, and oil.

The alchemical explanation was superficial, theoretical, in the wrong meaning of that word, and unworkable. It was superficial because it overlooked the fact that the primary calcination, the mortification, of the metals, and the other substances, was effected in the air, that is to say, in contact with something different from the thing which was calcined; the explanation was of the kind which people call theoretical, when they wish to condemn an explanation and put it out of court, because it was merely a re-statement of the facts in the language of a theory which had not been deduced from the facts themselves, or from facts like those to be explained, but from what were supposed to be facts without proper investigation, and, if facts, were of a totally different kind from those to which the explanation applied; and lastly, the explanation was unworkable, because it suggested no method whereby its accuracy could be tested, no definite line of investigation which might be pursued.

That great naturalist, the Honourable Robert Boyle (born in 1626, died in 1691), very perseveringly besought those who examined processes of calcination to pay heed to the action of everything which might take part in the processes. He was especially desirous they should consider what part the air might play in calcinations; he spoke of the air as a "menstruum or additament," and said that, in such operations as calcination, "We may well take the freedom to examine ... whether there intervene not a coalition of the parts of the body wrought upon with those of the menstruum, whereby the produced concrete may be judged to result from the union of both."

It was by examining the part played by the air in processes of calcination and burning that men at last became able to give approximately complete descriptions of these processes.

Boyle recognised that the air is not a simple or elementary substance; he spoke of it as "a confused aggregate of effluviums from such differing bodies, that, though they all agree in const.i.tuting by their minuteness and various motions one great ma.s.s of fluid matter, yet perhaps there is scarce a more heterogeneous body in the world."

Clement of Alexandria who lived in the end of the 2nd, and the early part of the 3rd, century A.D., seems to have regarded the air as playing a very important part in combustions; he said--"Airs are divided into two categories; an air for the divine flame, which is the soul; and a material air which is the nourisher of sensible fire, and the basis of combustible matter." Sentences like that I have just quoted are found here and there in the writings of the earlier and later alchemists; now and again we also find statements which may be interpreted, in the light of the fuller knowledge we now have, as indicating at least suspicions that the atmosphere is a mixture of different kinds of air, and that only some of these take part in calcining and burning operations. Those suspicions were confirmed by experiments on the calcination of metals and other substances, conducted in the 17th century by Jean Rey a French physician, and by John Mayow of Oxford. But these observations and the conclusions founded on them, did not bear much fruit until the time of Lavoisier, that is, towards the close of the 18th century. They were overshadowed and put aside by the work of Stahl (1660-1724). Some of the alchemists of the 14th, 15th and 16th centuries taught that combustion and calcination are processes wherein _the igneous principle_ is destroyed, using the word "destroyed" in its alchemical meaning. This description of processes of burning was much more in keeping with the ideas of the time than that given by Boyle, Rey and Mayow. It was adopted by Stahl, and made the basis of a general theory of those changes wherein one substance disappears and another, or others, very unlike it, are produced.

That he might bring into one point of view, and compare the various changes effected by the agency of fire, Stahl invented a new Principle, which he named _Phlogiston_, and constructed an hypothesis which is generally known as the phlogistic theory. He explained, and applied, this hypothesis in various books, especially in one published at Halle in 1717.

Stahl observed that many substances which differed much from one another in various respects were alike in one respect; they were all combustible. All the combustible substances, he argued, must contain a common principle; he named this supposed principle, _phlogiston_ (from the Greek word _phlogistos_ = burnt, or set on fire). Stahl said that the phlogiston of a combustible thing escapes as the substance burns, and, becoming apparent to the senses, is named fire or flame. The phlogiston in a combustible substance was supposed to be so intimately a.s.sociated with something else that our senses cannot perceive it; nevertheless, the theory said, it is there; we can see only the escaping phlogiston, we can perceive only the phlogiston which is set free from its combination with other things. The theory thought of phlogiston as imprisoned in the thing which can be burnt, and as itself forming part of the prison; that the prisoner should be set free, the walls of the prison had to be removed; the freeing of the prisoner destroyed the prison. As escaping, or free, phlogiston was called fire, or flame, so the phlogiston in a combustible substance was sometimes called combined fire, or flame in the state of combination. A peculiarity of the strange thing called phlogiston was that it preferred to be concealed in something, hidden, imprisoned, combined; free phlogiston* was supposed to be always ready to become combined phlogiston.

*Transcriber's Note: Original "phlogstion".

The phlogistic theory said that what remains when a substance has been burnt is the original substance deprived of phlogiston; and, therefore, to restore the phlogiston to the product of burning is to re-form the combustible substance. But how is such a restoration of phlogiston to be accomplished? Evidently by heating the burnt thing with something which is very ready to burn. Because, according to the theory, everything which can be burnt contains phlogiston, the more ready a substance is to burn the richer it is in phlogiston; burning is the outrush of phlogiston, phlogiston prefers to be combined with something; therefore, if you mix what remains after burning, with something which is very combustible, and heat the mixture, you are bringing the burnt matter under conditions which are very favourable for the reception of phlogiston by it, for you are bringing it into intimate contact with something from which freedom-hating phlogiston is being forced to escape.

Charcoal, sulphur, phosphorus, oils and fats are easily burnt; these substances were, therefore, chosen for the purpose of changing things which had been burnt into things which could again be burnt; these, and a few other substances like these, were cla.s.sed together, and called _phlogisticating agents_.

Very many of the substances which were dealt with by the experimenters of the last quarter of the 17th, and the first half of the 18th, century, were either substances which could be burned, or those which had been produced by burning; hence the phlogistic theory brought into one point of view, compared, and emphasised the similarities between, a great many things which had not been thought of as connected before that theory was promulgated. Moreover, the theory a.s.serted that all combustible, or incinerable, things are composed of phlogiston, and another principle, or, as was often said, another element, which is different in different kinds of combustible substances. The metals, for instance, were said to be composed of phlogiston and an earthy principle or element, which was somewhat different in different metals. The phlogisteans taught that the earthy principle of a metal remains in the form of ash, cinders, or calx, when the metal is calcined, or, as they expressed it, when the metal is deprived of its phlogiston.

The phlogistic theory savoured of alchemy; it postulated an undefined, undefinable, intangible Principle; it said that all combustible substances are formed by the union of this Principle with another, which is sometimes of an earthy character, sometimes of a fatty nature, sometimes highly volatile in habit. Nevertheless, the theory of Stahl was a step away from purely alchemical conceptions towards the accurate description of a very important cla.s.s of changes. The principle of phlogiston could be recognised by the senses as it was in the act of escaping from a substance; and the other principle of combustible things was scarcely a Principle in the alchemical sense, for, in the case of metals at any rate, it remained when the things which had contained it were burnt, and could be seen, handled, and weighed. To say that metals are composed of phlogiston and an earthy substance, was to express facts in such a language that the expression might be made the basis of experimental inquiry; it was very different from the a.s.sertion that metals are produced by the spiritual actions of the three Principles, salt, mercury and sulphur, the first of which is not salt, the second is not mercury, and the third is not sulphur.

The followers of Stahl often spoke of metals as composed of phlogiston and an _element_ of an earthy character; this expression also was an advance, from the hazy notion of _Element_ in purely alchemical writings, towards accuracy and fulness of description. An element was now something which could he seen and experimented with; it was no longer a semi-spiritual existence which could not be grasped by the senses.

The phlogistic theory regarded the calcination of a metal as the separation of it into two things, unlike the metal, and unlike each other; one of these things was phlogiston, the other was an earth-like residue. The theory thought of the re-formation of a metal from its calx, that is, the earthy substance which remains after combustion, as the combination of two things to produce one, apparently h.o.m.ogeneous, substance. Metals appeared to the phlogisteans, as they appeared to the alchemists, to be composite substances. Processes of burning were regarded by alchemists and phlogisteans alike, as processes of simplification.

The fact had been noticed and recorded, during the middle ages, that the earth-like matter which remains when a metal is calcined is heavier than the metal itself. From this fact, modern investigators of natural phenomena would draw the conclusion, that calcination of a metal is an addition of something to the metal, not a separation of the metal into different things. It seems impossible to us that a substance should be separated into portions, and one of these parts should weigh as much as, or more than, the whole.

The exact investigation of material changes called chemistry rests on the statement that _ma.s.s_, and ma.s.s is practically measured by _weight_, is the one property of what we call matter, the determination whereof enables us to decide whether a change is a combination, or coalescence, of different things, or a separation of one thing into parts. That any part of a material system can be removed without the weight of the portion which remains being less than the original weight of the whole system, is unthinkable, in the present state of our knowledge of material changes.

But in the 17th century, and throughout most of the 18th, only a few of those who examined changes in the properties of substances paid heed to changes of weight; they had not realised the importance of the property of ma.s.s, as measured by weight. The convinced upholder of the phlogistic theory had two answers to the argument, that, because the earth-like product of the calcination of a metal weighs more than the metal itself, therefore the metal cannot have lost something in the process; for, if one portion of what is taken away weighs more than the metal from which it has been separated, it is evident that the weight of the two portions into which the metal is said to have been divided must be considerably greater than the weight of the undivided metal. The upholders of the theory sometimes met the argument by saying, "Of course the calx weighs more than the metal, because phlogiston tends to lighten a body which contains it; and therefore the body weighs more after it has lost phlogiston than it did when the phlogiston formed part of it;" sometimes, and more often, their answer was--"loss or gain of weight is an accident, the essential thing is change of qualities."

If the argument against the separation of a metal into two const.i.tuents, by calcination, were answered to-day as it was answered by the upholders of the phlogistic theory, in the middle of the 18th century, the answers would justly be considered inconsequent and ridiculous. But it does not follow that the statements were either far-fetched or absurd at the time they were made. They were expressed in the phraseology of the time; a phraseology, it is true, sadly lacking in consistency, clearness, and appropriateness, but the only language then available for the description of such changes as those which happen when metals are calcined. One might suppose that it must always have sounded ridiculous to say that the weight of a thing can be decreased by adding something to it, that part of a thing weighs more than the whole of it. But the absurdity disappears if it can be admitted that ma.s.s, which is measured by weight, may be a property like colour, or taste, or smell; for the colour, taste, or smell of a thing may certainly be made less by adding something else, and the colour, taste, or smell of a thing may also be increased by adding something else. If we did not know that what we call _quant.i.ty of substance_ is measured by the property named _ma.s.s_, we might very well accept the proposition that the entrance of phlogiston into a substance decreases the quant.i.ty, hence the ma.s.s, and, therefore, the weight, of the substance.

Although Stahl and his followers were emerging from the trammels of alchemy, they were still bound by many of the conceptions of that scheme of nature. We have learned, in previous chapters, that the central idea of alchemy was expressed in the saying: "Matter must be deprived of its properties in order to draw out its soul." The properties of substances are everything to the modern chemist--indeed, such words as iron, copper, water, and gold are to him merely convenient expressions for certain definable groups of properties--but the phlogisteans regarded the properties of things, including ma.s.s, as of secondary importance; they were still trying to get beneath the properties of a thing, to its hypothetical essence, or substance.

Looking back, we cannot think of phlogiston as a substance, or as a thing, in the modern meanings of these terms as they are used in natural science. Nowadays we think, we are obliged to think, of the sum of the quant.i.ties of all the things in the universe as unchanging, and unchangeable by any agency whereof we have definite knowledge. The meaning we give to the word _thing_ rests upon the acceptance of this hypothesis. But the terms _substance_, _thing_, _properties_ were used very vaguely a couple of centuries ago; and it would be truly absurd to carry back to that time the meanings which we give to these terms to-day, and then to brand as ridiculous the attempts of the men who studied, then, the same problems which we study now, to express the results of their study in generalisations which employed the terms in question, in what seems to us a loose, vague, and inexact manner.

By a.s.serting, and to some extent experimentally proving, the existence of one principle in many apparently very different substances (or, as would be said to-day, one property common to many substances), the phlogistic theory acted as a very useful means for collecting, and placing in a favourable position for closer inspection, many substances which would probably have remained scattered and detached from one another had this theory not been constructed. A single a.s.sumption was made, that all combustible substances are alike in one respect, namely, in containing combined fire, or phlogiston; by the help of this a.s.sumption, the theory of phlogiston emphasised the fundamental similarity between all processes of combustion. The theory of phlogiston was extraordinarily simple, compared with the alchemical vagaries which preceded it. Hoefer says, in his _Histoire de la Chimie_, "If it is true that simplicity is the distinctive character of verity, never was a theory so true as that of Stahl."

The phlogistic theory did more than serve as a means for bringing together many apparently disconnected facts. By concentrating the attention of the students of material changes on one cla.s.s of events, and giving descriptions of these events without using either of the four alchemical Elements, or the three Principles, Stahl, and those who followed him, did an immense service to the advancement of clear thinking about natural occurrences. The principle of phlogiston was more tangible, and more readily used, than the Salt, Sulphur, and Mercury of the alchemists; and to accustom people to speak of the material substance which remained when a metal, or other combustible substance, was calcined or burnt, as one of the _elements_ of the thing which had been changed, prepared the way for the chemical conception of an element as a definite substance with certain definite properties.

In addition to these advantages, the phlogistic theory was based on experiments, and led to experiments, the results of which proved that the capacity to undergo combustion might be conveyed to an incombustible substance, by causing it to react with some other substance, itself combustible, under definite conditions. The theory thus prepared the way for the representation of a chemical change as an interaction between definite kinds of substances, marked by precise alterations both of properties and composition.

The great fault of the theory of phlogiston, considered as a general conception which brings many facts into one point of view, and leads the way to new and exact knowledge, was its looseness, its flexibility. It was very easy to make use of the theory in a broad and general way; by stretching it here, and modifying it there, it seemed to cover all the facts concerning combustion and calcination which were discovered during two generations after the publication of Stahl's books. But many of the subsidiary hypotheses which were required to make the theory cover the new facts were contradictory, or at any rate seemed to be contradictory, of the primary a.s.sumptions of the theory. The addition of this ancillary machinery burdened the mechanism of the theory, threw it out of order, and finally made it unworkable. The phlogistic theory was destroyed by its own c.u.mbersomeness.

A scientific theory never lasts long if its fundamental a.s.sumptions are stated so loosely that they may be easily modified, expanded, contracted, and adjusted to meet the requirements of newly discovered facts. It is true that the theories which have been of the greatest service in science, as summaries of the relations between established facts, and suggestions of lines of investigation, have been stated in terms whose full meaning has gradually unfolded itself. But the foundations of these theories have been at once so rigidly defined and clearly stated as to be incapable of essential modification, and so full of meaning and widely applicable as to cover large cla.s.ses of facts which were unknown when the theories were constructed. Of the founders of the lasting and expansible theories of natural science, it may be said, that "thoughts beyond their thoughts to those high bards were given."

CHAPTER XI.

THE EXAMINATION OF THE PHENOMENA OF COMBUSTION.

The alchemists thought that the most effectual method of separating a complex substance into more simple substances was to subject it to the action of heat. They were constantly distilling, incinerating, subliming, heating, in order that the spirit, or inner kernel of things, might be obtained. They took for granted that the action of fire was to simplify, and that simplification proceeded whatever might be the nature of the substance which was subjected to this action.

Boyle insisted that the effect of heating one substance may be, and often is, essentially different from the effect of heating another substance; and that the behaviour of the same substance when heated, sometimes varies when the conditions are changed. He takes the example of heating sulphur or brimstone: "Exposed to a moderate fire in subliming pots, it rises all into dry, and almost tasteless, flowers; whereas being exposed to a naked fire, it affords store of a saline and fretting liquor." Boyle thought that the action of fire was not necessarily to separate a thing into its principles or elements, but, in most cases, was either to rearrange the parts of the thing, so that new, and it might be, more complex things, were produced, or to form less simple things by the union of the substance with what he called, "the matter of fire." When the product of heating a substance, for example, tin or lead, weighed more than the substance itself, Boyle supposed that the gain in weight was often caused by the "matter of fire" adding itself to the substance which was heated. He commended to the investigation of philosophers this "subtil fluid," which is "able to pierce into the compact and solid bodies of metals, and add something to them that has no despicable weight upon the balance, and is able for a considerable time to continue fixed in the fire." Boyle also drew attention to the possibility of action taking place between a substance which is heated and some other substance, wherewith the original thing may have been mixed. In a word, Boyle showed that the alchemical a.s.sumption--fire simplifies--was too simple; and he taught, by precept and example, that the only way of discovering what the action of fire is, on this substance or on that, is to make accurate experiments. "I consider," he says, "that, generally speaking, to render a reason of an effect or phenomenon, is to deduce it from something else in nature more known than itself; and that consequently there may be divers kinds of degrees of explication of the same thing."

Boyle published his experiments and opinions concerning the action of fire on different substances in the seventies of the 17th century; Stahl's books, which laid the foundation of the phlogistic theory, and confirmed the alchemical opinion that the action of fire is essentially a simplifying action, were published about forty years later. But fifty years before Boyle, a French physician, named Jean Rey, had noticed that the calcination of a metal is the production of a more complex, from a less complex substance; and had a.s.signed the increase in weight which accompanies that operation to the attachment of particles of the air to the metal. A few years before the publication of Boyle's work, from which I have quoted, John Mayow, student of Oxford, recounted experiments which led to the conclusion that the air contains two substances, one of which supports combustion and the breathing of animals, while the other extinguishes fire. Mayow called the active component of the atmosphere _fiery air_; but he was unable to say definitely what becomes of this fiery air when a substance is burnt, although he thought that, in some cases, it probably attaches itself to the burning substances, by which, therefore, it may be said to be fixed. Mayow proved that the air wherein a substance is burnt, or an animal breathes, diminishes in volume during the burning, or the breathing. He tried, without much success, to restore to air that part of it which disappears when combustion, or respiration, proceeds in it.

What happens when a substance is burnt in the air? The alchemists answered this question by a.s.serting that the substance is separated or a.n.a.lysed into things simpler than itself. Boyle said: the process is not necessarily a simplification; it may be, and certainly sometimes is, the formation of something more complicated than the original substance, and when this happens, the process often consists in the fixation of "the matter of fire" by the burning substance. Rey said: calcination, of a metal at anyrate, probably consists in the fixation of particles of air by the substance which is calcined. Mayow answered the question by a.s.serting, on the ground of the results of his experiments, that the substance which is being calcined lays hold of a particular const.i.tuent of the air, not the air as a whole.

Now, it is evident that if Mayow's answer was a true description of the process of calcination, or combustion, it should be possible to separate the calcined substance into two different things, one of which would be the thing which was calcined, and the other would be that const.i.tuent of the air which had united with the burning, or calcining, substance. It seems clear to us that the one method of proving the accuracy of Mayow's supposition must be, to weigh a definite, combustible, substance--say, a metal; to calcine this in a measured quant.i.ty of air; to weigh the product, and to measure the quant.i.ty of air which remains; to separate the product of calcination into the original metal, and a kind of air or gas; to prove that the metal thus obtained is the same, and has the same weight, as the metal which was calcined; and to prove that the air or gas obtained from the calcined metal is the same, both in quality and quant.i.ty, as the air which disappeared in the process of calcination.

This proof was not forthcoming until about a century after the publication of Mayow's work. The experiments which furnished the proof were rendered possible by a notable discovery made on the 1st of August 1774, by the celebrated Joseph Priestley.

Priestley prepared many "airs" of different kinds: by the actions of acids on metals, by allowing vegetables to decay, by heating beef, mutton, and other animal substances, and by other methods. He says: "Having procured a lens of twelve inches diameter and twenty inches focal distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and fact.i.tious, would yield.... With this apparatus, after a variety of other experiments.... on the 1st of August, 1774, I endeavoured to extract air from _mercurius calcinatus per se_; and I presently found that, by means of this lens, air was expelled from it very readily.

Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprised me more than I can well express was, that a candle burned in this air with a remarkably vigorous flame.... I was utterly at a loss how to account for it."

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