Popular Science Monthly/Volume 9/September 1876/The Revived Theory of Phlogiston
By WILLIAM ODLING, M. B., F. R. S.
FULLERIAN PROFESSOR OF CHEMISTRY, ROYAL INSTITUTION.
IN 1781-'83, Cavendish showed that when inflammable air or hydrogen, and dephlogisticated air or oxygen, are exploded together in certain proportions, "almost the whole of the inflammable and dephlogisticated air is converted into pure water," or, as he elsewhere expresses it, "is turned into water."
On June 24, 1783, the experiment of Cavendish was repeated on a larger scale and in a somewhat different form by Lavoisier, who not only confirmed the synthesis of the English chemist, but drew from it the conclusion—at first strongly contested, then rapidly acknowledged, and since never called into question—"that water consists of inflammable air united to dephlogisticated air," or that it is a compound of hydrogen and oxygen.
This conclusion, so opposite to his own preconception on the matter, Lavoisier subsequently confirmed by an analysis of water. He found that iron, heated to redness and exposed to the action of water-vapor, became changed, by an abstraction of oxygen from the water, into the self-same oxide of iron procurable by burning the metal in oxygen gas—the other constituent of the water, namely, its hydrogen, being freely liberated.
With the demonstration by Lavoisier of the composition of water began the triumph of that antiphlogistic theory which he had conceived, in a necessarily imperfect form, so far back as 1772, or before the discovery of oxygen, and had brought to completion by the aid of every successive step in pneumatic chemistry, achieved by himself or by others.
In 1785, the relationship to one another of hydrogen and water being then conclusively established, Berthollet declared himself a convert to the new theory of combustion put forward by his countryman. Fourcroy next gave in his adhesion; and soon afterward De Morveau, invited to Paris expressly to be reasoned with by Lavoisier, succumbed to the reasons set before him. The four chemists then associated themselves together, and, in spite of a strong though short-lived opposition both in England and Germany, succeeded in obtaining for La Chimie Française an all but universal recognition.
The principal articles of the new or antiphlogistic theory of combustion propounded by Lavoisier are as follows: That combustible bodies in burning yield products of various kinds, solid in the case of phosphorus and the metals, liquid in the case of hydrogen, gaseous in the case of carbon and sulphur. That in every case the weight of the products formed by the burning is greater than the weight of the combustible burned. That the increase of weight is due to an addition of matter furnished to the combustible by the air in which its burning takes place. That bodies of which the weights are made up of the weights of two or more distinct kinds of matter are of necessity compound; whereas bodies of which the weights cannot be shown to be made up of the weights of two or more distinct kinds of matter are in effect simple or elementary. That, inasmuch as the weights of the products furnished by the burning of different combustibles are made up of the weights of the combustible burned and of the oxygen consumed in the burning, these products are compound bodies—oxides in fact of the substances burned. That, inasmuch as given weights of many combustibles, as of hydrogen, sulphur, phosphorus, carbon, and the metals, are not apparently made up of the weights of two or more distinct kinds of matter, these particular combustibles are in effect elementary; as for the same reason is the oxygen with which in the act of burning they enter into combination. And, lastly, that combustion or burning consists in nothing else than in the union of combustible matter, simple or compound, with the empyreal matter, oxygen—the act of union being somehow attended by an evolution of light and heat. And, except that it would be necessary nowadays to explain how, in certain cases of combustion, the combustible enters into union not with oxygen, but with some analogue of oxygen, the above precise statement might equally well have been made by Lavoisier in 1785, or be made by one of ourselves at the present day.
Lavoisier's theory of combustion being known as the antiphlogistic theory, the question arises, What was the phlogistic theory to which it was opposed, and which it succeeded so completely in displacing? This phlogistic theory was founded and elaborated at the close of the seventeenth century by two German physicians, Beccher and Stahl. Having exercised a scarcely-disputed authority over men's minds until the notorious defection in 1785, it preserved for some years longer a resolute though tortuous existence, and was to the last defended and approved by our own Priestley and Cavendish—who died, the former in 1804, and the latter in 1810.
The importance attached to the refutation of this theory may be judged of from the circumstance that, after the early experiments of Lavoisier on the composition and decomposition of water had been successfully repeated by a committee of the French Academy in 1790, a congratulatory meeting was held in Paris, at which Madame Lavoisier, attired as a priestess, burned on an altar Stahl's celebrated "Fundamenta Chemiæ Dogmaticæ et Experimentalis," solemn music playing a requiem the while. And the sort of estimation in which the Stahlian doctrines have since been held by chemists is fairly illustrated by a criticism of Sir J. Herschel, who, speaking of the phlogistic theory of chemistry, says that it "impeded the progress of the science, as far as a science of experiment can be impeded by a false theory, . . . . by involving the subject in a mist of visionary and hypothetical causes in place of the true acting principles." Possibly, however, this much-abused theory may yet prove to contain an element of permanent vitality and truth; anyhow the study of this earliest and most enduring of chemical theories can never be wholly devoid of interest to chemists.
To appreciate the merit of the phlogistic theory it is necessary to bear in mind the period of its announcement. Its originator, Beccher, was born in 1625, and died a middle-aged but worn-out man in 1682, a few years before the publication of the "Principia." His more fortunate disciple, Stahl, who was born in 1660, and died in 1734, in his seventy-fifth year, though afforded a possibility of knowing, seems equally with Beccher to have remained throughout his long career indifferent to the Newtonian principle that the weight of a body is proportionate to its quantity of matter—that loss of weight implies of necessity abstraction of matter, and increase of weight addition of matter. Whether or not the founders of the phlogistic theory conceived that change of matter in the way of kind might, equally with its change in point of quantity, be associated with an alteration in weight—and it must not be forgotten what pains Newton thought it necessary to take in order to show the contrary—certain it is, they attached very little importance to the changes of weight manifested by bodies undergoing the metamorphosis of combustion. It might be that when combustible charcoal was burned the weight of incombustible residue was less than the original weight of charcoal it might be that when combustible lead was burned the weight of incombustible residue was greater than the original weight of metal—this was far too trifling an unlikeness to stand in the way of the paramount likeness presented by the two bodies. For the lead and charcoal had the common property of manifesting the wonderful energy of fire; they could alike suffer a loss of light and heat—that is, of phlogiston—by the deprivation of which they were alike changed into greater or less weights of inert incombustible residue.
And not only were these primitive students of the philosophy of combustion unconscious of the fact and meaning of the relationship in weight subsisting between the consuming and the consumed body, but they were altogether ignorant of the part played by the air in the phenomena which they so boldly and successfully attempted to explain. Torricelli's invention of the barometer, and Guericke's invention of the air-pump, were both, indeed, made during Beccher's early boyhood; but years had to elapse before the consequent idea of-the materiality of air could be domiciled, as it were, in human understandings. And not until more than a century after Torricelli's discovery of the weight of air—not, indeed, until the time of the great pneumatic chemists, Black, Cavendish, Priestley, and Scheele—was it ever imagined that the aërial state, like the solid or liquid state, was a state common to many distinct kinds of matter; and that the weight or substance of a rigid solid might be largely contributed to by the weight or substance of some constituent having its independent existence in the aerial or gaseous form. The notion that 100 pounds of smithy-scales might consist of 73 pounds of iron and 27 pounds of a particular kind of air, and that 100 pounds of marble might consist of 56 pounds of lime and 44 pounds of another kind of air, was a notion utterly foreign to the elder philosophy. Air, it was allowed, might be rendered mephitic by one kind of contamination, and sulphurous by another, and inflammable by a third; it might even be absorbed in, and so add to the weight of, a porous solid, as water is absorbable by sand; but still air was ever indisputably air, essentially alike and unalterable in its mechanical and chemical oneness. This familiar conception had to be overcome, and the utterly strange notion of the largely aërial constitution of solid matter to be established in its stead, by the early pneumatic chemists, Black, Cavendish, and Bergmann, before the deficiencies rather than positive errors of the phlogistic theory could be perceived.
But long ere the foundation of modern chemistry had thus been laid, in 1756, by Black's discovery of fixed air or carbonic acid as a constituent of mild alkalies and limestone, those old German doctors, Beccher and Stahl, though ignorant of the nature of air, and neglectful of the import of gravity, had yet found something to say about the chemistry of combustion worthy of being defended a century afterward by men like Priestley and Cavendish—worthy, it is believed, of being recognized nearly two centuries afterward as the expression of a fundamental doctrine in chemical and cosmical philosophy. They pointed out, for example, that the different and seemingly unlike processes of burning, smouldering, calcining, rusting, and decaying, by which combustible is changed into incombustible matter, have a community of character; that combustible bodies possess in common a power or energy capable of being elicited and used, whereas incombustible bodies are devoid of any such energy or power; and, lastly, that the energy pertaining to combustible bodies is the same in all of them, and capable of being transferred from the combustible body which has it to an incombustible body which has it not, rendering the body that was energetic and combustible inert and incombustible, and the body that was inert and incombustible energetic and combustible, and further rendering some particular body combustible over and over again. That this is a fair representation of the views held by phlogistic chemists is readily recognizable by a study of chemical works written before the outbreak of the antiphlogistic revolution. After Lavoisier's challenge, the advocates of phlogiston, striving to make it account for a novel order of facts with which it had little or nothing to do, were driven to the most incongruous of positions; for, while Priestley wrote of inert nitrogen as phlogisticated air, Kirwan and others regarded inflammable hydrogen as being phlogiston itself in the isolated state. Very different is the view of phlogiston to be gathered from the writings of Dr. Watson, for example, who was appointed Professor of Chemistry at Cambridge in 1764, became Regius Professor of Divinity in 1771, and Bishop of Llandaff in 1782. This cultivated divine, indifferent, it is true, to the novel questions by which in less placid regions men's minds were so deeply stirred, amused the leisure of his dignified university life by writing scholarly accounts of the chemistry it had formerly been his province to teach; and in the first volume of his well-known "Chemical Essays," published in 1781, the following excellent account of phlogiston is to be found:
"Notwithstanding all that perhaps can be said upon this subject, I am sensible the reader will be still ready to ask, What is phlogiston? You do not surely expect that chemistry should be able to present you with a handful of phlogiston, separated from an inflammable body; you may just as reasonably demand a handful of magnetism, gravity, or electricity, to be extracted from a magnetic, weighty, or electric body. There are powers in Nature which cannot otherwise become the objects of sense than by the effects they produce; and of this kind is phlogiston. But the following experiments will tend to render this perplexed subject somewhat more clear:
"If you take a piece of sulphur and set it on fire it will burn entirely away, without leaving any ashes or yielding any soot. During the burning of the sulphur a copious vapor, powerfully affecting the organs of sight and smell, is dispersed. Means have been invented for collecting this vapor, and it is found to be a very strong acid. The acid thus procured from the burning of sulphur is incapable of being either burned by itself or of contributing toward the support of fire in other bodies; the sulphur, from which it was procured, was capable of both: there is a remarkable difference, then, between the acid procured from the sulphur and the sulphur itself. The acid cannot be the only constituent part of sulphur; it is evident that something else must have entered into its composition, by which it was rendered capable of combustion. This something is, from its most remarkable property, that of rendering a body combustible, properly enough denominated the food of fire, the inflammable principle, the phlogiston. . . . This inflammable principle or phlogiston is not one thing in animals, another in vegetables, another in minerals; it is absolutely the same in them all. This identity of phlogiston may be proved from a variety of decisive experiments; I will select a few, which may at the same time confirm what has been advanced concerning the constituent parts of sulphur.
"From the analysis or decomposition of sulphur effected by burning, we have concluded that the constituent parts of sulphur are two—an acid which may be collected, and an inflammable principle which is dispersed. If the reader has yet acquired any real taste for chemical truths, he will wish to see this analysis confirmed by synthesis; that is, in common language, he will wish to see sulphur actually made by combining its acid with an inflammable principle. It seldom happens that chemists can reproduce the original bodies, though they combine together all the principles into which they have analyzed them; in the instance, however, before us, the reproduction of the original substance will be found complete.
"As the inflammable principle cannot be obtained in a palpable form separate from all other bodies, the only method by which we can attempt to unite it with the acid of sulphur must be by presenting to that acid some substance in which it is contained. Charcoal is such a substance; and by distilling powdered charcoal and the acid of sulphur together, we can procure a true yellow sulphur, in no wise to be distinguished from common sulphur. This sulphur is formed from the union of the acid with the phlogiston of the charcoal; and the charcoal may by this means be so entirely robbed of its phlogiston that it will be reduced to ashes, as if it had been burned. . . .
"I will in this place, by way of further illustration of the term phlogiston, add a word or two concerning the necessity of its union with a metallic earth, in order to constitute a metal. Lead, it has been observed, when melted in a strong fire, burns away like rotten wood; all its properties as a metal are destroyed, and it is reduced to ashes. If you expose the ashes of lead to a strong fire they will melt; but the melted substance will not be a metal, it will be a yellow or orange-colored glass. If you pound the glass, and mix it with charcoal-dust, or if you mix the ashes of the lead with charcoal-dust, and expose either mixture to a melting heat, you will obtain, not a glass, but a metal, in weight, color, consistency, and every other property, the same as lead. The ashes of lead melted without charcoal become glass; the ashes of lead melted with charcoal become a metal. The charcoal, then, must have communicated something to the ashes of lead, by which they are changed from a glass to a metal. Charcoal consists of but two things—of ashes and of phlogiston; the ashes of charcoal, though united with the ashes of lead, would only produce glass; it must, therefore, be the other constituent part of charcoal or phlogiston which is communicated to the ashes of lead, and by a union with which the ashes are restored to their metallic form. The ashes of lead can never be restored to their metallic form without their being united with some matter containing phlogiston, and they may be reduced in their metallic form by being united with any substance containing phlogiston in a proper state, whether that substance be derived from the animal, vegetable, or mineral kingdom; and thence we conclude, not only that phlogiston is a necessary part of a metal, but that phlogiston has an identity belonging to it, from whatever substance in Nature it be extracted. And this assertion still becomes more general, if we may believe that metallic ashes have been reduced to their metallic form, both by the solar rays and the electrical fire."
The foregoing account by Dr. Watson is almost a translation from Stahl's "Zymotechnica Fundamentalis, simulque experimentum novum sulphur verum arte producendi," in which he establishes what may be called the permanency of chemical substance—that metallic lead is reproducible from the ashes of lead, sulphur verum from the acid of sulphur. And, whether or not taking note of the oxidations and deoxidations effected, how little differently, even at the present day, would the actions referred to be described and explained! Is it not our habit to say that charcoal and sulphur and lead are bodies possessing potential chemical energy—that is, phlogiston; that, in the act of burning, their energy which was potential becomes kinetic or dynamical, and is dissipated in the form of light and heat; that the products of their burning (including the gaseous product now known to be furnished by the burning of charcoal) are substances devoid of chemical energy—that is, of phlogiston; that, when the acid substance furnished by burning sulphur is heated with charcoal, some energy of the unburnt charcoal is transferred to the burnt sulphur, just as some energy of a raised weight may be transferred to a fallen one, whereby the burnt sulphur is unburnt, provided with energy, and enabled to burn again, and the fallen weight is lifted up, provided with energy, and enabled to fall again; that the potential chemical energy of metallic lead did not originate in the lead, but is energy or phlogiston transferred thereto from the charcoal by which it was smelted; and, lastly, that the chemical energy of the charcoal itself, its capability of burning, its power of doing work, in one word, its phlogiston, is merely a portion of energy appropriated directly from the solar rays?
If this be a correct interpretation of the phlogistic doctrine, it is evident that the Stahlians, though ignorant of much that has since become known, were nevertheless cognizant of much that became afterward forgotten. For most of what has since become known mankind are indebted to the surpassing genius of Lavoisier; but the truth which he established, alike with that which he subverted, is now recognizable as a partial truth only; and the merit of his generalization is now perceived to consist in its addition to—its demerit to consist in its supercession of—the not less grand generalization established by his scarcely-remembered predecessors. This being so, the relationship to one another of the Stahlian and Lavoisierian theories of combustion furnishes an apt illustration of the general truth set forth by a great modern writer, that "in the human mind one-sidedness has always been the rule, and many-sidedness the exception. Hence, even in revolutions of opinion, one part of the truth usually sets while another rises. Even progress, which ought to superadd, for the most part only substitutes one partial and incomplete truth for another; improvement consisting chiefly in this, that the new fragment of truth is more wanted, more adapted to the needs of the time, than that which it displaces."
The partial truth contributed by Lavoisier was indeed more wanted, more adapted to the needs of the time, than the partial truth which it displaced. To him chemists are indebted for their present conception of material elements; and especially for their knowledge of the part played by the air in the phenomena of combustion, whereby oxygenated compounds are produced. The phlogistians, indeed, were not unaware of the necessity of air to combustion, but, being ignorant of the nature of air, were necessarily ignorant of the functions which it fulfilled. To burn and throw off phlogiston being with them synonymous expressions, the air was conceived to act by somehow or other enabling the combustible to throw its phlogiston off; and a current of air was conceived to promote combustion by enabling the combustible to throw its phlogiston off more easily. Moreover, contact of air was not essential to combustion, provided there was present instead some substance, such as nitre, which, equally with or even more effectively than air, could enable the combustible to discharge itself of phlogiston. But, while the phlogistians, on the one hand, were unaware that the burnt product differed from the original combustible otherwise than as ice differs from water, by loss of energy, Lavoisier, on the other hand, disregarded the notion of energy, and showed that the burnt product included not only the stuff of the combustible, but also the stuff of the oxygen it had absorbed in the burning. But, as well observed by Dr. Crum-Brown, we now know "that no compound contains the substances from which it was produced, but that it contains them minus something. We now know what this something is, and can give it the more appropriate name of potential energy; but there can be no doubt that this is what the chemists of the seventeenth century meant when they spoke of phlogiston."
Accordingly, the phlogistic and antiphlogistic views are in reality complementary and not, as suggested by their names and usually maintained, antagonistic to one another. It has been said, for example, that, according to Stahl, the product of combustion is simple, and the combustible a compound of the product with imaginary phlogiston—which is false; whereas, according to Lavoisier, the combustible is simple, and the product a compound of the combustible with actual oxygen—which is true. But in this case, as in so many others, everything turns upon the use of the same word in a different sense at different periods of time. When Lavoisier spoke of red lead as being metallic lead combined with oxygen, he meant that the matter or stuff of the red lead consisted of the matter or stuff of lead plus the matter or stuff of oxygen. But, when the Stahlians spoke of metallic lead being burnt lead combined with phlogiston, they had the same sort of idea of combination in this instance as others have expressed by saying that the weight of a body is compounded of its matter and its gravity; or that steam is a compound of water and heat; or, to use a yet more Lavoisierian expression, that oxygen gas itself is a compound of the basis of oxygen with caloric. It is not, then, that the one statement, Stahlian or Lavoisierian, is false and the other true, but that both of them are distorted, because incomplete. Chemists nowadays are both Stahlian and Lavoisierian in their notions, or have regard both to energy and matter. But Lavoisierian ideas still interfere very little with our use of the Stahlian language. While we acknowledge that in the act of burning the combustible and the oxygen take equal part, just as in the act of falling the weight and the earth take equal part, yet in our common language we alike disregard the abundant atmosphere and abundant earth as being necessarily understood, and speak only of the energy of the combustible and of the weight, which burn and fall respectively. Whatever may be the fault of language, however, chemists do not omit to superpose the Lavoisierian on the Stahlian notion. They recognize fully that it is by the union of the combustible with oxygen that phlogiston is dissipated in the form of heat; and, further, that phlogiston can only be restored to the burnt combustible on condition of separating the combustible from the oxygen with which it has united, just as energy of position can only be restored to a fallen weight on condition of separating it to a distance from the surface on which it has fallen.
That Stahl and his followers regarded phlogiston as a material substance, if they did so regard it, should interfere no more with our recognition of the merit due to their doctrine, than the circumstance of Black and Lavoisier regarding caloric as a material substance, if they did so regard it, should interfere with our recognition of the merit due to the doctrine of latent heat. But, though defining phlogiston as the principle or matter of fire, it is not at all clear that the phlogistians considered this matter of fire as constituting a real body or ponderable substance; but rather that they thought and spoke of it as many philosophers nowadays think and speak of the electric fluid and luminiferous ether. The nondescript character, properly ascribable to phlogiston, is indicated by the following quotation taken from Macquer's "Élémens de Chymie Théorique" (1749). It must not, of course, be forgotten that the popular impression as to phlogiston having been conceived by its advocates as a material substance having a negative weight or levity, is erroneous, and is based on an innovation that was introduced during the struggling decadence of the phlogistic theory, and advocated more particularly by Lavoisier's subsequent colleague, Guyton de Morveau, in his "Dissertation sur le Phlogistique, considéré comme Corps grave, et par Rapport aux Changemens de Pesanteur qu'il produit dans les Corps auxquels il est uni" (1762). Macquer writes as follows:
"'Matter of the sun, or of light,' 'phlogiston,' 'fire,' 'sulphur-principle,' 'inflammable matter'—such are the names usually employed to designate the element fire. But no precise distinction appears to have been drawn between fire viewed as a principle in the composition of a body, and fire when it stands alone and in its natural state. Viewed under the latter aspect, the terms 'fire,' 'matter of the sun, of light, and of heat,' are specially appropriate to it. Under such conditions, it is a substance which may be regarded as made up of infinitesimal particles, agitated by a very rapid and continuous motion, and hence essentially fluid. This substance, of which the sun is, as it were, the general reservoir, is emanating thence constantly, and is universally distributed throughout all bodies known to us, though not as a principle, or as essential to their constitution, inasmuch as we may deprive them of it—at least in great measure—witbout their suffering the least decomposition in consequence. . . . Yet the phenomena presented by inflammable substances in burning show that they really contain the matter of fire as one of their principles. . . . Let us, therefore, investigate the properties of this fire which has become fixed, and entered as a principle into bodies. To it we will specially assign the name of 'inflammable matter,' 'sulphur-principle,' and 'phlogiston,' to distinguish it from pure fire."
Again, much the same thing is to be found in Baumé's "Manuel de Chymie" (1765); as, for example:
"We consider fire in two different states: when it is pure, isolated, and forming no part of any compound. . . . when it is combined with other substances, forming one of the constituent principles of compound bodies. . . . We have no certainty whether or not fire possesses weight. There are experiments pro and contra. . . . During the combustion of substances, combined fire is reduced to elementary fire, and is dissipated as the process goes on. The famous Boerhaave, however, is not of this opinion; he says that, were this the case, the amount of elementary fire in Nature must increase ad infinitum. . . . But it is easy to reply to this objection by saying that, as we have the right to presume, the elementary fire discharged from bodies combines with other substances, and that it loses all its properties as free fire on becoming a constituent principle of bodies into the composition of which it enters. . . . The principle here spoken of is that to which Stahl has given the name of phlogiston."
In interpreting the above and other phlogistic writings by the light of modern doctrine, it is not meant to attribute to their several authors the precise notion of energy that now prevails. It is contended only that the phlogistians had, in their time, possession of a real truth in Nature which, altogether lost sight of in the intermediate period, has since crystallized out in a definite form. "I trust," said Beccher, "that I have got hold of my pitcher by the right handle." And what he and his followers got hold of and retained so tenaciously, though it may be shiftingly and ignorantly, we now hold to knowingly, definitely, and quantitatively, as part and parcel of the grandest generalization in science that has ever yet been established.