Chapter II (1)

ESTABLISHMENT OF CHEMISTRY AS A SCIENCE--PERIOD OF BLACK, PRIESTLEY AND LAVOISIER.

_Joseph Black_, 1728-1799. _Joseph, Priestley_,
1733-1804. _Antoine Laurent Lavoisier_, 1743-1794.

During this period of advance, which may be broadly stated as comprising the last half of the eighteenth century, the aim and scope of chemical science were clearly indicated by the labours of Black, Priestley and Lavoisier. The work of these men dealt chiefly with the process of combustion. Black and Priestley finally proved the existence of airs or gases different from common air, and Lavoisier applied these discoveries to give a clear explanation of what happens when a substance burns.

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JOSEPH BLACK was born near Bordeaux in the year 1728. His father was of Scottish family, but a native of Belfast; his mother was the daughter of Mr. Gordon, of Hilhead in Aberdeenshire. We are told by Dr. Robison, in his preface to Black's Lectures, that John Black, the father of Joseph, was a man "of most amiable manners, candid and liberal in his sentiments, and of no common information."

At the age of twelve Black was sent home to a school at Belfast; after spending six years there he went to the University of Glasgow in the year 1746. Little is known of his progress at school or at the university, but judging from his father's letters, which his son preserved, he seems to have devoted himself to study. While at Glasgow he was attracted to the pursuit of physical science, and chose medicine as a profession. Becoming a pupil of Dr. Cullen, he was much impressed with the importance of chemical knowledge to the student of medicine. Dr. Cullen appears to have been one of the first to take large and philosophical views of the scope of chemical science, and to attempt to raise chemistry from the rank of a useful art to that of a branch of natural philosophy. Such a man must have been attracted by the young student, whose work was already at once accurate in detail and wide in general scope.

In the notes of work kept by Black at this time are displayed those qualities of methodical arrangement, perseverance and thoroughness which are so prominent in his published investigations and lectures. In one place we find, says his biographer, many disjointed facts and records of diverse observations, but the next time he refers to the same subjects we generally have analogous facts noted and some conclusions drawn--we have the beginnings of knowledge. Having once entered on an investigation Black works it out steadily until he gets definite results.

His earlier notes are concerned chiefly with heat and cold; about 1752 he begins to make references to the subject of "fixed air."

About 1750 Black went to Edinburgh University to complete his medical studies, and here he was again fortunate in finding a really scientific student occupying the chair of natural philosophy.

The attention of medical men was directed at this time to the action of limewater as a remedy for stone in the bladder. All the medicines which were of any avail in mitigating the pain attendant on this disease more or less resembled the "caustic ley of the soap-boilers" (or as we should now call it caustic potash or soda). These caustic medicines were mostly prepared by the action of quicklime on some other substance, and quicklime was generally supposed to derive its caustic, or corrosive properties from the fire which was used in changing ordinary limestone into quicklime.

When quicklime was heated with "fixed alkalis" (_i.e._ with potassium or sodium carbonate), it changed these substances into caustic bodies which had a corrosive action on animal matter; hence it was concluded that the quicklime had derived a "power"--or some said had derived "igneous matter"--from the fire, and had communicated this to the fixed alkalis, which thereby acquired the property of corroding animal matter.

Black thought that he might be able to lay hold of this "igneous matter" supposed to be taken by the limestone from the fire; but he found that limestone loses weight when changed into quicklime. He then dissolved limestone (or chalk) in spirits of salt (hydrochloric acid), and compared the loss of weight undergone by the chalk in this process with the loss suffered by an equal quantity of chalk when strongly heated. This investigation led Black to a fuller study of the action of heat on chalk and on "mild magnesia" (or as we now say, magnesium carbonate).

In order that his experiments might be complete and his conclusions well established, he delayed taking the degree of Doctor of Medicine for three years. He graduated as M. D. in 1755, and presented his thesis on "Magnesia Alba, Quicklime and other Alkaline Substances," which contained the results of what is probably the first accurately quantitative examination of a chemical action which we possess.

Black prepared mild magnesia (magnesium carbonate) by boiling together solutions of Epsom salts (magnesium sulphate) and fixed alkali (potassium carbonate). He showed that when mild magnesia is heated--

1. It is much decreased in bulk.

2. It loses weight (twelve parts become five, according to Black).

3. It does not precipitate lime from solutions of that substance in acids (Black had already shown that mild magnesia does precipitate lime).

He then strongly heated a weighed quantity of mild magnesia in a retort connected with a receiver; a few drops of water were obtained in the receiver, but the magnesia lost six or seven times as much weight as the weight of the water produced. Black then recalls the experiments of Hales, wherein airs other than common air had been prepared, and concludes that the loss of weight noticed when mild magnesia is calcined is probably due to expulsion, by the heat, of some kind of air. Dissolving some of his mild magnesia in acid he noticed that effervescence occurred, and from this he concluded that the same air which, according to his hypothesis, is expelled by heat, is also driven out from the mild magnesia by the action of acid. He then proceeded to test this hypothesis. One hundred and twenty grains of mild magnesia were strongly calcined; the calcined matter, amounting to seventy grains, was dissolved in dilute oil of vitriol, and this solution was mixed with common fixed alkali (potassium carbonate). The solid which was thus produced was collected, washed and weighed; it amounted to a trifle less than one hundred and twenty grains, and possessed all the properties--detailed by Black--of the original mild magnesia. But this is exactly the result which ought to have occurred according to his hypothesis.

The next step in the investigation was to collect the peculiar air which Black had proved to be evolved during the calcination of mild magnesia. To this substance he gave the name of "fixed air," because it was fixed or held by magnesia. Black established the existence of this air in the expired breath of animals, and also showed that it was present in the air evolved during vinous fermentation. He demonstrated several of its properties; among these, the fact that animals die when placed in this air. An air with similar properties was obtained by calcining chalk. Black held that the chemical changes which occur when chalk is calcined are exactly analogous to those which he had proved to take place when magnesia is strongly heated. Chalk ought therefore to lose weight when calcined; the residue ought to neutralize an acid without evolution of any gas, and the quantity of acid thus neutralized ought to be the same as would be neutralized by the uncalcined chalk; lastly, it ought to be possible to recover the uncalcined chalk by adding a fixed alkali to a solution of the calcined chalk or quicklime.

The actual results which Black obtained were as follows:--

One hundred and twenty grains of chalk were dissolved in dilute muriatic (hydrochloric) acid; 421 grains of the acid were needed to neutralize the chalk, and 48 grains of fixed air were evolved. One hundred and twenty grains of the same specimen of chalk were strongly calcined, and then dissolved in dilute muriatic acid; 414 grains of the acid were required to neutralize the calcined chalk. The difference between 421 and 414 is very slight; considering the state of practical chemistry at Black's time, we may well agree with him that he was justified in the conclusion that equal weights of calcined and of uncalcined chalk neutralize the same amount of acid. One hundred and twenty grains of the same specimen of chalk were again strongly heated; the calcined chalk, amounting to 68 grains, was digested with a solution of fixed alkali in water. The substance thus obtained, when washed and dried, weighed 118 grains, and had all the properties of ordinary chalk. Therefore, said Black, it is possible to recover the whole of the chalk originally present before calcination, by adding a fixed alkali to the calcined chalk or quicklime.

At this time it was known that water dissolves quicklime, but it was generally held that only about one-fourth (or perhaps a little more) of any specimen of quicklime could be dissolved by water, however much water was employed. Black's researches had led him to regard quicklime as a homogeneous chemical compound; he concluded that as water undoubtedly dissolves quicklime to some extent, any specimen of this substance, provided it be pure, must be wholly soluble in water. Carefully conducted experiments proved that Black's conclusion was correct. Black had thus proved that quicklime is a definite substance, with certain fixed properties which characterize it and mark it off from all other substances; that by absorbing, or combining with another definite substance (fixed air), quicklime is changed into a third substance, namely chalk, which is also characterized by properties as definite and marked as those of quicklime or fixed air.

Black, quite as much as the alchemists, recognized the fact that change is continually proceeding in Nature; but he clearly established the all-important conclusion that these natural changes proceed in definite order, and that it is possible by careful experiment and just reasoning to acquire a knowledge of this order. He began the great work of showing that, as in other branches of natural science, so also in chemistry, which is pre-eminently the study of the changes of Nature, "the only distinct meaning of that word" (natural) "is _stated_, _fixed_, or _settled_" (Butler's "Analogy," published 1736).

This research by Black is a model of what scientific work ought to be. He begins with a few observations of some natural phenomenon; these he supplements by careful experiments, and thus establishes a sure basis of fact; he then builds on this basis a general hypothesis, which he proceeds to test by deducing from it certain necessary conclusions, and proving, or disproving, these by an appeal to Nature. This is the scientific method; it is common sense made accurate.

Very shortly after the publication of the thesis on magnesia and quicklime, a vacancy occurred in the chemical chair in Glasgow University, and Black was appointed Professor of Anatomy and Lecturer on Chemistry. As he did not feel fully qualified to lecture on anatomy, he made an arrangement to exchange subjects with the Professor of Medicine, and from this time he delivered lectures on chemistry and on "The Institutes of Medicine."

Black devoted a great deal of care and time to the teaching duties of his chair. His chemical experimental researches were not much advanced after this time; but he delivered courses of lectures in which new light was thrown on the whole range of chemical science.

In the years between 1759 and 1763 Black examined the phenomena of heat and cold, and gave an explanation, founded on accurate experiments, of the thermal changes which accompany the melting of solids and the vaporization of liquids.

If pieces of wood, lead and ice be taken by the hand from a box in which they have been kept cold, the wood feels cold to the touch, the lead feels colder than the wood, and the ice feels colder than the lead; hence it was concluded that the hand receives cold from the wood, more cold from the lead, and most cold from the ice.

Black however showed that the wood really takes away heat from the hand, but that as the wood soon gets warmed, the process stops before long; that the lead, not being so quickly warmed as the wood, takes away more heat from the hand than the wood does, and that the ice takes away more heat than either wood or lead.

Black thought that the heat which is taken by melting ice from a warm body remains in the water which is produced; as soon as winter came he proceeded to test this supposition by comparing the times required to melt one pound of ice and to raise the temperature of one pound of water through one degree, the source of heat being the same in each case. He also compared the time required to lower the temperature of one pound of water through one degree with that required to freeze one pound of ice-cold water. He found that in order to melt one pound of ice without raising its temperature, as much heat had to be added to the ice as sufficed to raise the temperature of one pound of water through about 140 degrees of Fahrenheit's thermometer. But this heat which has been added to the ice to convert it into water is not indicated by the thermometer. Black called this "_latent heat_."

The experimental data and the complete theory of latent heat were contained in a paper read by Black to a private society which met in the University of Glasgow, on April 23, 1762; but it appears that Black was accustomed to teach the theory in his ordinary lectures before this date.

The theory of latent heat ought also to explain the phenomena noticed when liquid water is changed into steam. Black applied his theory generally to this change, but did not fully work out the details and actually measure the quantity of heat which is absorbed by water at the boiling point before it is wholly converted into steam at the same temperature, until some years later when he had the assistance of his pupil and friend James Watt.

Taking a survey of the phenomena of Nature, Black insisted on the importance of these experimentally established facts--that before ice melts it must absorb a large quantity of heat, and before water is vaporized it must absorb another large quantity of heat, which amounts of heat are restored to surrounding substances when water vapour again becomes liquid water and when liquid water is congealed to ice. He allows his imagination to picture the effects of these properties of water in modifying and ameliorating the climates of tropical and of Northern countries. In his lectures he says, "Here we can also trace another magnificent train of changes which are nicely accommodated to the wants of the inhabitants of this globe. In the equatorial regions, the oppressive heat of the sun is prevented from a destructive accumulation by copious evaporation. The waters, stored with their vaporific heat, are then carried aloft into the atmosphere till the rarest of the vapour reaches the very cold regions of the air, which immediately forms a small portion of it into a fleecy cloud. This also further tempers the scorching heat by its opacity, performing the acceptable office of a screen. From thence the clouds are carried to the inland countries, to form the sources in the mountains which are to supply the numberless streams that water the fields. And by the steady operation of causes, which are tolerably uniform, the greater part of the vapours passes on to the circumpolar regions, there to descend in rains and dews; and by this beneficent conversion into rain by the cold of those regions, each particle of steam gives up the heat which was latent in it. This is immediately diffused, and softens the rigour of those less comfortable climates."

In the year 1766 Black was appointed Professor of Chemistry in the University of Edinburgh, in which position he remained till his death in 1799. During these thirty-three years he devoted himself chiefly to teaching and to encouraging the advance of chemical science. He was especially careful in the preparation of his elementary lectures, being persuaded that it was of the utmost importance that his pupils should be well grounded in the principles of chemistry.

His health had never been robust, and as he grew old he was obliged to use great care in his diet; his simple and methodical character and habits made it easy for him to live on the plainest food, and to take meals and exercise at stated times and in fixed quantities.

Black's life closed, as was fitting, in a quiet and honoured old age. He had many friends, but lived pretty much alone--he was never married.

On the 26th of November 1799, "being at table with his usual fare, some bread, a few prunes and a measured quantity of milk diluted with water, and having the cup in his hand when the last stroke of his pulse was to be given, he had set it down on his knees, which were joined together, and kept it steady with his hand, in the manner of a person perfectly at ease; and in this attitude he expired, without spilling a drop, and without a writhe in his countenance, as if an experiment had been required to show to his friends the facility with which he departed."

Black was characterized by "moderation and sobriety of thought;" he had a great sense of the fitness of things--of what is called by the older writers "propriety." But he was by no means a dull companion; he enjoyed general society, and was able to bear a part in any kind of conversation. A thorough student of Nature, he none the less did not wish to devote his whole time to laboratory work or to the labours of study; indeed he seems to have preferred the society of well-cultivated men and women to that of specialists in his own or other branches of natural science. But with his true scientific peers he doubtless appeared at his best. Among his more intimate friends were the famous political economist Adam Smith, and the no less celebrated philosopher David Hume. Dr. Hutton, one of the earliest workers in geology, was a particular friend of Black; his friendship with James Watt began when Watt was a student in his class, and continued during his life.

With such men as his friends, and engaged in the study of Nature--that boundless subject which one can never know to the full, but which one can always know a little more year by year--Black's life could not but be happy. His example and his teaching animated his students; he was what a university professor ought to be, a student among students, but yet a teacher among pupils. His work gained for him a place in the first rank of men of science; his clearness of mind, his moderation, his gentleness, his readiness to accept the views of others provided these views were well established on a basis of experimentally determined facts, fitted him to be the centre of a circle of scientific students who looked on him as at once their teacher and their friend.

As a lecturer Black was eminently successful. He endeavoured to make all his lectures plain and intelligible; he enlivened them by many experiments designed simply to illustrate the special point which he had in view. He abhorred ostentatious display and trickiness in a teacher.

Black was strongly opposed to the use of hypotheses in science. Dr. Robison (the editor of his lectures) tells that when a student in Edinburgh he met Black, who became interested in him from hearing him speak somewhat enthusiastically in favour of one of the lecturers in the university. Black impressed on him the necessity of steady experimental work in natural science, gave him a copy of Newton's "Optics" as a model after which scientific work ought to be conducted, and advised him "to reject, even without examination, any hypothetical explanation, as a mere waste of time and ingenuity." But, when we examine Black's own work, we see that by "hypothetical explanations" he meant vague guesses. He himself made free use of scientific (_i.e._ of exact) hypotheses; indeed the history of science tells us that without hypotheses advance is impossible. Black taught by his own researches that science is not an array of facts, but that the object of the student of Nature is to explain facts. But the method generally in vogue before the time of Black was to gather together a few facts, or what seemed to be facts, and on these to raise a vast superstructure of "vain imaginings." Naturalists had scarcely yet learned that Nature is very complex, and that guessing and reasoning on guesses, with here and there an observation added, was not the method by which progress was to be made in learning the lessons written in this complex book of Nature.

In place of this loose and slipshod method Black insisted that the student must endeavour to form a clear mental image of every phenomenon which he studied. Such an image could be obtained only by beginning with detailed observation and experiment. From a number of definite mental images the student must put together a picture of the whole natural phenomenon under examination; perceiving that something was wanted here, or that the picture was overcrowded there, he must again go to Nature and gain fresh facts, or sometimes prove that what had been accepted as facts had no real existence, and so at length he would arrive at a true representation of the whole process.

So anxious was Black to define clearly what he knew and professed to teach, that he preferred to call his lectures "On the Effects of Heat and Mixtures," rather than to announce them as "A Systematic Course on Chemistry."

His introductory lecture on "Heat in General" is very admirable; the following quotation will serve to show the clearness of his style and the methodical but yet eminently suggestive manner of his teaching:--

_"Of Heat in General._

"That this extensive subject may be treated in a profitable manner, I propose--

"First. To ascertain what I mean by the word _heat_ in these lectures.

"Secondly. To explain the meaning of the term _cold_, and ascertain the real difference between heat and cold.

"Thirdly. To mention some of the attempts which have been made to discover the nature of heat, or to form an idea of what may be the immediate cause of it.

"Fourthly and lastly. I shall begin to describe sensible effects produced by heat on the bodies to which it is communicated.

"Any person who reflects on the ideas which we annex to the word _heat_ will perceive that this word is used for two meanings, or to express two different things. It either means a sensation excited in our organs, or a certain quality, affection, or condition of the bodies around us, by which they excite in us that sensation. The word is used in the first sense when we say, we feel heat; in the second, when we say, there is heat in the fire or in a hot stone. There cannot be a sensation of heat in the fire, or in the hot stone, but the matter of the fire, or of the stone, is in a state or condition by which it excites in us the sensation of heat.

"Now, in beginning to treat of heat and its effects, I propose to use the word in this second sense only; or as expressing that state, condition, or quality of matter by which it excites in us the sensation of heat. This idea of heat will be modified a little and extended as we proceed, but the meaning of the word will continue at bottom the same, and the reason of the modification will be easily perceived."

Black's manner of dealing with the phenomenon of combustion illustrates the clearness of the conceptions which he formed of natural phenomena, and shows moreover the thoroughly unbiased nature of his mind. As soon as he had convinced himself that the balance of evidence was in favour of the new (antiphlogistic) theory, he gave up those doctrines in which he had been trained, and accepted the teaching of the French chemists; but he did not--as some with less well-balanced minds might do--regard the new theory as a final statement, but rather as one stage nearer the complete explanation which future experiments and future reasoning would serve to establish.

In his lectures on combustion Black first of all establishes the facts, that when a body is burned it is changed into a kind (or kinds) of matter which is no longer inflammable; that the presence of air is needed for combustion to proceed; that the substance must be heated "to a certain degree" before combustion or inflammation begins; that this degree of heat (or we should now say this degree of temperature) differs for each combustible substance; that the supply of air must be renewed if the burning is to continue; and that the process of burning produces a change in the quality of the air supplied to the burning body.

He then states the phlogistic interpretation of these phenomena: that combustion is caused by the outrush from the burning body of a something called the _principle of fire_, or _phlogiston_.

Black then proceeds to demonstrate certain other facts:--When the substances produced by burning phosphorus or sulphur are heated with carbon (charcoal) the original phosphorus or sulphur is reproduced. This reproduction is due, according to the phlogistic chemists, to the giving back, by carbon, of the phlogiston which had escaped during the burning. Hence carbon contains much phlogiston. But as a similar reproduction of phosphorus or sulphur, from the substances obtained by burning these bodies, can be accomplished by the use of substances other than carbon, it is evident that these other substances also contain much phlogiston, and, moreover, that the phlogiston contained in all these substances is one and the same _principle_. What then, he asks, is this "principle" which can so escape, and be so restored by the action of various substances? He then proceeds as follows:--

"But when we inquire further, and endeavour to learn what notion was formed of the nature of this principle, and what qualities it was supposed to have in its separate state, we find this part of the subject very obscure and unsatisfactory, and the opinions very unsettled.

"The elder chemists, and the alchemists, considered sulphur as the universal inflammable principle, or at least they chose to call the inflammable part of all bodies, that are more or less inflammable, by the name of their sulphur.... The famous German chemist Becher was, I believe, the first who rejected the notion of sulphur being the principle of inflammability in bodies.... His notion of the nature of the pure principle of inflammability was afterwards more fully explained and supported by Professor Stahl, who, agreeably to the doctrine of Becher, represented the principle of inflammability as a dry substance, or of an earthy nature, the particles of which were exquisitely subtile, and were much disposed to be agitated and set in motion with inconceivable velocity.... The opinion of Becher and Stahl concerning this _terra secunda_, or _terra inflammabilis_, or _phlogiston_, was that the atoms of it are, more than all others, disposed to be affected with an excessively swift whirling motion (_motus vorticillaris_). The particles of other elementary substances are likewise liable to be affected with the same sort of motion, but not so liable as those of _terra secunda_; and when the particles of any body are agitated with this sort of motion, the body exhibits the phenomena of heat, or ignition, or inflammation according to the violence and rapidity of the motion.... Becher and Stahl, therefore, did not suppose that heat depended on the abundance of a peculiar matter, such as the matter of heat or fire is now supposed to be, but on a peculiar motion of the particles of matter....

"This very crude opinion of the earthy nature of the principle of inflammability appears to have been deduced from a quality of many of the inflammable substances, by which they resist the action of water as a solvent. The greater number of the earthy substances are little, or not at all, soluble in water.... And when Becher and Stahl found those compounds, which they supposed contained phlogiston in the largest quantity, to be insoluble in water, although the other matter, with which the phlogiston was supposed to be united, was, in its separate state, exceedingly soluble in that fluid, they concluded that _a dry nature, or an incapability to be combined with water_, was an eminent quality of their phlogiston; and this was what they meant by calling it an earth or earthy substance.... But these authors supposed, at the same time, that the particles of this dry and earthy phlogiston were much disposed to be excessively agitated with a whirling motion; which whirling motion, exerted in all directions from the bodies in which phlogiston is contained, produced the phenomena of inflammation. This appears to have been the notion formed by Becher and Stahl, concerning the nature of the principle of inflammability, or the phlogiston; a notion which seems the least entitled to the name of explanation of anything we can think of. I presume that few persons can form any clear conception of this whirling motion, or, if they can, are able to explain to themselves how it produces, or can produce, anything like the phenomena of heat or fire."

Black then gives a clear account of the experiments of Priestley and Lavoisier (see pp. 58, 59, and 87-89), which established the presence, in common air, of a peculiar kind of gas which is especially concerned in the processes of combustion; he emphasizes the fact that a substance increases in weight when it is burned; and he gives a simple and clear statement of that explanation of combustion which is now accepted by all, and which does not require that the existence of any principle of fire should be assumed.

It is important to note that Black clearly connects the _physical_ fact that heat is absorbed, or evolved, by a substance during combustion, with the _chemical_ changes which are brought about in the properties of the substance burned. He concludes with an admirable contrast between the phlogistic theory and the theory of Lavoisier, which shows how wide, and at the same time how definite, his conceptions were. Black never speaks contemptuously of a theory which he opposes.

"According to this theory" (_i.e._ the theory of Lavoisier), "the inflammable bodies, sulphur for example, or phosphorus, are simple substances. The acid into which they are changed by inflammation is a compound. The chemists, on the contrary" (_i.e._ the followers of Stahl), "consider the inflammable bodies as compounds, and the uninflammable matter as more simple. In the common theory the heat and light are supposed to emanate from, or to be furnished by, the burning body. But, in Mr. Lavoisier's theory, both are held to be furnished by the air, of which they are held to be constituent parts, or ingredients, while in its state of fire-supporting air."

Black was not a brilliant discoverer, but an eminently sound and at the same time imaginative worker; whatever he did he did well, but he did not exhaust any field of inquiry. Many of the facts established by him have served as the basis of important work done by those who came after him. The number of new facts added by Black to the data of chemistry was not large; but by his lectures--which are original dissertations of the highest value--he did splendid service in advancing the science of chemistry. Black possessed that which has generally distinguished great men of science, a marked honesty of character; and to this he added comprehensiveness of mental vision: he saw beyond the limits of the facts which formed the foundations of chemical science in his day. He was not a fact-collector, but a philosopher.

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JOSEPH PRIESTLEY, the son of Jonas Priestley, "a maker and dresser of woollen cloth," was born at Fieldhead, near Leeds, in the year 1733. His mother, who was the daughter of a farmer near Wakefield, died when he was seven years old. From that time he was brought up by a sister of his father, who was possessed of considerable private means.

Priestley's surroundings in his young days were decidedly religious, and evidently gave a tone to his whole after life. We shall find that Priestley's work as a man of science can scarcely be separated from his theological and metaphysical work. His cast of mind was decidedly metaphysical; he was altogether different from Black, who, as we have seen, was a typical student of natural phenomena.

The house of Priestley's aunt was a resort for all the Dissenting ministers of that part of the county. She herself was strictly Calvinistic in her theological views, but not wholly illiberal.

Priestley's early schooling was chiefly devoted to learning languages; he acquired a fair knowledge of Latin, a little Greek, and somewhat later he learned the elements of Hebrew. At one time he thought of going into trade, and therefore, as he tells us in his "Memoirs," he acquired some knowledge of French, Italian and High Dutch. With the help of a friend, a Dissenting minister, he learned something of geometry, mathematics and natural philosophy, and also got some smattering of the Chaldee and Syriac tongues.

At the age of nineteen Priestley went to an "academy" at Daventry. The intellectual atmosphere here seems to have been suitable to the rapid development of Priestley's mind. Great freedom of discussion was allowed; even during the teachers' lectures the students were permitted "to ask whatever questions and to make whatever remarks" they pleased; and they did it, Priestley says, "with the greatest, but without any offensive, freedom."

The students were required to read and to give an account of the more important arguments for and against the questions discussed in the teachers' lectures. Theological disputations appear to have been the favourite topics on which the students exercised their ingenuity among themselves. Priestley tells us that he "saw reason to embrace what is generally called the heterodox side of almost every question."

Leaving this academy, Priestley went, in 1755, as assistant to the Dissenting minister at Needham, in Suffolk. Here he remained for three years, living on a salary of about L30 a year, and getting more and more into bad odour because of his peculiar theological views.

From Needham he moved to Nantwich, in Cheshire, where he was more comfortable, and, having plenty of work to do, he had little time for abstruse speculations. School work engaged most of his time at Nantwich; he also began to collect a few scientific instruments, such as an electrical machine and an air-pump. These he taught his scholars to use and to keep in good order. He gave lectures on natural phenomena, and encouraged his scholars to make experiments and sometimes to exhibit their experiments before their parents and friends. He thus extended the reputation of his school and implanted in his scholars a love of natural knowledge.

In the year 1761 Priestley removed to Warrington, to act as tutor in a newly established academy, where he taught languages--a somewhat wide subject, as it included lectures on "The Theory of Languages," on "Oratory and Criticism," and on "The History, Laws, and Constitution of England." He says, "It was my province to teach elocution, and also logic and Hebrew. The first of these I retained, but after a year or two I exchanged the two last articles with Dr. Aikin for the civil law, and one year I gave a course of lectures on anatomy."

During his stay at Warrington, which lasted until 1767, Priestley married a daughter of Mr. Isaac Wilkinson, an ironmaster of Wrexham, in Wales. He describes his wife as "a woman of an excellent understanding much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous, feeling strongly for others and little for herself, also greatly excelling in everything relating to household affairs."

About this time Priestley met Dr. Franklin more than once in London. His conversation seems to have incited Priestley to a further study of natural philosophy. He began to examine electrical phenomena, and this led to his writing and publishing a "History of Electricity," in the course of which he found it necessary to make new experiments. The publication of the results of these experiments brought him more into notice among scientific men, and led to his election as a Fellow of the Royal Society, and to his obtaining the degree of LL.D. from the University of Edinburgh. In the year 1767 Priestley removed to Leeds, where he spent six years as minister of Millhill Chapel.

He was able to give freer expression to his theological views in Leeds than could be done in smaller places, such as Needham and Nantwich. During this time he wrote and published many theological and metaphysical treatises. But, what is of more importance to us, he happened to live near a brewery. Now, the accidental circumstances, as we call them, of Priestley's life were frequently of the greatest importance in their effects on his scientific work. Black had established the existence and leading properties of fixed air about twelve or thirteen years before the time when Priestley came to live near the brewery in Leeds. He had shown that this fixed air is produced during alcoholic fermentation. Priestley knowing this used to collect the fixed air which came off from the vats in the neighbouring brewery, and amuse himself with observing its properties. But removing from this part of the town his supplies of fixed air were stopped. As however he had become interested in working with airs, he began to make fixed air for himself from chalk, and in order to collect this air he devised a very simple piece of apparatus which has played a most important part in the later development of the chemistry of gases, or pneumatic chemistry. Priestley's _pneumatic trough_ is at this day to be found in every laboratory; it is extremely simple and extremely perfect. A dish of glass, or earthenware, or wood is partly filled with water; a shelf runs across the dish at a little distance beneath the surface of the water; a wide-mouthed bottle is filled with water and placed, mouth downwards, over a hole in this shelf. The gas which is to be collected in this bottle is generated in a suitable vessel, from which a piece of glass or metal tubing passes under the shelf and stops just where the hole is made. The gas which comes from the apparatus bubbles up into the bottle, drives out the water, and fills the bottle. When the bottle is full of gas, it is moved to one side along the shelf, and another bottle filled with water is put in its place. As the mouth of each bottle is under water there is no connection between the gas inside and the air outside the bottle; the gas may therefore be kept in the bottle until the experimenter wants it. (See Fig. 1. which is reduced from the cut in Priestley's "Air.")

Priestley tells us that at this time he knew very little chemistry, but he thinks that this was a good thing, else he might not have been led to make so many new discoveries as he did afterwards make.

Experimenting with fixed air, he found that water could be caused to dissolve some of the gas. In 1772 he published a pamphlet on the method of impregnating water with fixed air; this solution of fixed air in water was employed medicinally, and from this time we date the manufacture of artificial mineral waters.

The next six years of Priestley's life (1773-1779) are very important in the history of chemistry; it was during these years that much of his best work on various airs was performed. During this time he lived as a kind of literary companion (nominally as librarian) with the Earl of Shelburne (afterwards Marquis of Lansdowne.) His wife and family--he had now three children--lived at Calne, in Wiltshire, near Lord Shelburne's seat of Bowood. Priestley spent most of the summer months with his family, and the greater part of each winter with Lord Shelburne at his London residence; during this time he also travelled in Holland and Germany, and visited Paris in 1774.

In a paper published in November 1772, Priestley says that he examined a specimen of air which he had extracted from saltpetre above a year before this date. This air "had by some means or other become noxious, but," he supposed, "had been restored to its former wholesome state, so as to effervesce with nitrous air" (in modern language, to combine with nitric oxide) "and to admit a candle to burn in it, in consequence of agitation with water." He tells us, in his "Observations on Air" (1779), that at this time he was altogether in the dark as to the nature of this air obtained from saltpetre. In August 1774, he was amusing himself by observing the action of heat on various substances--"without any particular view," he says, "except that of extracting air from a variety of substances by means of a burning lens in quicksilver, which was then a new process with me, and which I was very proud of"--when he obtained from _red precipitate_ (oxide of mercury) an air in which a candle burned with a "remarkably vigorous flame." The production of this peculiar air "surprised me more than I can well express;" "I was utterly at a loss how to account for it." At first he thought that the specimen of _red precipitate_ from which the air had been obtained was not a proper preparation, but getting fresh specimens of this salt, he found that they all yielded the same kind of air. Having satisfied himself by experiment that this peculiar air had "all the properties of common air, only in much greater perfection," he gave to it the name of _dephlogisticated air_. Later experiments taught him that the same air might be obtained from red lead, from manganese oxide, etc., by the action of heat, and from various other salts by the action of acids.

Priestley evidently regards the new "dephlogisticated air" simply as very pure ordinary air; indeed, he seems to look on all airs, or gases, as easily changeable one into the other. He always interprets his experimental results by the help of the theory of phlogiston. One would indeed think from Priestley's papers that the existence of this substance phlogiston was an unquestioned and unquestionable fact. Thus, he says in the preface to his "Experiments on Air:" "If any opinion in all the modern doctrine concerning air be well founded, it is certainly this, that nitrous air is highly charged with phlogiston, and that from this quality only it renders pure air noxious.... If I have completely ascertained anything at all relating to air it is this." Priestley thought that "very pure air" would take away phlogiston from some metals without the help of heat or any acid, and thus cause these metals to rust. He therefore placed some clean iron nails in _dephlogisticated air_ standing over mercury; after three months he noticed that about one-tenth of the air in the vessel had disappeared, and he concluded, although no rust appeared, that the dephlogisticated air had as a fact withdrawn phlogiston from the iron nails. This is the kind of reasoning which Black described to his pupils as "mere waste of time and ingenuity." The experiment with the nails was made in 1779; at this time, therefore, Priestley had no conception as to what his _dephlogisticated air_ really was.

Trying a great many experiments, and finding that the new air was obtained by the action of acids on earthy substances, Priestley was inclined to regard this air, and if this then all other airs, as made up of an acid (or acids) and an earthy substance. We now know how completely erroneous this conclusion was, but we must remember that in Priestley's time chemical substances were generally regarded as of no very definite or fixed composition; that almost any substance, it was supposed, might be changed into almost any other; that no clear meaning was attached to the word "element;" and that few, if any, careful measurements of the quantities of different kinds of matter taking part in chemical actions had yet been made.

But at the same time we cannot forget that the books of Hooke and Mayow had been published years before this time, and that twenty years before Priestley began his work on airs, Black had published his exact, scientific investigation on fixed air.

Although we may agree with Priestley that, had he made himself acquainted with what others had done before he began his own experiments, he might not have made so many new discoveries as he did, yet one cannot but think that his discoveries, although fewer, would have been more accurate.

We are told by Priestley that, when he was in Paris in 1774, he exhibited the method of obtaining dephlogisticated air from _red precipitate_ to Lavoisier and other French chemists. We shall see hereafter what important results to science followed from this visit to Lavoisier.

Let us shortly review Priestley's answer to the question, "What happens when a substance burns in air?"

Beginning to make chemical experiments when he had no knowledge of chemistry, and being an extremely rapid worker and thinker, he naturally adopted the prevalent theory, and as naturally interpreted the facts which he discovered in accordance with this theory.

When a substance burns, phlogiston, it was said, rushes out of it. But why does rapid burning only take place in air? Because, said Priestley, air has a great affinity for phlogiston, and draws it out of the burning substance. What then becomes of this phlogiston? we next inquire. The answer is, obviously it remains in the air around the burning body, and this is proved by the fact that this air soon becomes incapable of supporting the process of burning, it becomes phlogisticated. Now, if phlogisticated air cannot support combustion, the greater the quantity of phlogiston in air, the less will it support burning; but we know that if a substance is burnt in a closed tube containing air, the air which remains when the burning is quite finished at once extinguishes a lighted candle. Priestley also proved that an air can be obtained by heating _red precipitate_, characterized by its power of supporting combustion with great vigour. What is this but common air completely deprived of phlogiston? It is dephlogisticated air. Now, if common air draws phlogiston out of substances, surely this dephlogisticated air will even more readily do the same. That it really does this Priestley thought he had proved by his experiment with clean iron nails (see p. 60).

Water was regarded as a substance which, like air, readily combined with phlogiston; but Priestley thought that a candle burned less vigorously in dephlogisticated air which had been shaken with water than in the same air before this treatment; hence he concluded that phlogiston had been taken from the water.

After Cavendish had discovered (or rather rediscovered) hydrogen, and had established the fact that this air is extremely inflammable, most chemists began to regard this gas as pure or nearly pure phlogiston, or, at least, as a substance very highly charged with phlogiston. "Now," said Priestley, "when a metal burns phlogiston rushes out of it; if I restore this phlogiston to the metallic calx, I shall convert it back into the metal." He then showed by experiment that when calx of iron is heated with hydrogen, the hydrogen disappears and the metal iron is produced.

He seemed, therefore, to have a large experimental basis for his answer to the question, "What happens when a substance burns?" But at a later time it was proved that iron was also produced by heating the calx of iron with carbon. The antiphlogistic chemists regarded fixed air as composed of carbon and dephlogisticated air; the phlogisteans said it was a substance highly charged with phlogiston. The antiphlogistic school said that calx of iron is composed of iron and dephlogisticated air; the phlogisteans said it was iron deprived of its phlogiston. Here was surely an opportunity for a crucial experiment: when calx of iron is heated with carbon, and iron is produced, there must either be a production of fixed air (which is a non-inflammable gas, and forms a white solid substance when brought into contact with limewater), or there must be an outrush of phlogiston from the carbon. The experiment was tried: a gas was produced which had no action on limewater and which was very inflammable; what could this be but phlogiston, already recognized by this very property of extreme inflammability? Thus the phlogisteans appeared to triumph. But if we examine these experiments made by Priestley with the light thrown on them by subsequent research, we find that they bear the interpretation which he put on them only because they were not accurate; thus, two gases are inflammable, but it by no means follows that these gases are one and the same. We must have more accurate knowledge of the properties of these gases.

The air around a burning body, such as iron, after a time loses the power of supporting combustion; but this is merely a qualitative fact. Accurately to trace the change in the properties of this air, it is absolutely necessary that exact measurements should be made; when this is done, we find that the volume of air diminishes during the combustion, that the burning body gains weight, and that this gain in weight is just equal to the loss in weight undergone by the air. When the inflammable gas produced by heating calx of iron with carbon was carefully and _quantitatively_ analyzed, it was found to consist of carbon and oxygen (dephlogisticated air), but to contain these substances in a proportion different from that in which they existed in fixed air. It was a new kind of air or gas; it was _not_ hydrogen.

This account of Priestley's experiments and conclusions regarding combustion shows how easy it is in natural science to interpret experimental results, especially when these results are not very accurate, in accordance with a favourite theory; and it also illustrates one of the lessons so emphatically taught by all scientific study, viz. the necessity of suspending one's judgment until accurate measurements have been made, and the great wisdom of then judging cautiously.

About 1779 Priestley left Lord Shelburne, and went as minister of a chapel to Birmingham, where he remained until 1791.

During his stay in Birmingham, Priestley had a considerable amount of pecuniary help from his friends. He had from Lord Shelburne, according to an agreement made when he entered his service, an annuity of L150 a year for life; some of his friends raised a sum of money annually for him, in order that he might be able to prosecute his researches without the necessity of taking pupils. During the ten years or so after he settled in Birmingham, Priestley did a great deal of chemical work, and made many discoveries, almost entirely in the field of pneumatic chemistry.

Besides the discovery of dephlogisticated air (or oxygen) which has been already described, Priestley discovered and gave some account of the properties of _nitrous air_ (nitric acid), _vitriolic acid air_ (sulphur dioxide), _muriatic acid air_ (hydrochloric acid), and _alkaline air_ (ammonia), etc.

In the course of his researches on the last-named air he showed, that when a succession of electric sparks is passed through this gas a great increase in the volume of the gas occurs. This fact was further examined at a later time by Berthollet, who, by measuring the increase in volume undergone by a measured quantity of ammonia gas, and determining the nature of the gases produced by the passage of the electric sparks, proved that ammonia is a compound of hydrogen and nitrogen, and that three volumes of the former gas combine with one volume of the latter to produce two volumes of ammonia gas.

Priestley's experiments on "inflammable air"--or hydrogen--are important and interesting. The existence of this substance as a definite kind of air had been proved by the accurate researches of Cavendish in 1766. Priestley drew attention to many actions in which this inflammable air is produced, chiefly to those which take place between acids and metals. He showed that inflammable air is not decomposed by electric sparks; but he thought that it was decomposed by long-continued heating in closed tubes made of lead-glass. Priestley regarded inflammable air as an air containing much phlogiston. He found that tubes of lead-glass, filled with this air, were blackened when strongly heated for a long time, and he explained this by saying that the lead in the glass had a great affinity for phlogiston, and drew it out of the inflammable air.