of the straitest orthodoxy, the boy’s striking natural ability led to
his being devoted to the profession of a minister of religion; and, in
1752, he was sent to the Dissenting Academy at Daventry—an institution
which authority left undisturbed, though its existence contravened the
law. The teachers under whose instruction and influence the young man
came at Daventry, carried out to the letter the injunction to “try all
things: hold fast that which is good,” and encouraged the discussion
of every imaginable proposition with complete freedom, the leading
professors taking opposite sides; a discipline which, admirable as
it may be from a purely scientific point of view, would seem to be
calculated to make acute, rather than sound, divines. Priestley tells
us, in his “Autobiography,” that he generally found himself on the
unorthodox side: and, as he grew older, and his faculties attained
their maturity, this native tendency towards heterodoxy grew with his
growth and strengthened with his strength. He passed from Calvinism to
Arianism; and finally, in middle life, landed in that very broad form
of Unitarianism, by which his craving after a credible and consistent
theory of things was satisfied.
On leaving Daventry, Priestley became minister of a congregation, first
at Needham Market, and secondly at Nantwich; but whether on account
of his heterodox opinions, or of the stuttering which impeded his
expression of them in the pulpit, little success attended his efforts
in this capacity. In 1761, a career much more suited to his abilities
became open to him. He was appointed “tutor in the languages” in the
Dissenting Academy at Warrington, in which capacity, besides giving
three courses of lectures, he taught Latin, Greek, French, and Italian,
and read lectures on the Theory of Language and Universal Grammar, on
Oratory, Philosophical Criticism, and Civil law. And it is interesting
to observe that, as a teacher, he encouraged and cherished in those
whom he instructed, the freedom which he had enjoyed, in his own
student days, at Daventry. One of his pupils tells us that,
“At the conclusion of his lecture, he always encouraged
his students to express their sentiments relative to the
subject of it, and to urge any objections to what he had
delivered, without reserve. It pleased him when any one
commenced such a conversation. In order to excite the
freest discussion, he occasionally invited the students
to drink tea with him, in order to canvass the subjects
of his lectures. I do not recollect that he ever showed
the least displeasure at the strongest objections that
were made to what he delivered, but I distinctly remember
the smile of approbation with which he usually received
them: nor did he fail to point out, in a very encouraging
manner, the ingenuity or force of any remarks that were
made, when they merited these characters. His object,
as well as Dr. Aikin’s, was to engage the students to
examine and decide for themselves, uninfluenced by the
sentiments of any other persons.”[11]
It would be difficult to give a better description of a model teacher
than that conveyed in these words.
From his earliest days, Priestley had shown a strong bent towards the
study of nature; and his brother Timothy tells us that the boy put
spiders into bottles, to see how long they would live in the same air—a
curious anticipation of the investigations of his later years. At
Nantwich, where he set up a school, Priestley informs us that he bought
an air pump, an electrical machine, and other instruments, in the use
of which he instructed his scholars. But he does not seem to have
devoted himself seriously to physical science until 1766, when he had
the great good fortune to meet Benjamin Franklin, whose friendship he
ever afterwards enjoyed. Encouraged by Franklin, he wrote a “History of
Electricity,” which was published in 1767, and appears to have met with
considerable success.
In the same year, Priestley left Warrington to become the minister of
a congregation at Leeds; and, here, happening to live next door to a
public brewery, as he says,
“I, at first, amused myself with making experiments on
the fixed air which I found ready-made in the process
of fermentation. When I removed from that house I was
under the necessity of making fixed air for myself; and
one experiment leading to another, as I have distinctly
and faithfully noted in my various publications on the
subject, I by degrees contrived a convenient apparatus
for the purpose, but of the cheapest kind.
“When I began these experiments I knew very little
of _chemistry_, and had, in a manner, no idea on the
subject before I attended a course of chemical lectures,
delivered in the Academy at Warrington, by Dr. Turner of
Liverpool. But I have often thought that, upon the whole,
this circumstance was no disadvantage to me; as, in this
situation, I was led to devise an apparatus and processes
of my own, adapted to my peculiar views; whereas, if I
had been previously accustomed to the usual chemical
processes, I should not have so easily thought of any
other, and without new modes of operation, I should
hardly have discovered anything materially new.”[12]
The first outcome of Priestley’s chemical work, published in 1772, was
of a very practical character. He discovered the way of impregnating
water with an excess of “fixed air,” or carbonic acid, and thereby
producing what we now know as “soda water”—a service to naturally,
and still more to artificially, thirsty souls, which those whose
parched throats and hot heads are cooled by morning draughts of
that beverage, cannot too gratefully acknowledge. In the same year,
Priestley communicated the extensive series of observations which his
industry and ingenuity had accumulated, in the course of four years,
to the Royal Society, under the title of “Observations on Different
Kinds of Air”—a memoir which was justly regarded of so much merit and
importance, that the Society at once conferred upon the author the
highest distinction in their power, by awarding him the Copley Medal.
In 1771 a proposal was made to Priestley to accompany Captain Cook
in his second voyage to the South Seas. He accepted it, and his
congregation agreed to pay an assistant to supply his place during
his absence. But the appointment lay in the hands of the Board of
Longitude, of which certain clergymen were members; and whether these
worthy ecclesiastics feared that Priestley’s presence among the ship’s
company might expose his Majesty’s Sloop _Resolution_ to the fate which
aforetime befell a certain ship that went from Joppa to Tarshish; or
whether they were alarmed lest a Socinian should undermine that piety
which, in the days of Commodore Trunnion, so strikingly characterised
sailors, does not appear; but, at any rate, they objected to Priestley
“on account of his religious principles,” and appointed the two
Forsters, whose “religious principles,” if they had been known to these
well-meaning but not far-sighted persons, would probably have surprised
them.
In 1772 another proposal was made to Priestley. Lord Shelburne,
desiring a “literary companion,” had been brought into communication
with Priestley by the good offices of a friend of both, Dr. Price;
and offered him the nominal post of librarian, with a good house
and appointments, and an annuity in case of the termination of the
engagement. Priestley accepted the offer, and remained with Lord
Shelburne for seven years, sometimes residing at Calne, sometimes
travelling abroad with the Earl.
Why the connection terminated has never been exactly known; but it
is certain that Lord Shelburne behaved with the utmost consideration
and kindness towards Priestley; that he fulfilled his engagements to
the letter; and that, at a later period, he expressed a desire that
Priestley should return to his old footing in his house. Probably
enough, the politician, aspiring to the highest offices in the state,
may have found the position of the protector of a man who was being
denounced all over the country as an infidel and an atheist somewhat
embarrassing. In fact, a passage in Priestley’s “Autobiography” on the
occasion of the publication of his “Disquisitions relating to Matter
and Spirit,” which took place in 1777, indicates pretty clearly the
state of the case:—
“(126) It being probable that this publication would be
unpopular, and might be the means of bringing odium on
my patron, several attempts were made by his friends,
though none by himself, to dissuade me from persisting
in it. But being, as I thought, engaged in the cause
of important truth, I proceeded without regard to any
consequences, assuring them that this publication should
not be injurious to his lordship.”
It is not unreasonable to suppose that his lordship, as a keen,
practical man of the world, did not derive much satisfaction from this
assurance. The “evident marks of dissatisfaction” which Priestley says
he first perceived in his patron in 1778, may well have arisen from
the peer’s not unnatural uneasiness as to what his domesticated, but
not tamed, philosopher might write next, and what storm might thereby
be brought down on his own head; and it speaks very highly for Lord
Shelburne’s delicacy that, in the midst of such perplexities, he
made not the least attempt to interfere with Priestley’s freedom of
action. In 1780, however, he intimated to Dr. Price that he should be
glad to establish Priestley on his Irish estates: the suggestion was
interpreted, as Lord Shelburne probably intended it should be, and
Priestley left him, the annuity of £150 a year, which had been promised
in view of such a contingency, being punctually paid.
After leaving Calne, Priestley spent some little time in London, and
then, having settled in Birmingham at the desire of his brother-in-law,
he was soon invited to become the minister of a large congregation.
This settlement Priestley considered, at the time, to be “the happiest
event of his life.” And well he might think so; for it gave him
competence and leisure; placed him within reach of the best makers
of apparatus of the day; made him a member of that remarkable “Lunar
Society,” at whose meetings he could exchange thoughts with such men
as Watt, Wedgewood, Darwin, and Boulton; and threw open to him the
pleasant house of the Galtons of Barr, where these men, and others of
less note, formed a society of exceptional charm and intelligence.[13]
But these halcyon days were ended by a bitter storm. The French
Revolution broke out. An electric shock ran through the nations;
whatever there was of corrupt and retrograde, and, at the same time,
a great deal of what there was of best and noblest, in European
society shuddered at the outburst of long-pent-up social fires.
Men’s feelings were excited in a way that we, in this generation,
can hardly comprehend. Party wrath and virulence were expressed in
a manner unparalleled, and it is to be hoped impossible, in our
times; and Priestley and his friends were held up to public scorn,
even in Parliament, as fomenters of sedition. A “Church-and-King” cry
was raised against the Liberal Dissenters; and, in Birmingham, it
was intensified and specially directed towards Priestley by a local
controversy, in which he had engaged with his usual vigour. In 1791,
the celebration of the second anniversary of the taking of the Bastille
by a public dinner, with which Priestley had nothing whatever to do,
gave the signal to the loyal and pious mob, who, unchecked, and indeed
to some extent encouraged, by those who were responsible for order, had
the town at their mercy for three days. The chapels and houses of the
leading Dissenters were wrecked, and Priestley and his family had to
fly for their lives, leaving library, apparatus, papers, and all their
possessions, a prey to the flames.
Priestley never returned to Birmingham. He bore the outrages and losses
inflicted upon him with extreme patience and sweetness,[14] and betook
himself to London. But even his scientific colleagues gave him a cold
shoulder; and though he was elected minister of a congregation at
Hackney, he felt his position to be insecure, and finally determined on
emigrating to the United States. He landed in America in 1794; lived
quietly with his sons at Northumberland, in Pennsylvania, where his
posterity still flourish; and, clear-headed and busy to the last, died
on the 6th of February 1804.
Such were the conditions under which Joseph Priestley did the work
which lay before him, and then, as the Norse Sagas say, went out of the
story. The work itself was of the most varied kind. No human interest
was without its attraction for Priestley, and few men have ever had
so many irons in the fire at once; but, though he may have burned his
fingers a little, very few who have tried that operation have burned
their fingers so little. He made admirable discoveries in science; his
philosophical treatises are still well worth reading; his political
works are full of insight and replete with the spirit of freedom; and
while all these sparks flew off from his anvil, the controversial
hammer rained a hail of blows on orthodox priest and bishop. While
thus engaged, the kindly, cheerful doctor felt no more wrath or
uncharitableness towards his opponents than a smith does towards his
iron. But if the iron could only speak!—and the priests and bishops
took the point of view of the iron.
No doubt what Priestley’s friends repeatedly urged upon him—that he
would have escaped the heavier trials of his life and done more for the
advancement of knowledge, if he had confined himself to his scientific
pursuits and let his fellow-men go their way—was true. But it seems to
have been Priestley’s feeling that he was a man and a citizen before
he was a philosopher, and that the duties of the two former positions
are at least as imperative as those of the latter. Moreover, there are
men (and I think Priestley was one of them) to whom the satisfaction
of throwing down a triumphant fallacy is as great as that which
attends the discovery of a new truth; who feel better satisfied with
the government of the world, when they have been helping Providence
by knocking an imposture on the head; and who care even more for
freedom of thought than for mere advance of knowledge. These men are
the Carnots who organise victory for truth, and they are, at least, as
important as the generals who visibly fight her battles in the field.
Priestley’s reputation as a man of science rests upon his numerous and
important contributions to the chemistry of gaseous bodies; and to form
a just estimate of the value of his work—of the extent to which it
advanced the knowledge of fact and the development of sound theoretical
views—we must reflect what chemistry was in the first half of the
eighteenth century.
The vast science which now passes under that name had no existence.
Air, water, and fire were still counted among the elemental bodies;
and though Van Helmont, a century before, had distinguished different
kinds of air as _gas ventosum_ and _gas sylvestre_, and Boyle and
Hales had experimentally defined the physical properties of air, and
discriminated some of the various kinds of aëriform bodies, no one
suspected the existence of the numerous totally distinct gaseous
elements which are now known, or dreamed that the air we breathe and
the water we drink are compounds of gaseous elements.
But, in 1754, a young Scotch physician, Dr. Black, made the first
clearing in this tangled backwood of knowledge. And it gives one a
wonderful impression of the juvenility of scientific chemistry to think
that Lord Brougham, whom so many of us recollect, attended Black’s
lectures when he was a student in Edinburgh. Black’s researches gave
the world the novel and startling conception of a gas that was a
permanently elastic fluid like air, but that differed from common air
in being much heavier, very poisonous, and in having the properties of
an acid, capable of neutralising the strongest alkalies; and it took
the world some time to become accustomed to the notion.
A dozen years later, one of the most sagacious and accurate
investigators who has adorned this, or any other, country, Henry
Cavendish, published a memoir in the “Philosophical Transactions,” in
which he deals not only with the “fixed air” (now called carbonic acid
or carbonic anhydride) of Black, but with “inflammable air,” or what we
now term hydrogen.
By the rigorous application of weight and measure to all his processes,
Cavendish implied the belief subsequently formulated by Lavoisier,
that, in chemical processes, matter is neither created nor destroyed,
and indicated the path along which all future explorers must travel.
Nor did he himself halt until this path led him, in 1784, to the
brilliant and fundamental discovery that water is composed of two gases
united in fixed and constant proportions.
It is a trying ordeal for any man to be compared with Black and
Cavendish, and Priestley cannot be said to stand on their level.
Nevertheless, his achievements are not only great in themselves, but
truly wonderful, if we consider the disadvantages under which he
laboured. Without the careful scientific training of Black, without the
leisure and appliances secured by the wealth of Cavendish, he scaled
the walls of science as so many Englishmen have done before and since
his day; and trusting to mother wit to supply the place of training,
and to ingenuity to create apparatus out of washing tubs, he discovered
more new gases than all his predecessors put together had done. He
laid the foundations of gas analysis; he discovered the complementary
actions of animal and vegetable life upon the constituents of the
atmosphere; and, finally, he crowned his work, this day one hundred
years ago, by the discovery of that “pure dephlogisticated air” to
which the French chemists subsequently gave the name of oxygen. Its
importance, as the constituent of the atmosphere which disappears in
the processes of respiration and combustion, and is restored by green
plants growing in sunshine, was proved somewhat later. For these
brilliant discoveries, the Royal Society elected Priestley a fellow and
gave him their medal, while the Academies of Paris and St. Petersburg
conferred their membership upon him. Edinburgh had made him an
honorary doctor of laws at an early period of his career; but, I need
hardly add, that a man of Priestley’s opinions received no recognition
from the universities of his own country.
That Priestley’s contributions to the knowledge of chemical fact were
of the greatest importance, and that they richly deserve all the praise
that has been awarded to them, is unquestionable; but it must, at the
same time, be admitted that he had no comprehension of the deeper
significance of his work; and, so far from contributing anything to the
theory of the facts which he discovered, or assisting in their rational
explanation, his influence to the end of his life was warmly exerted
in favour of error. From first to last, he was a stiff adherent of the
phlogiston doctrine which was prevalent when his studies commenced;
and, by a curious irony of fate, the man who by the discovery of what
he called “dephlogisticated air” furnished the essential datum for the
true theory of combustion, of respiration, and of the composition of
water, to the end of his days fought against the inevitable corollaries
from his own labours. His last scientific work, published in 1800,
bears the title, “The Doctrine of Phlogiston established, and that of
the Composition of Water refuted.”
When Priestley commenced his studies, the current belief was, that
atmospheric air, freed from accidental impurities, is a simple
elementary substance, indestructible and unalterable, as water was
supposed to be. When a combustible burned, or when an animal breathed
in air, it was supposed that a substance, “phlogiston,” the matter of
heat and light, passed from the burning or breathing body into it,
and destroyed its powers of supporting life and combustion. Thus, air
contained in a vessel in which a lighted candle had gone out, or a
living animal had breathed until it could breathe no longer, was called
“phlogisticated.” The same result was supposed to be brought about by
the addition of what Priestley called “nitrous gas” to common air.
In the course of his researches, Priestley found that the quantity of
common air which can thus become “phlogisticated,” amounts to about
one-fifth the volume of the whole quantity submitted to experiment.
Hence it appeared that common air consists, to the extent of
four-fifths of its volume, of air which is already “phlogisticated;”
while the other fifth is free from phlogiston, or “dephlogisticated.”
On the other hand, Priestley found that air “phlogisticated” by
combustion or respiration could be “dephlogisticated,” or have the
properties of pure common air restored to it, by the action of green
plants in sunshine. The question, therefore, would naturally arise—as
common air can be wholly phlogisticated by combustion, and converted
into a substance which will no longer support combustion, is it
possible to get air that shall be less phlogisticated than common air,
and consequently support combustion better than common air does?
Now, Priestley says that, in 1774, the possibility of obtaining air
less phlogisticated than common air had not occurred to him.[15] But
in pursuing his experiments on the evolution of air from various bodies
by means of heat, it happened that, on the 1st of August 1774, he threw
the heat of the sun, by means of a large burning glass which he had
recently obtained, upon a substance which was then called _mercurius
calcinatus per se_, and which is commonly known as red precipitate.
“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, very much like that enlarged
flame with which a candle burns in nitrous air, exposed
to iron or lime of sulphur; but as I had got nothing like
this remarkable appearance from any kind of air besides
this particular modification of nitrous air, and I knew
no nitrous acid was used in the preparation of _mercurius
calcinatus_, I was utterly at a loss how to account for
it.
“In this case also, though I did not give sufficient
attention to the circumstance at that time, the flame
of the candle, besides being larger, burned with more
splendour and heat than in that species of nitrous air;
and a piece of red-hot wood sparkled in it, exactly like
paper dipped in a solution of nitre, and it consumed very
fast—an experiment which I had never thought of trying
with nitrous air.”[16]
Priestley obtained the same sort of air from red lead, but, as he says
himself, he remained in ignorance of the properties of this new kind of
air for seven months, or until March 1775, when he found that the new
air behaved with “nitrous gas” in the same way as the dephlogisticated
part of common air does;[17] but that, instead of being diminished to
four-fifths, it almost completely vanished, and, therefore, showed
itself to be “between five and six times as good as the best common
air I have ever met with.”[18] As this new air thus appeared to be
completely free from phlogiston, Priestley called it “dephlogisticated
air.”
What was the nature of this air? Priestley found that the same kind of
air was to be obtained by moistening with the spirit of nitre (which he
terms nitrous acid) any kind of earth that is free from phlogiston, and
applying heat; and consequently he says: “There remained no doubt on
my mind but that the atmospherical air, or the thing that we breathe,
consists of the nitrous acid and earth, with so much phlogiston as is
necessary to its elasticity, and likewise so much more as is required
to bring it from its state of perfect purity to the mean condition in
which we find it.”[19]
Priestley’s view, in fact, is that atmospheric air is a kind of
saltpetre, in which the potash is replaced by some unknown earth.
And in speculating on the manner in which saltpetre is formed,
he enunciates the hypothesis, “that nitre is formed by a real
_decomposition of the air itself_, the _bases_ that are presented to
it having, in such circumstances, a nearer affinity with the spirit
of nitre than that kind of earth with which it is united in the
atmosphere.”[20]
It would have been hard for the most ingenious person to have wandered
farther from the truth than Priestley does in this hypothesis; and,
though Lavoisier undoubtedly treated Priestley very ill, and pretended
to have discovered dephlogisticated air, or oxygen, as he called it,
independently, we can almost forgive him, when we reflect how different
were the ideas which the great French chemist attached to the body
which Priestley discovered.