indulgence of my readers for any oversights they may discover in me as
a _philosopher_, or imperfections as a _writer_. I am far from
pretending to infallibility; but I have the satisfaction to reflect
that, imperfect as my works may be found to be, they are each as
perfect as I was able to make them....
"Upon this, as upon other occasions, I can only repeat that it is not
my _opinions_ on which I would be understood to lay any stress. Let
the _new facts_, from which I deduce them, be considered as my
_discoveries_, and let other persons draw better inferences from them
if they can. This is a new and a wide field of experiment and
speculation, and a premature attachment to hypothesis is the greatest
obstruction we are likely to meet with in our progress through it; and
as I think I have been pretty much upon my guard myself, I would
caution others to be upon their guard too."
These passages evidently were written under the influence of the feeling
of resentment with which he viewed the criticism to which his
speculations were subjected abroad. Fontana, Lavoisier and others were,
indeed, zealously engaged in using Priestley's own _facts_ to destroy
the conception by which he explained them. An appeal to the balance was
felt to be necessary, and Priestley, as a logician, could not resist it.
But he was no quantitative chemist: the habits of a Cavendish were quite
foreign to his genius: patient, scrupulous attention to numerical
accuracy was not one of his characteristics: he was one of the most
industrious of experimenters--delighting, indeed, in manipulation for
the mere sake of it, but withal hasty and superficial. It is nowhere
evident in his writings that his problems were attacked according to any
carefully-thought-out plan. He confesses indeed, on more than one
occasion, he tested the inflammability of one of his numerous "airs"
_because_ he had a lighted candle near him: had the candle not been
lighted it would not have occurred to him to do it. Priestley was, in
fact, a pioneer: he showed the existence of a new world for science, and
he himself roamed over a portion of it, like a second Joshua; but he had
not the experience or the aptitude to accurately map out even that
fraction.
There is little in the third volume of permanent value. It is largely an
account of a series of disconnected observations on the action of nitric
acid upon a variety of substances, which, however, led to no general
conclusions. It is, however, certain that if Priestley could have
induced himself to follow up certain of his observations he would have
arrived at _facts_ of far greater importance than those he actually
narrates. "_Speculation_," he said, by way of rejoinder to Lavoisier,
"is a cheap commodity. _New and important facts_ are most wanted, and
therefore of most value," and the new and important facts were within
his grasp if he had only reached out for them.
Another portion of the work is concerned with supplementary observations
on the gases treated of in the preceding volumes, partly by way of
correction and partly additional. Here and there we have a suggestive
passage, as in the paper on "Experiments on the Mixture of Different
Kinds of Air that have no Mutual Action," in which he thus clearly
indicates the principle of the intra-diffusion of gases.
"The result of my trials has been this general conclusion: that when
two kinds of air have been mixed it is not possible to separate them
again by any method of _decanting_ or pouring them off, though the
greatest possible care be taken in doing it. They may not properly
_incorporate_, so as to form a _third species of air_, possessed of
new properties; but they will remain equally diffused through the mass
of each other; and whether it be the upper or the lower part of the
air that is taken out of the vessel, without disturbing the rest, it
will contain an equal mixture of them both."
Another suggestive paper is on "Respiration and the Use of the Blood,"
which was read to the Royal Society on January 25, 1776, and appears in
the _Phil. Trans._, vol. lxvi. Priestley, of course, regarded
respiration as a _phlogistic process_, and "that the use of the lungs is
to carry off a putrid _effluvium_, or to discharge that phlogiston,
which had been taken into the system with the aliment, and has become,
as it were, _effete_, the air that is respired serving as a menstruum
for that purpose." This he thinks he has "proved to be effected by means
of the _blood_, in consequence of its coming so nearly into contact with
the air in the lungs, the blood appearing to be a fluid wonderfully
formed to imbibe and part with that principle which the chemists call
phlogiston, and changing its colour in consequence of being charged with
it or being freed from it." The _facts_ in this paper are for the most
part correctly stated, but the discoverer of oxygen led the world
woefully astray as to the part played by that gas in the phenomena of
respiration.
The fourth volume made its appearance in March 1779, with a dedication
to Sir George Savile, who had rendered Priestley the service of
introducing him and his invention of soda-water to the notice of the
Admiralty. In the preface, which is commendably short, he makes some
reference to the respite which he had promised himself and his readers,
but trusts, by way of extenuation, "it may be sufficient to allege the
instability of human purposes and pursuits." He had intended to devote
himself to metaphysics.
"But that kind of writing," he says, "is a thing of a very different
nature from this. I can truly say ... that single sections in this
work have cost me more than whole volumes of the other; so great is
the difference between writing from the head only and writing, as it
may be called, from the hands."
The fact was Priestley could not keep away from his laboratory.
"Having acquired a fondness for experiments, even slighter inducements
than I have had would have been sufficient to determine my conduct."
The preface is noteworthy for its plea for the position of experimental
science in the scheme of general education.
"If we wish to lay a good foundation for a philosophical taste, and
philosophical pursuits, persons should be accustomed to the sight of
experiments and processes in early life. They should, more especially,
be early initiated in the theory and practice of _investigation_, by
which many of the old discoveries may be made to be really _their
own_; on which account they will be much more valued by them. And, in
a great variety of articles, very young persons may be made so far
acquainted with everything necessary to be previously known as to
engage (which they will do with peculiar alacrity) in pursuits truly
original."
In the course of some observations on the effect "of impregnating oil of
vitriol with nitrous acid vapour" he discovered _nitrosulphuric acid_,
the so-called "Leaden Chamber Crystals," whose properties and behaviour
with water he describes with accuracy and even eloquence. Of these
crystals he says: "A more beautiful appearance can hardly be imagined,
and I am afraid I shall never see the like again." He also noticed the
formation of the dark brown compound which nitric oxide forms with a
solution of green vitriol, and adds:--
"To determine whether the phenomena attending the impregnation of the
solution of green vitriol with nitrous air depended in any measure
upon the seeming _astringency_ of that solution ... I impregnated a
quantity of _green tea_, which is also said to be astringent, with
nitrous air, but no sensible change of colour was produced in it."
He several times noticed the deep blue liquid which nitrogen peroxide
forms with cold water. He made many attempts to use nitric oxide as an
antiseptic, especially for culinary purposes. But the gastronomic
results with fowls and pigeons were not to his liking, although he says,
"my friend Mr Magellan ... had not so bad an opinion of this piece of
cookery as I had." One cannot read Priestley's description of his
multifarious experiments without being struck with the number of
occasions in which he just missed making discoveries of first-rate
importance. It is obvious that he had obtained chlorine without
recognising it, even before the news of Scheele's discovery reached this
country. He had also prepared, without knowing it, phosphoretted
hydrogen and phosphorous acid. At times, however, he can follow a clue
with remarkable perspicacity; as in his observation of the cause of the
"flouring" of mercury, and in his discovery of a method of removing lead
and tin from that metal.
The subject of "dephlogisticated air" naturally continued to interest
him, and he again returns to it in this volume, for he says:--
"As it sometimes amuses myself it may perhaps amuse others to look
back with me to the several steps in the actual progress of this
investigation, some of which I overlooked in my last account of it."
He points out, as already stated, that he must have had the new gas in
his hands as far back as November 1771, having obtained it from nitre.
He admits that he had no particular view in making his crucial
experiment of August 1, 1774,
"excepting 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 fond of."
He explains how he was led to his speculation that "this kind of air,
and consequently of atmospherical air, which is the same thing but in a
state of inferior purity," consists "of earth and spirit of nitre."
"But," he adds, "I have since seen reason to suspect that hypothesis,
plausible as it appears. Indeed, some of my late experiments would
lead me to conclude that there is no acid at all in pure air."
He then experiments with manganese, which Scheele, who independently
discovered oxygen, had already employed, and finds that it yields the
new air both when heated alone or with oil of vitriol. The production of
oxygen from manganese was contrary to his expectations as the substances
he had hitherto used, the _precipitate per se_ and the _red lead_ and
the nitre, had all been subjected to "the influence of the atmosphere,"
whereas "here was pure air from a substance which for anything that
appeared had always been in the bowels of the earth, and never had had
any communication with the external air." This led to the surmise that
possibly the expulsion of dephlogisticated air from such mineral
substances
"might assist in sustaining subterraneous fires.... The solution of
the phenomena of subterraneous fires would certainly be much easier on
the supposition of their supplying their own _pabulum_, by means of
dephlogisticated air contained in substances exposed to their heat. I
therefore desired Mr Landriani, who being in Italy had a good
opportunity of making inquiries on the subject, to inform me whether
any of those substances, and particularly _manganese_ be found in
their volcanoes; and his answer makes it rather probable that those
fires are, in part, sustained by this means."
The ease with which nitre parts with its oxygen on heating furnished
Priestley with the true explanation of its so-called "detonation,"
"concerning which," he says, "the most improbable conjectures have been
advanced by the most eminent philosophers and chemists." After a
reference to the hypothesis of Macquer, who assumes that what he calls
"a _nitrous sulphur_" is produced, Priestley points out that
"the doctrine of dephlogisticated air supplies the easiest solution
imaginable of this very difficult phenomenon. Let any person but
attend to the phenomena of the detonation of charcoal in nitre, and
that of dipping a piece of hot charcoal into a jar of dephlogisticated
air, and I think it will be impossible for him not to conclude that
the appearances are the very same and must have the same cause."
Of all the quantitative exercises performed by Priestley, by far the
most numerous depended upon his application of nitric oxide to measure
the "goodness" of air.
"When," he says, "I first discovered the property of nitrous air as a
test of the wholesomeness of common air, I flattered myself that it
might be of considerable practical use, and particularly that the air
of distant places and countries might be brought and examined together
with great ease and satisfaction; but I own that hitherto I have
rather been disappointed in my expectations from it.... I gave several
of my friends the trouble to send me air from distant places,
especially from manufacturing towns, and the worst they could find to
be actually breathed by the manufacturers, such as is known to be
exceedingly offensive to those who visit them; but when I examined
those specimens of air in Wiltshire, the difference between them and
the very best air in this county, which is esteemed to be very good,
as also the difference between them and specimens of the best air in
the counties in which these manufacturing towns are situated, was very
trifling.... I have frequently taken the open air in the most exposed
places in this country at _different times of the year_, and in
different states of the _weather_, etc., but never found the
difference so great as the inaccuracy arising from the method of
making the trial might easily amount to or exceed."
Other observers, less careful or more sanguine than Priestley, were,
however, successful in detecting the differences which prejudice led
them to anticipate. Thus Signor Marsilio Landriani of Milan, whose name
has already been mentioned in connection with the theory of
subterraneous fires, in the course of a tour through Italy had the
satisfaction of convincing himself
"that the air of all those places, which from the long experience of
the inhabitants has been reputed unwholesome, is found to be so to a
very great degree of exactness by the _eudiometer_.... The air of the
Pontine lakes, that of the Sciroccho at Rome (so very unwholesome),
that of the Campagna Romana, of the Grotto del Cane, of the Zolfatara
at Naples, of the baths of Nero at Baja, of the seacoast of Tuscany,
were all examined by me and found to be in such a state as daily
experience led me to expect."
Modern eudiometry, making use of methods of far greater precision than
were possible to Priestley, has confirmed his supposition that
atmospheric air is remarkably constant in composition, and that its
wholesomeness depends upon other causes than the relative amount of the
dephlogisticated air contained in it.
Perhaps the most important of the many papers contained in this volume
are those which relate to the "Melioration of Air by the Growth of
Plants," a subject to which Priestley gave attention, even whilst at
Leeds, in 1771. In these papers he clearly proves that this
"melioration" is connected with the green matter of leaves and that it
is dependent upon sunlight. This observation is of fundamental
importance and attracted much attention.
In the fifth volume, which was published in the spring of 1781, with a
dedication to Dr Heberden, when Priestley had moved to Birmingham, he
again returns to this subject. Practically all the experimental work to
which it relates was done whilst he was with Lord Shelburne, and mainly
at Calne. During the former parts of the summer of 1780 he suffered from
an illness which greatly interfered with his work, although he thinks
that during his incapacity for making experiments his "hints for the
farther prosecution of them are greatly accumulated." It cannot be said
that the five papers on the relations of vegetation to air, with which
the volume opens, added very materially to the fundamental fact which
Priestley had discovered. They furnished, however, additional evidence
of it and no doubt stimulated further inquiry. If his facts could not be
controverted, his explanations and surmises were at least open to
attack, and a number of observers, both here and abroad, busied
themselves with the problems of physiological botany thereby suggested.
As regards the subject of "air" in general, although a large number of
isolated observations are recorded in somewhat tedious detail, no new
fact of first-rate importance is apparent. The experiments are largely
supplementary to those in the preceding volumes and are for the most
explanatory or corroborative of them. Perhaps the most important are
those dealing with "the production of nitrous air in which a candle will
burn," by which is signified the gas we now know as nitrous oxide, but
which Priestley eventually termed _dephlogisticated nitrous air_. The
process he employed is no longer used in the production of this gas, but
it sufficed in his hands to determine its individuality without doubt.
Priestley's methods of experiment with his various "airs" were very
uniform. He tried their solubility in water, their power of supporting
or extinguishing flame, whether they were respirable, how they behaved
with acid and alkaline air, and with nitric oxide and inflammable air,
and lastly how they were affected by the electric spark. He occasionally
made attempts to weigh them, but his determinations of their relative
density were altogether untrustworthy. Indeed, it is evident from the
terms in which he speaks of these efforts that he was conscious of their
inadequacy. The result of submitting alkaline air (ammonia) to the
electric spark, whereby it is resolved into nitrogen and hydrogen,
surprised him not a little.
"There are few experiments the _rationale_ of which I less pretend to
understand than the production of genuine and permanent inflammable
air from alkaline air by means of the electric spark.... One query on
this subject is, whence comes the phlogiston, which is certainly a
principal ingredient in the constitution of inflammable air. Alkaline
air, indeed, contains phlogiston, because in the manner in which I
have generally produced it, it is itself partially inflammable; but it
is not nearly so much so as the inflammable air which is produced by
means of it. Besides, it will appear by the following experiments that
the quantity of the inflammable air far exceeds that of the alkaline."
Although Priestley clearly recognised the production of the inflammable
air, "in no respect to be distinguished from that which is extracted
from metals by acids," and inferred it must come from the alkaline air
("the production having its limits"), he failed to detect the other
constituent of ammonia. His determination of the actual increase in
volume was inaccurate, and his attempt to explain the phenomenon wholly
fallacious.
At the instigation of Mr Woulfe, whose name mainly lives in connection
with a useful piece of chemical apparatus, Priestley was encouraged to
hope that he would
"find something remarkable in the solution of _manganese_ in spirit of
salt. Mr Woulfe, however, in a very friendly manner, at the same time,
cautioned me with respect of the vapour that would issue from it, as
from his own experience he apprehended it was of a very dangerous
nature.... I cannot say that it was the apprehension of danger, but
rather having other things in view, that prevented my giving much
attention to the subject."
Priestley's experiments led to no decisive result: he of course
recognised the
"peculiar smell, exactly resembling that which is procured by
dissolving _red lead_ in the same acids.... On the application of heat
it was easy to perceive that air, or vapour, was expelled; but it was
instantly seized by the quicksilver.... This is a new field that is
yet before me."
Priestley never occupied that field. It is tolerably certain that both
Woulfe and he had unknowingly prepared _chlorine gas_, but the glory of
its discovery belongs to Scheele.
The paper "Of Sound in Different Kinds of Air" is worth quoting as
showing Priestley at his best:--
"Almost all the experiments that have hitherto been made relating to
_sound_ have been made in common air, of which it is known to be a
vibration, though it is likewise known to be capable of being
transmitted by other substances. There could be little doubt, however,
of the possibility of sound _originating_ in any other kind of air, as
well as being _transmitted_ by them; but the trial had not been
actually made, and I had an easy opportunity of making it.
"Besides, the experiments promised to ascertain whether the
_intensity_ of sound was affected by any other property of the air in
which it was made than the mere _density_ of it. For the different
kinds of air in which I was able to make the same sound, besides
differing in specific gravity, have likewise other remarkable chemical
differences, the influence of which with respect to sound would, at
the same time, be submitted to examination.
"Being provided with a piece of clock-work, in which was a bell, and a
hammer to strike upon it (which I could cover with a receiver, and
which, when it was properly covered up, I could set in motion by the
pressure of a brass rod going through a collar of leather), I placed
it on some soft paper on a transfer. Then taking a receiver, the top
of which was closed with a plate of brass, through which the brass rod
and collar of leathers was inserted, I placed the whole on the plate
of an air-pump, and exhausted the receiver of all the air that it
contained. Then removing this exhausted receiver, containing the piece
of clock-work, I filled it with some of those kinds of air that are
capable of being confined by water.... Then by forcing down the brass
rod through the collar of leathers I made the hammer strike the bell,
which it would do more than a dozen times after each pressure. And the
instrument was contrived to do the same thing many times successively
after being once wound up.
"Everything being thus prepared, I had nothing to do, after filling
the same receiver with each of the kinds of air in its turn, but
receding from the apparatus, while an assistant produced the sound, to
observe at what distance I could distinctly hear it. The result of all
my observations, as far as I could judge, was that the intensity of
sound depends solely upon the _density_ of the air in which it is
made, and not at all upon any chemical principle in its constitution.
"In inflammable air the sound of the bell was hardly to be
distinguished from the same in a pretty good vacuum; and this air is
ten times rarer than common air.
"In fixed air the sound was much louder than in common air, so as to
be heard about half as far again; and this air is in about the same
proportion denser than common air.
"In dephlogisticated air the sound was also sensibly louder than in
common air, and, as I thought, rather more than in the proportion of
its superior density; but of this I cannot pretend to be quite sure.
"In all these experiments the common standard was the sound of the
same bell in the same receiver, every other circumstance also being
the same; the air only being changed by removing the receiver from the
transfer and blowing through it, etc."
The sixth and last volume appeared in 1786 with a dedication to William
Constable, Esq., of Barton Constable.
In the preface Priestley is concerned to defend himself against the
charge that he occupies himself too much with Theology to the detriment
of Natural Philosophy. Theology, he pleads, is his original and proper
province, and for which, therefore, he may be allowed to have a
justifiable predilection. But as with Metaphysics, so with Theology.
Neither subject engrossed so much of his time as some persons imagined.
"I am particularly complained of at present as having thrown away so
much time on the composition of my _History of the Corruptions of
Christianity_, and of the _Opinions Concerning Christ_. But I can
assure them, and the nature of the thing, if they consider it, may
satisfy them, that the time I must necessarily have bestowed upon the
experiments, of which an account is contained in this single volume,
is much more than I have given to the _six_, of which the
above-mentioned works consist, and to all the controversial pieces
that I have written in defence of the former of them. The labour and
attention necessary to enable me to write single paragraphs in this
work have been more than was requisite to compose whole sections or
chapters of the former.... Besides, these different studies so relieve
one another that I believe I do more in each of them, by applying to
them alternately, than I should do if I gave my whole attention to one
of them only."
But Priestley's main defence rests "on the superior dignity and
importance of _theological studies_ to any other whatever." The whole
preface must be read in the light of Priestley's altered circumstances
and of his relations to the theological world, which, since his removal
to Birmingham, had greatly increased in weight and importance. As
already stated, he regarded himself as ordained to champion the cause of
religion among the persons to whom his writings as a natural philosopher
specially appealed. The author of the _Institutes of Natural and
Revealed Religion_ was the writer of the _Letters to a Philosophical
Unbeliever_ and, in an age of unbelief, the doughty antagonist of
Gibbon. Otherwise the incongruous mixture of Theology and Natural
Philosophy, of which the preface is made up, seems inexplicable.
To the historian of chemistry the last volume of the series is hardly
less interesting than any one of its predecessors, not so much as
affording knowledge of new "airs" as by reason of Priestley's relation
to the waning doctrine of phlogiston, and on account of the part that
his own work was playing, in spite of himself, in completing its
overthrow. The volume indeed significantly opens with "Experiments
relating to Phlogiston," a reprint with notes of his paper in the 73rd
volume of the _Philosophical Transactions_. Priestley truly says:--
"There are few subjects, perhaps none, that have occasioned more
perplexity to chemists than that of _phlogiston_, or, as it is
sometimes called, the principle _of inflammability_. It was the great
discovery of Stahl that this principle, whatever it be, is
transferable from one substance to another, how different soever in
their other properties, such as sulphur, wood, and all the metals, and
therefore is the same thing in them all. But what has given an air of
mystery to this subject has been that it was imagined that this
principle, or substance, could not be exhibited except in combination
with other substances, and could not be made to assume separately
either a fluid or solid form. It was also asserted by some that
phlogiston was so far from adding to the weight of bodies that the
addition of it made them really lighter than they were before; on
which account they chose to call it _the principle of levity_. This
opinion had great patrons.
"Of late it has been the opinion of many celebrated chemists, Mr
Lavoisier among others, that the whole doctrine of phlogiston has been
founded on mistake, and that in all cases in which it was thought that
bodies parted with the principle of phlogiston, they in fact lost
nothing, but on the contrary acquired something; and in most cases an
addition of some kind of air; that a _metal_, for instance, was not a
combination of two things, viz., an _earth_ and _phlogiston_, but was
probably a simple substance in its metallic state; and that the calx
is produced not by the loss of phlogiston, or of anything else, but by
the acquisition of air."
He then goes on to say that the arguments in favour of this opinion,
especially those which were drawn from the experiments of Lavoisier on
mercury, were "so specious" that he owns he was much inclined to adopt
it. But he was evidently loth to part company with a conception which
had hitherto been the central idea of his chemical creed, the very
key-stone of the structure which he was pleased to regard as his
philosophy. As an abstract conception, as the principle of levity, as
something which was the negation of mass and which gravity repelled,
phlogiston was eminently unsatisfactory. But what if phlogiston were an
entity? A ponderable substance, no matter how light? In that case
Stahl's generalisation might still afford salvation. "My friend, Mr
Kirwan"--a clever, ingenious Irishman, with a nimble wit and a facile
pen--supplied the hint--"Phlogiston was inflammable air"--and Priestley
by a series of experiments, faultless as to execution but utterly
fallacious as to interpretation, persuades himself that Kirwan is right
and that Mr Lavoisier's opinion and his "specious arguments" are
therefore to be discountenanced. The paper, in certain respects, is one
of the most noteworthy of Priestley's productions. The experiments are
original, ingenious and striking, but as an example of his inductive
capacity, or as an indication of its author's logical power, or of his
ability to try judicially the very issue he has raised, it is
significant only of the profound truth of his own words that
"we may take a maxim so strongly for granted that the plainest
evidence of sense will not entirely change, and often hardly modify,
our persuasions; and the more ingenious a man is, the more effectually
he is entangled in his errors, his ingenuity only helping him to
deceive himself by evading the force of truth."
The next paper in the volume, on "The Seeming Conversion of Water into
Air," is a record of experiments which cost Priestley much labour and
the Lunar Society, for a time, much mystification. Priestley eventually
detected the fallacy in the observation which originally induced him to
believe that it was possible to transmute water into a permanently
elastic fluid, but he got no further in his explanation than that air
has a faculty of passing through the pores of an earthern vessel "by
means of a power very different from that of pressure."
This and the third paper in the series are classical, and this partly by
reason of, and partly in spite of, their blunders, for they are the
record of the work upon which James Watt largely based his conjectures
concerning the real chemical nature of water, whereby his name has been
associated with that of Cavendish and Lavoisier as the true discoverer
of its composition. In the course of his inquiry Priestley studied the
action of steam upon red-hot iron by an arrangement generally similar to
that employed by Lavoisier, but his explanation of the phenomena is
essentially different from that of the French chemist, as may be seen
from the following quotation:--
"Since iron gains the same addition of weight by melting it in
_dephlogisticated air_, and also by the addition of _water_ when
red-hot, and becomes, as I have already observed, in all respects the
same substance, it is evident that this air or water, as existing in
the iron, is the very same thing; and this can hardly be explained but
upon the supposition that water consists of two kinds of air, _viz._,
inflammable and dephlogisticated."
This, however, is how Priestley actually does explain it:--
"When iron is melted in dephlogisticated air we may suppose that,
though part of its phlogiston escapes to enter into the composition of
the small quantity of fixed air which is then procured, yet enough
remains to form _water_ with the addition of the dephlogisticated air
which it has imbibed, so that this _calx_ of iron consists of the
intimate union of the pure _earth of iron_ and of _water_; and
therefore when the same calx, thus saturated with water, is exposed to
heat in inflammable air, this air enters into it, destroys the
attraction between the water and the earth, and revives the iron while
the water is expelled in its proper form.
"Consequently, in the process with _steam_, nothing is necessary to be
supposed but the entrance of the water and the expulsion of the
phlogiston belonging to the iron, no more phlogiston remaining in it
than what the water brought along with it, and which is retained as a
constituent part of the water or of the new compound."
No more striking illustration of how a man's ingenuity may help him to
deceive himself could be given than is afforded by this passage.
Priestley to the end of his days never got a just conception of the real
chemical constitution of water.
The remaining papers call for little comment. In the course of some
further inquiries Priestley discovered _sulphuretted hydrogen_, termed
by him _sulphurated inflammable air_, and which he prepared by the
action of oil of vitriol upon ferrous sulphide. This gas must of course
have been frequently obtained or perceived by him, and possibly by
others, as it is produced by a number of processes. Its characteristic
smell was associated with sulphur: it was thought to be nothing but
inflammable air modified or polluted by the accidental presence of
sulphur. It cannot be held that Priestley drew the same sharp
distinctions between the various kinds of inflammable air that we draw
to-day. To us they are essentially different substances. Priestley,
however, regarded them as in the main phlogiston combined or associated
with other substances which affected the character of their flames or
gave them different properties. In his opinion they were essentially the
same. This fact serves to explain what is otherwise incomprehensible,
and accounts for many of his mistakes.
The last paper in the volume, excluding the "Supplementary
Observations," has a special interest. It is entitled "Observations
relating to Theory," and is in fact Priestley's Confession of Faith in
the doctrine which enslaved and misled him throughout the whole of his
scientific career. But he makes it so hesitatingly and with so many
reservations that one wonders why he is constrained to make it at all.
He appears to think, however, that it is expected of him.
"It is always our endeavour, after making experiments, to generalise
the conclusions we draw from them, and by this means to form a
_theory_, or _system of principles_, to which all the facts may be
reduced, and by means of which we may be able to foretell the results
of future experiments.... In my former publications I have frequently
promised to give such a _general theory_ of the experiments in which
the different kinds of air are concerned, as the present state of our
knowledge of them will enable me to do. But, like Simonides with
respect to the question that was proposed to him concerning God, I
have deferred it from time to time; and indeed I am more than ever
disposed to defer it still longer, as I own that I am at present even
less able to give such a theory as shall satisfy myself than I was
some years ago; new difficulties having arisen, which unhinge former
theories, and more experiments being necessary to establish new ones.
"Fluctuating, however, as the present state of this branch of
knowledge is, I do not think that I can, on this occasion, entirely
decline giving some observations of a theoretical nature, and though I
cannot pretend to perform the whole of my promise, I shall give a
summary view of what appears to me to be the constituent parts of all
the kinds of air with which we are acquainted, and a more particular
account of the hypothesis concerning phlogiston, which is at present
more an object of discussion than anything else of a theoretical
nature."
Priestley then passes in review all the "airs" of which the chemistry of
his time had any knowledge, giving the _elements_ or constituent
principles of which he imagined them to be composed.
The only kind of air that he thinks to be properly _elementary_, and to
consist of a simple substance, is _dephlogisticated air_, with possibly
the addition of the principle of heat, which, as it is not probable that
it adds to the _weight_ of bodies, can hardly be called an _element_ in
their composition.
"Dephlogisticated air appears to be one of the elements of water, of
fixed air, of all the acids, and of many other substances which, till
lately, have been thought to be simple. The air of the atmosphere,
exclusive of a great variety of foreign impregnations, appears to
consist of dephlogisticated and phlogisticated air."
As regards _phlogisticated air_--the mephitic air of Rutherford, the
azote of Lavoisier, the nitrogen of Chaptal--Priestley, reasoning from
Cavendish's work, concluded that it was probably not elementary, but
"that it consists of nitrous acid and phlogiston; this acid having
always been produced by decomposing it with ... dephlogisticated air."
He is conscious, however, of the insufficiency of this hypothesis, and
suggests
"that the _acid principle_ is supplied by the dephlogisticated air,
while the nitrous air gives the base of the nitrous acid and
phlogiston; and then this [phlogisticated] air may perhaps be
considered as phlogiston combined not with all the necessary elements
of nitrous acid, but only what may be called the base of it, _viz._,
the dephlogisticated nitrous vapour, or something which when united to
dephlogisticated air will constitute nitrous acid."
"_Fixed air_ (carbonic acid) seems to be a compound of phlogiston and
dephlogisticated air." In other words, carbonic acid and water have,
according to Priestley, "the same elementary composition." "It is
something remarkable that two substances so different from each other as
_fixed air_ and _water_ should be analysed into the same principles. But
there is this difference between them, that water is the union not of
pure phlogiston but of inflammable air and dephlogisticated air."
Of the true nature of _inflammable air_, Priestley, as we have more than
once had occasion to point out, had only the vaguest notions.
"Inflammable air," he says, "seems now to consist of water and
inflammable air, which however seems extraordinary, as the two
substances are hereby made to involve each other, one of the
constituent parts of water being inflammable air, and one of the
constituent parts of inflammable air being water; and therefore, if
the experiments would favour it (but I do not see that they do so) it
would be more natural to suppose that water, like fixed air, consists
of phlogiston and dephlogisticated air in some different mode of
combination."
That Priestley to the last imagined that the various kinds of
inflammable air known to him were at bottom one and the same substance,
modified or affected by other substances, accidental and unessential,
might be proved by a number of passages. He says with respect to
inflammable air generally:--
"There is an astonishing variety in the different kinds of inflammable
air, the cause of which is very imperfectly known. The lightest, and
therefore, probably, the purest kind seems to consist of phlogiston
and water only. But it is probable that _oil_, and that of different
kinds, may be held in solution in several of them, and be the reason
of their burning with a lambent flame, and also of their being so
readily resolved into fixed air when they are decomposed with
dephlogisticated air; though _why_ this should be the case I cannot
imagine."
_Nitrous air_ (nitric oxide) he conceives to be a combination of a
dephlogisticated nitrous air and phlogiston, and that by adding to it
dephlogisticated air and water it is converted into nitrous acid.
_Dephlogisticated nitrous air_ (nitrous oxide) he conceives may, like
dephlogisticated air, be an elementary substance and to be formed by
depriving nitrous air of its phlogiston.
The various _acid airs_ (_e.g._, marine acid air, vitriolic acid air,
etc.) consist of the peculiar acids as vapours combined with phlogiston.
The _Alkaline air_ (ammonia) he thought to consist of inflammable air
and phlogisticated air (nitrogen),
"or of something capable of being converted into phlogisticated
air.... That water enters into the composition of alkaline air seems
necessary to be admitted, because it is decomposed into inflammable
air, which I cannot help thinking necessarily requires water. It
seems, however, clearly to be inferred ... that there is no occasion
to admit the _alkaline principle_ into the number of _elements_; the
_alkalinity_, as I may say, some way or other, arising from
phlogiston, or phlogisticated air, as _acidity_ arises from
dephlogisticated air."
After these theoretical speculations, "in which," he says, "I fear I
have not communicated much light, though it is as much as I have been
able to get," Priestley proceeds to make some observations relating to
_phlogiston_, "the existence of which is at present a great subject of
discussion with philosophers; some maintaining that there is no such
thing, and others holding the doctrine of Stahl on the subject."
"According to Stahl, phlogiston is a real substance, capable of being
transferred from one body to another; its presence or absence making a
remarkable difference in the properties of bodies, whether it add to
their _weight_ or not. Thus he concluded that oil of vitriol deprived
of water, and united to phlogiston, becomes sulphur; and that the
calces of metals, by the addition of the same substance, become
metals.... What is now contended for is that in the oil of vitriol
changing into sulphur something is lost and nothing gained, and also
that a calx becomes a metal by the loss of air only. And did facts
correspond to this theory it would certainly be preferable to that of
Stahl, as being more simple; there being one principle less to take
into our account in explaining the changes of bodies. But I do not
know of any case in which phlogiston has been supposed to enter into a
body, but there is room to suppose that something does enter into
it....
"What has been insisted upon, as most favourable to the exclusion of
phlogiston, is the revival of mercury without the addition of any
other substance from the precipitate _per se_. In this case it is
evident that mere _heat_ ... is sufficient to revive the metal. And as
what is expelled from this calx is the purest dephlogisticated air, it
has been said that mercury is changed into this calx by imbibing pure
air, and therefore becomes a metal again, merely in consequence of
parting with that air."
The dexterous Mr Kirwan, not long before he himself embraced the French
doctrine, furnished Priestley with an argument which satisfied him that
this cardinal fact can be accounted for without excluding phlogiston.
"Since therefore the supposition is exceedingly convenient, if not
absolutely necessary, to the explanation of many other facts in
chemistry, it is at least advisable not to abandon it."
"That calces do not become metals merely by parting with the air they
contain, is evident from my experiments on heating them in contact
with inflammable air, in which the inflammable air, or some necessary
part of it, is undoubtedly absorbed; and though a little moisture be
deposited in the process, it may well be supposed to be that which in
conjunction with phlogiston constituted the inflammable air. And what
can the other principle that is absorbed by the calx be but the same
thing which, when united to water, is recovered again from the metal
and found to be inflammable air having all the same properties with
that which was employed in the revival of it. Metals therefore are not
simple substances, but consist of their calces, and something else
which they take from inflammable air. And as the same may also be
taken from any combustible substance, it corresponds exactly to
Stahl's phlogiston, and therefore the doctrine of it is confirmed by
these experiments; that is, we must still say that in all combustible
substances there is a principle capable of being transferred to other
substances, which when united to the calces of metals makes them to be
metals, and which, united to oil of vitriol (deprived of its water)
makes it to be sulphur."
Thus was the ingenious man effectually entangled in his errors, his
ingenuity helping him to deceive himself by evading the force of truth.
To err is human. If Priestley saw through a glass darkly, and but dimly
discerned the truth, he at least strove, so far as in him lay, to reach
the light. Posterity forgives, and may well forget, his errors in
grateful recognition of the many noble services he rendered to our
common humanity, and in humbling recollection of the suffering and
sacrifice with which those services were requited.
FOOTNOTES
[1]The Gregorian Calendar was not adopted in Great Britain until 1751.
In 1752 eleven days were left out of the Calendar, September 3rd
being counted the 14th. The change of style probably accounts for
the confusion in the various dates of Priestley's birth given by
different writers. In Chalmers's _General Biographical Dictionary_
the date is given as March 18; in Allen's _American Biographical and
Historical Dictionary_ and in Thomson's _History of the Royal
Society_ as March 24; Corry, in his _Life of Priestley_, gives March
24; Hoefer, in his _Histoire de la Chimie_, gives March 30, probably
following Dumas's _Philosophie de Chimie_; Cuvier, in his _Eloge_,
says that he was born near Bristol in 1728! In a letter to Wedgwood,
dated March 23, 1783, Priestley says in a postscript "This day I
complete my half century."
[2]T. Wemyss Reid, _Memoir of John Deskin Heaton_, p. 7 _et seq._
[3]The "Great Frost," as it was called, which, beginning on December 26,
1739, continued with the greatest intensity till February 17, 1740.
Above London Bridge the Thames was completely frozen over, and
numerous booths were erected on it for selling liquor, etc., to the
multitudes who daily flocked there.
[4]_The Inquirer_, January 16, 1904.
[5]Dr Andrew Kippis, an eminent Presbyterian, was the minister of the
Prince's Street Chapel, Westminster, and had at his disposal funds
which he could employ in assisting young ministers in their
education and first settlement. Priestley enjoyed his friendship
through life. Kippis, who was the editor of the _Biographia
Britannia_, was elected into the Royal Society in 1779, and served
on its council.
[6]William Eyres of Warrington, who was one of the most remarkable
printers of his day, produced a number of works noted for their
typographic excellence and beauty. He printed, in addition to the
works above mentioned, the first editions of Mrs Barbauld's poems,
Gilbert Wakefield's _Lucretius_, and other well-known classics.
[7]She was then sixty-two, and lived twenty years longer.
[8]The lines were the well-known stanza:--
"Life! We've been long together
Through pleasant and through cloudy weather;
'Tis hard to part when friends are dear,
Perhaps 'twill cost a sigh, a tear;
Then steal away, give little warning.
Choose thine own time;
Say not good-night, but in some brighter clime
Bid me good-morning."
[9]He lies buried near Castlehead, in Cartmel, Lancashire, where his
monument, a pyramidal mausoleum containing some twenty tons of iron,
is a notable feature in the landscape. On it is the following
epitaph written by himself:--
"Delivered from persecution of malice and envy here rests John
Wilkinson, Iron Master, in certain hopes of a better state and
heavenly mansion, as promulgated by Jesus Christ, in whose Gospel he
was a firm believer. His life was spent in action for the benefit of
man, and he trusts in some degree to the glory of God."
[10]This portrait was formerly in the possession of Mrs Crouch,
Priestley's youngest sister, and, according to Mrs Bilbrough of
Gildersome (_née_ Ellen Priestley), was brought by Mrs Crouch,
"along with the old family clock from her father's, Fieldhead, when
she came to live here in 1787." The picture was once placed in the
window of a carver and gilder's shop at Leeds, when Priestley
stopped to look at it in passing by. A woman happened to be doing
the same, and, on seeing him, exclaimed, "Why, here's the fellow
himself!" A photographic copy of it was presented to the subscribers
to the Stephen Statue in the Oxford Museum.
[11]There is a pencil-drawing of the house, made by the son of Dr
Kenrick of Warrington, among the Yates papers in the possession of
the Royal Society.
[12]The Chapel, or "Meeting House" as it was called in Priestley's time,
adjoined the Alms House Garth and was erected in 1673, after the
passage of the Act of Indulgence. It was pulled down in 1847 and the
present Mill Hill Chapel erected on its site.
[13]"_Lectures on the Memory of the Just._ A Series of Discourses on the
Lives and Times of the Ministers of Mill Hill Chapel, Leeds."
[14]He had in John Lee, a native of Leeds and a man of about his own
age, who became Solicitor-General in 1782, a friend who offered to
further his interests in that matter. Priestley, in his
autobiography, says: "Mr Lee showed himself particularly my friend
at the time I left Lord Shelburne, assisting me in the difficulties
with which I was then pressed, and continuing to befriend me
afterwards by seasonable benefactions."
[15]Richard Brinsley Sheridan at that time represented Stafford in the
House of Commons. Both he and Fox sympathised with Priestley and
sought to secure him compensation for his losses.
[16]Withering's _Botanical Arrangement_, 2nd Ed. 3 vols. 1792.
[17]Mercuric oxide made by heating quicksilver in air.
[18]Nitrous oxide: _see_ p. 182.
[19]Mercuric oxide made by heating mercuric nitrate.
[20]See the author's _Essays in Historical Chemistry_--"Priestley,
Cavendish, Lavoisier and La Révolution Chimique."
INDEX
A
Aikin, Anna Lætitia, 33, 43;
her poem _Corsica_, 70, 75.
Aikin, John, 32, 33, 36, 121.
Aikin, Lucy, 38.
_Air, Experiments and Observations on Different Kinds of_, Priestley's
treatise on, 77, 103, 167 _et seq._
Air, Fixed, 175, 176;
Priestley's view of its nature, 220.
Air, Inflammable, 179, 220.
Alkaline Air (Ammonia), Priestley's isolation of, 185;
its properties, 186;
decomposition of by electricity, 210;
nature of, 221.
Ammonia Gas, Priestley's discovery of, 185;
its properties, 186.
Ammonium Sesquicarbonate, Synthesis of, by Priestley, 186.
B
Barbauld, Mrs (Anna Lætitia Aikin), 33, 160.
Belloc, Madame, account of the Priestleys, 2;
of the times of Priestley, 5;
her account of Mrs Priestley, 49.
Bewley, Richard, his mephitic julep, 79.
Birmingham Riots of 1791, 120 _et seq._
Bolton, H. Carrington, his account of the Lunar Society, 96.
Boulton, Matthew, founds the Lunar Society of Birmingham, 94.
Boyle, Robert, his recognition that different elastic fluids exist,
174.
Bright, Henry A., his account of the Warrington Academy, 34.
Burke, Edmund, his denunciation of Priestley, 147.
C
Canton, John, his school, 9, 62.
"Conversion of Water into Air, Seeming," Priestley's work on, 216.
_Corruptions of Christianity_, Priestley's, 106.
D
Darwin, Erasmus, his connection with the Birmingham Lunar Society, 94,
95.
Daventry Academy, 18.
Dephlogisticated Air (Oxygen), Priestley's discovery of, 184, 192 _et
seq._
_Doctrine of Phlogiston Established_, Priestley's last scientific
work, 162.
E
Edgeworth, Richard Lovell, his account of the Lunar Society, 96.
Electricity, History of, Priestley's, 63.
Enfield, William, 33, 43.
Eudiometry, 182, 207, 208.
Eyres, William, of Warrington, 38.
F
Faujar St Fond, his visits to Priestley, 103.
Fieldhead, 3.
Fluor Acid Air, 198.
Forster, John Reinhold, 33, 81.
Franklin Benjamin, 63, 91, 92;
his opinion of Priestley, 93.
G
Galton, Samuel, 94, 96.
Gases, diffusion of, 202.
"General History of the Christian Church to the Fall of the Western
Empire," 109, 163.
Gordon, Rev. Alexander, his account of Priestley as a theologian, 109.
H
Harrison, Frederic, on Priestley, 1.
Horner, Leonard, his account of the Lunar Society, 102.
Hutton, Miss, her account of Priestley as a preacher, 105.
I
India-rubber, its use for erasing lead-pencil marks, 72.
J
"Jesus and Socrates compared," Priestley's Essay, 163.
K
Keighley, Mrs Sarah, 3, 8, 12.
Keir, Captain, 94, 127.
Kippis, Dr Andrew, 24, 26.
Kirwan, Richard, 215, 223.
L
Lavoisier, his relations to the discovery of oxygen, 195, 196, 198,
201, 215.
"Letters to a Philosophical Unbeliever," Priestley's, 85.
Lindsey, Theophilus, his friendship for Priestley, 73, 106.
Lunar Society of Birmingham, 94 _et seq._
M
Magellan, or Magalhæns, 84, 205.
Marine Acid Air (Hydrogen Chloride), Priestley's discovery of, 184.
"Melioration of Air, by the Growth of Plants," Priestley's paper on,
208.
N
Nitrosulphuric Acid, discovery of, 204.
Nitrous Air (Nitric Oxide), Priestley's work on, 181;
his application of it to Eudiometry, 182;
nature of, 221.
Nitrous Acid, discovery of, 182;
nature of, 221.
O
Oxygen, Priestley's discovery of, 184, 192 _et seq._
P
Paoli, Pascal, 70 _et seq._
Parr, Dr Samuel, 113, 144.
Phlogisticated Air (Nitrogen), Priestley's theory of its constitution,
220.
Phlogiston, 98, 100, 162, 214, 222.
Pneumatic Trough, the, 176.
Price, Dr Richard, 61.
Pringle, Sir John, address on award of Copley Medal to Priestley, 80.
Priestley, Jonas, father of Joseph Priestley, 2.
Priestley, Joseph, his birth, 1;
ancestry, 2;
education, 8;
becomes an Arminian, 14;
goes to Daventry, 17;
his studies there, 18;
begins his _Institutes of Natural and Revealed Religion_, 20, 68;
his "thorn in the flesh," 21;
goes to Needham, 22;
his privations there, 23;
goes to Nantwich, 30;
starts a school, 31;
removes to Warrington, 32;
marries, 47;
is ordained, 49;
his _English Grammar_, 32, 51;
_Essay on Education_, 53;
_Lectures on History and General Policy_, 55;
his _Chart of Biography_, 60;
is made Doctor of Laws of Edinburgh, 60;
publishes his _History and Present State of Electricity_, 64;
goes to Leeds, 66;
becomes a Unitarian, 69;
_Theory and Practice of Perspective_, 72;
_History of Discoveries Relating to Vision, Light and Colours_,
73;
his literary characteristics, 74;
his invention of soda-water, 77 _et seq._;
is awarded the Copley Medal, 80;
becomes librarian to Lord Shelburne, 82;
visits the continent, 84;
his _Disquisitions relating to Matter and Spirit_, 86;
_Experiments and Observations on Different Kinds of Air_, 77, 103;
leaves Lord Shelburne, 88;
removes to Birmingham, 93;
his merits as a preacher, 105;
his _History of the Corruptions of Christianity_, 106;
his merits as a theologian, 109;
his love of literature, 111;
his physical characteristics, 114;
his habits of life, 114;
his mental characteristics, 117;
destruction of his house at Birmingham, 120 _et seq._;
his address to the inhabitants of Birmingham, 134;
his account of the riots, 143;
succeeds Dr Price as minister at Hackney, 146;
withdraws from the Royal Society, 147;
determines to leave the country, 149;
his arrival in New York, 155;
settles in Northumberland, 155;
death of Mrs Priestley, 158;
his last days, 162;
his _Doctrine of Phlogiston Established_, 162;
his death, 164.
Priestley, Timothy, 2, 7.
R
Respiration, and the use of the Blood, Priestley's paper on, 203.
Robinson, Henry Crabb, his account of Mrs Barbauld, 46.
Russell, Martha, her account of the Birmingham Riots of 1791, 128.
Russell, William, 105, 126, 149.
S
Sal Ammoniac, Synthesis of, by Priestley, 186.
Scheele, an independent discoverer of Oxygen, 206;
discovers Chlorine, 211.
Schimmelpenninck's, Mrs, account of Priestley, 96.
Seddon, John, 34, 42.
Shelburne, Lord (Marquis of Lansdowne), 82.
Silicon Fluoride, Priestley's preparation of, 198;
its decomposition by water, 199.
Soda-water, invention of, 77.
Sound in Air, 211.
Sozzini, Fausto, 69, 108.
Stahl, author of the hypothesis of phlogiston, 222.
Sulphur dioxide (Vitriolic Acid Air), Priestley's discovery of, 190.
Sulphurated inflammable air (sulphuretted hydrogen), Priestley's
isolation of, 218.
Sulphuretted hydrogen, Priestley's isolation of, 218.
T
Taylor, Dr John, of Norwich, 36.
Theological Repository, the, 68, 105.
Turner, Matthew, teacher of chemistry at the Warrington Academy, 51,
76.
V
Vegetation, relation of to Air, Priestley's discovery of, 178, 208.
Vitriolic Acid Air (sulphur dioxide), Priestley's isolation of, 189.
W
Warrington Academy, the, 33 _et seq._
Water Controversy, the, 97.
Watt, James, his connection with the Lunar Society, 94;
discovery of Composition of Water, 97.
Wedgwood, Josiah, his friendship for Priestley, 55, 83, 137.
Wilkes and Liberty, Priestley's pamphlet on, 73, 152.
Wilkinson, John, 48, 138.
Wilkinson, Mary (Mrs Priestley), 49, 158;
Madame Belloc's account of her, 49.
Withering, Dr, 94, 101, 148.
Wordsworth, William, his admiration of Mrs Barbauld's literary work,
46.
_Colston & Coy. Limited, Printers, Edinburgh._
Transcriber's Notes
--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--Created cover and spine images based on elements in the book.
--Silently corrected a few palpable typos.
--In the text versions only, text in italics is delimited by
_underscores_.
End of Project Gutenberg's Joseph Priestley, by Thomas Edward Thorpe