Priestley as a man of science--His characteristics as a
philosopher--_Experiments and Observations on Different Kinds of
Air_--His discovery of the influence of vegetation on vitiated
air--Atmospheric air not elementary--His researches on nitric
oxide--Eudiometry--Nitrous oxide--Discovers hydrogen
chloride--Prepares oxygen from nitre (1771)--Isolates ammonia
gas--Discovers sulphur dioxide--Dephlogisticated air
(oxygen)--Discovers silicon fluoride--Intra-diffusion of
gases--Respiration--Priestley's opinions of the value of experimental
science in education--Discovers nitrosulphuric acid--Notes the
constancy of composition of the atmosphere--Prepares chlorine--Sound
in "air"--Experiments relating to phlogiston--The seeming conversion
of water into air--Watt and the compound nature of water--Discovers
sulphuretted hydrogen--Priestley's confession of faith in phlogiston.
Priestley's position in the history of science mainly rests on his
discoveries in pneumatic chemistry. The course of inquiry which he began
at Leeds was continued by him, with characteristic assiduity and
conspicuous success, at Calne, and his labours added largely to the
number of the aeriform bodies which were clearly recognised as distinct
substances, essentially differing from each other, and not merely
modifications of a common principle, modified or affected by properties
more or less fortuitous and accidental. The old idea of the nature of
"air" had its origin in the doctrine of the Four Elements. It is
Priestley's merit that he, more than any man of his time, contributed to
the overthrow of this conception as the basis of a philosophical system
of the constitution of the material universe. Although Priestley could
not be unmindful that his claim to scientific fame was to be found in
the succession of volumes which he called _Experiments and Observations
on Different Kinds of Air_, the very title suggests that he, at all
events in the outset, was hardly conscious of the magnitude and true
significance of his work. Priestley was in no real sense a speculative
philosopher: he was indeed pre-eminently the type of man whom Hobbes
disparaged as an "experimentarian philosopher," and an experimentarian
philosopher he remained to the end of his days. He was aware of his
limitations, and many passages from his works, and especially from his
correspondence, might be quoted in proof of this fact. His simple,
unaffected candour was indeed one of the charms of his character and the
secret of much of his influence. It is reflected in every page of his
scientific writings. His own discoveries, taken collectively, did more
than those of any one of his contemporaries to uproot and destroy the
only generalisation by which his immediate predecessors had sought to
group and connect the phenomena of chemistry, but he was wholly unable
to perceive this fact. A patient and industrious observer, absolutely
truthful, and, as he hoped and believed, unbiassed and impartial, he was
nevertheless entirely lacking in the higher qualities of the imagination
or in that power of divination which is the characteristic of men of the
type of Newton. The contrast between Priestley--the social, political
and theological reformer, always in advance of his times, receptive,
fearless and insistent; and Priestley the man of science--timorous and
halting when he might well be bold, conservative and orthodox when
almost every other active worker was heterodox and progressive--is most
striking. And yet, such is the irony of circumstance, Priestley's name
mainly lives as that of a chemical philosopher. When men have desired to
do him honour, and have sought to perpetuate his memory by statues in
public places, he is generally represented as making a chemical
experiment. In reality, great as Priestley's merit is as an
experimentarian philosopher, his greater claim on our regard and esteem
rests upon his struggles and his sufferings in the cause of civil,
political and religious liberty.
The years which Priestley spent at Calne constitute the most fruitful
period of his scientific career. Practically all that he did in the way
of solid achievement and of addition to the armoury of science was
effected during that time. Although, after leaving Lord Shelburne, he
continued to pursue scientific inquiry with his wonted zeal and
industry, doubtless adding thereby to his fame among his contemporaries,
posterity has set the true measure of appreciation to his later efforts.
He doubtless made many hundreds of experiments in connection with more
or less well-defined trains of inquiry; nevertheless, it cannot be
maintained that during his subsequent period he added many first-rate
facts to our knowledge, or indeed discovered any facts at all comparable
in importance with those he ascertained during his life in Wiltshire. On
the contrary, what he did observe--as for example the seeming conversion
of water into air--too frequently led him astray and was the cause of
error to himself and others. Thus Watt's claim to be considered as an
independent, if not the first and true, discoverer of the real chemical
nature of water is based upon Priestley's experimental blunders. Watt
was undoubtedly accurate in his surmise, but the surmise was right in
spite of, and not by reason of, Priestley's experimental evidence.
Priestley recorded his experiments with such fulness that it is now easy
to perceive where he went wrong. He was constantly on the verge of a
discovery, sometimes indeed of a discovery of cardinal importance, but
as constantly it eluded his grasp. The experiments on the seeming
conversion of water into air might have led him, when he got over his
chagrin on the detection of the real cause of his error, to the
recognition of the underlying truth in it, namely, the principle of the
diffusion of gases. He was, of course, familiar with the fact that the
various gases he discovered, or which were known to him, differed in
relative density, and he knew perfectly well that they tended to escape
from the bottles in which they were contained if these were uncovered
and freely exposed to the air. But, so far as we can learn, he never
seems to have pondered on these facts, or noted their connection with
the phenomena he observed in the course of his many experiments with
Wedgwood's retorts, and of the interchange of the water vapour he
introduced into them with the gases of the fire which heated them. And
yet, had he perceived even a glimmer of the truth he had sufficient
means at his disposal, and sufficient knowledge from his own work and
that of his contemporaries, to make the great step which it was reserved
to Graham to accomplish half a century later.
Whilst the chief importance of the _Experiments and Observations on
Different Kinds of Air_ is that it is Priestley's _magnum opus_, to his
biographer it has the additional interest of affording an insight into
the personal character and intellectual attributes of its author. Few
writers on scientific subjects have ever taken their readers so
completely into their confidence as Priestley. Whatever he knows or
thinks he tells: doubts, perplexities, blunders are set down with the
most refreshing candour; one forgives the prolixity and occasional
tediousness, even the little touches of self-satisfaction, in view of
the transparent honesty of purpose, the single-minded pursuit of truth
for its own sake, wholly apart from preconception or bias of dogma which
shine on every page. As key-notes to character, even the dedications and
prefaces to the several volumes have their peculiar value and charm, as
evidence of the workings of an ingenuous mind.
The publication of the six volumes comprising the original work--the
edition of greatest value to Priestley's biographer--extended from 1775
to 1786. Although the space at our disposal precludes any attempt at a
full account of the contents, it is necessary to set these out in such
detail as may serve to afford a just idea of their value, and with such
comment as may be necessary to elucidate their significance.
In the preface to the first volume, which made its appearance in 1775,
with a dedication to Lord Shelburne, Priestley thinks it necessary to
explain why he has decided, contrary to his original intention, but with
the approbation of the President and of his friends in the Royal
Society, not to send them any more papers on the subject of "Air" at
present but to make immediate publication of all he has done with
respect to it. In view, he says, of the rapid progress that has been
made and may be expected to be made in this branch of knowledge,
"unnecessary delays in the publication of experiments relating to it are
peculiarly unjustifiable."
"When, for the sake of a little more reputation, men can keep brooding
over a new fact, in the discovery of which they might possibly have
very little real merit, till they think they can astonish the world
with a system as complete as it is new, and give mankind a prodigious
idea of their judgment and penetration, they are justly punished for
their ingratitude to the fountain of all knowledge, and for the want
of a genuine love of science and of mankind in finding their boasted
discoveries anticipated and the field of honest fame pre-occupied by
men who, from a natural ardour of mind, engage in philosophical
pursuits, and with an ingenuous simplicity immediately communicate to
others whatever occurs to them in their inquiries."
Priestley's productions, from the very nature of the case make no
pretensions to completeness.
"In completing one discovery we never fail to get an imperfect
knowledge of others of which we could have no idea before, so that we
cannot solve one doubt without creating several new ones."
He farther observes that a person who means to serve the cause of
science effectually must hazard his own reputation so far as to risk
even _mistakes_ in things of less moment.
"Among a multiplicity of new objects and new relations some will
necessarily pass without sufficient attention; but if a man be not
mistaken in the principal objects of his pursuits he has no occasion
to distress himself about lesser things.
"In the progress of his inquiries he will generally be able to rectify
his own mistakes; or if little and envious souls should take a
malignant pleasure in detecting them for him and endeavouring to
expose him, he is not worthy of the name of a philosopher if he has
not strength of mind sufficient to enable him not to be disturbed at
it. He who does not foolishly affect to be above the failings of
humanity will not be mortified when it is proved that he is but a
man."
He made it a rule to disclose the real views with which he made his
experiments. Although, he says, by following a contrary maxim he might
have acquired a character of greater sagacity, he thought that two good
ends were secured by his method--one as tending to make his narrative
more interesting, and the other as encouraging other adventurers in
experimental philosophy by showing them that by pursuing even false
lights real and important truths may be discovered, and that in seeking
one thing we often find another. He believes, however, that he writes
more concisely than is usual with those who publish accounts of their
experiments, and in thus refraining from swelling his book "to a pompous
and respectable size" he trusts he will earn the gratitude of those
philosophers who, having but little time to spare for _reading_, which
is always the case with those who _do_ much themselves, will thereby be
kept not too long from their own pursuits. He then comments on what he
justly considers the amazing improvements in natural knowledge which
have been made within the century, and contrasts these with the
comparative poverty as regards scientific results of the many preceding
ages, which yet abounded with men who had no other object but study; and
he rejoices to think that this rapid progress of knowledge, extending
itself not this way or that way only, but in all directions, will be the
means of extirpating all error and prejudice and of putting an end to
all undue and usurped authority in the business of religion as well as
of science.
"It was ill policy in Leo the Tenth to patronise polite literature. He
was cherishing an enemy in disguise. And the English hierarchy (if
there be anything unsound in its constitution) has equal reason to
tremble even at an air-pump or an electrical machine."
He regrets that the rich and great in this country, unmindful of the
example of Bacon, give less attention to these matters than do men of
rank and fortune in other countries: he contrasts the pleasure of the
pursuit of science with the pains and penalties of the pursuit of
politics.
"If extensive and lasting fame be at all an object, literary, and
especially scientifical, pursuits are preferable to political ones in
a variety of respects.... If extensive usefulness be the object,
science has the same advantage over politics. The greatest success in
the latter seldom extends farther than one particular country and one
particular age, whereas a successful pursuit of science makes a man
the benefactor of all mankind and of every age. How trifling is the
fame of any statesman that this country has ever produced to that of
Lord Bacon, of Newton, or of Boyle; and how much greater are our
obligations to such men as these than to any other in the whole
_Biographia Britannica_."
It would be interesting to know the sentiments of Lord Shelburne, then
in the cold shade of retirement, as he perused these passages, and
whether he realised the truth of the little homily from his "tame
philosopher."
The preface is followed by an introduction, in which Priestley gives a
rapid and confessedly imperfect survey of the state of knowledge
concerning "air" prior to 1774. He gives to Boyle the credit of first
clearly recognising that elastic fluids exist differing essentially from
the air of the atmosphere, but agreeing with it in the properties of
weight, elasticity and transparency. But he also points out that two
remarkable kinds of factitious air had long been known to miners, viz.,
_choke damp_, which is heavier than air, which lies at the bottom of
pits, extinguishes flame and kills animals; and the other, called _fire
damp_, which is lighter than common air, is found, therefore, near the
roofs of subterraneous places and is liable to take fire and explode
like gunpowder. "The word _damp_ signifies _vapour_ or _exhalation_ in
the German and Saxon languages."
"Air of the former kind, besides having been discovered in various
caverns, particularly the Grotta del Cane in Italy, had also been
observed on the surface of fermenting liquors, and had been called
_gas_ (which is the same with _geist_, or _spirit_) by Van Helmont and
other German chemists; but afterwards it obtained the name of _fixed
air_, especially after it had been discovered by Dr Black of Edinburgh
to exist, in a fixed state, in alkaline salts, chalk, and other
calcareous substances."
Black's work is dealt with in half a dozen lines, and a passing
reference is made to Macbride and Brownrigg. A very imperfect account is
given of the work of Hales, although it is stated that "his experiments
are so numerous and various that they are justly esteemed to be the
solid foundation of all our knowledge of this subject." This section
concludes with the mention of Cavendish's determinations of the relative
weights of fixed air (carbon dioxide), and inflammable air from metals
(hydrogen), and of Lane's observations that water charged with carbonic
acid will dissolve iron, "and thereby become a strong chalybeate."
Priestley was the last man in the world to seek to disparage the work of
his predecessors or to minimise what was due to them. In reality he had
the intention, as he distinctly states, to write at his leisure the
history and present state of discoveries relating to air, in a manner
similar to his _History of Electricity_, and of the _Discoveries
Relating to Vision, Light and Colours_, when no doubt he would have done
full justice to all concerned. In the meantime he gives only such
particulars as are necessary, in his judgment, to the understanding of
his own work.
The remaining section of the introduction deals with his method of
experimenting and with the apparatus he employed. It is of historical
interest as containing a description of that most useful article of
chemical furniture, his well-known pneumatic trough. He explains its use
and gives details of his modes of manipulation. What an advance these
were in simplicity, ingenuity and convenience can only be fully realised
by comparing his methods with those of Hales. Not the least of
Priestley's services to science were the improvements he effected in
that section of operative chemistry which is concerned with the
preparation, collection and storage of gaseous substances.
The main body of the volume is divided into two parts--the first dealing
with observations made in and before 1772, the second with observations
made in the year 1773 and in the beginning of 1774. In the outset
Priestley finds himself at a disadvantage in regard to the only terms at
that time in vogue for the factitious airs, viz., _fixed_, _mephitic_
and _inflammable_, which, he rightly says, are not sufficiently
characteristic and distinct. Strictly speaking, any two of these terms
might be applied to any one of the "airs" then known. The inflammable
air from metals, as well as choke damp, is noxious, and therefore
mephitic, as is fixed air, and since the inflammable airs are,
apparently, capable of being imbibed by certain substances they may
equally be considered fixable. The term _fixed air_ had, however,
acquired a distinctive meaning, and rather than introduce a new term or
change the signification of an old one, he would, with his
contemporaries, restrict the term to the air which had been made the
subject of Black's memorable investigation. The first paper in this
section deals with fixed air; it is practically a reprint of that in the
_Phil. Trans._ and which has already been described in sufficient
detail. In the course of his experiments he says he once thought that
the readiest method of procuring fixed air, and in sufficient purity,
would be to heat pounded lime-stone in a gun barrel, "making it pass
through the stem of a tobacco pipe or a glass tube carefully luted to
the orifice of it."
"In this manner I found that air is produced in great plenty; but,
upon examining it, I found to my great surprise that little more than
one half of it was fixed air, capable of being absorbed by water; and
that the rest was inflammable, sometimes very weakly, but sometimes
pretty highly so."
He surmised that this "air" must come from the iron, and yet, he noted,
it differed from the ordinary inflammable air from iron by the
remarkable blue colour of its flame, and he concludes that "this
inflammable principle may come from some remains of the animals from
which it is thought that all calcareous matter proceeds." Priestley, we
now know, had incidentally converted some of the fixed air into the only
other oxide of carbon, but he failed to appreciate the significance of
his observation, and the credit of the discovery of carbon monoxide
belongs to Cruikshank.
In his next paper on "Air in which Candles have burned," Priestley made
a discovery of the very highest importance. He had attempted to verify
without success the allegation by the Count de Saluce, made in the
memoirs of the Philosophical Society of Turin, that air vitiated by the
combustion of candles could be restored by exposure to cold.
"Though this experiment failed," he says, "I have been so happy as by
accident to have hit upon a method of restoring air which has been
injured by the burning of candles, and to have discovered at least one
of the restoratives which Nature employs for this purpose. It is
_vegetation_. This restoration of vitiated air, I conjecture, is
effected by plants imbibing the phlogistic matter with which it is
overloaded by the burning of inflammable bodies. But whether there be
any foundation for this conjecture or not, the fact is, I think,
indisputable."
He then proceeds to give an account of his observations on the growing
of plants in confined air which led to his discovery.
"One might have imagined," he says, "that since common air is
necessary to vegetable as well as to animal life, both plants and
animals had affected it in the same manner; and I own I had that
expectation when I first put a sprig of mint into a glass jar standing
inverted in a vessel of water: but when it had continued there for
some months I found the air would neither extinguish the candle, nor
was it at all inconvenient to a mouse, which I put into it.... Finding
that candles would burn very well in air in which plants had grown a
long time, and having had some reason to think that there was
something attending vegetation which restored air that had been
injured by respiration, I thought it was possible that the same
process might also restore the air that had been injured by the
burning of candles.
"Accordingly, on the 17th of August 1771, I put a sprig of mint into a
quantity of air in which a wax candle had burned out, and found that
on the 27th of the same month another candle burned perfectly well in
it. This experiment I repeated, without the least variation in the
event, not less than eight or ten times in the remainder of the
summer.
"Several times I divided the quantity of air in which the candle had
burned out into two parts, and putting the plant into one of them left
the other in the same exposure, contained also in a glass vessel
immersed in water, but without any plant, and never failed to find
that a candle would burn in the former but not in the latter.... This
remarkable effect does not depend upon anything peculiar to _mint_,
which was the plant that I always made use of till July 1772; for on
the 16th of that month I found a quantity of this kind of air to be
perfectly restored by sprigs of _balm_, which had grown in it from the
7th of the same month.
"That this restoration of air was not owing to any aromatic effluvia
of these two plants not only appeared by the _essential oil of mint_
having no sensible effect of this kind, but from the equally complete
restoration of this vitiated air by the plant called _groundsel_,
which is usually ranked among the weeds and has an offensive smell.
Besides, the plant which I have found to be the most effectual of any
that I have tried for this purpose is _spinach_, which is of quick
growth, but will seldom thrive long in water."
The next paper on "Inflammable Air" is of slight importance, and indeed
is full of errors. Priestley made no distinction between the inflammable
air obtained by the action of acids on metals (hydrogen) and that formed
by the destructive distillation of coal and other organic substances
(marsh gas or carbonic oxide, or mixtures of the two), and his inability
to distinguish these different gases accounts for many of the phenomena
he observed and which he confesses himself unable to explain. The most
sagacious observation in the memoir has reference to the colour of the
electric spark in the different gases which he accurately describes.
The paper on "Air Infected with Animal Respiration or Putrefection" may
be considered as the complement of that on "Air in which a Candle has
burned out," and is no less valuable.
"That candles will burn only a certain time in a given quantity of air
is a fact not better known than it is that animals can live only a
certain time in it; but the cause of the death of the animal is not
better known than that of the extinction of flame in the same
circumstances; and when once any quantity of air has been rendered
noxious by animals breathing in it as long as they could, I do not
know that any methods have been discovered of rendering it fit for
breathing again. It is evident, however, that there must be some
provision in Nature for this purpose, as well as for that of rendering
the air fit for sustaining flame; for without it the whole mass of the
atmosphere would, in time, become unfit for the purpose of animal
life; and yet there is no reason to think that it is, at present, at
all less fit for respiration than it has ever been. I flatter myself,
however, that I have hit upon two of the methods employed by Nature
for this great purpose. How many others there may be I cannot tell."
One of these methods he eventually finds to be, as in the first case,
the action of vegetation, and he proves by a number of decisive
experiments
"that plants, instead of affecting the air in the same manner with
animal respiration, reverse the effects of breathing and tend to keep
the atmosphere sweet and wholesome when it is become noxious in
consequence of animals either living and breathing, or dying and
putrefying in it."
The other method he conceived to be the action of water, since he found
that by vigorous agitation with water, air which breathing had rendered
noxious could again be breathed for a further period.
"I do not think it improbable but that the agitation of the sea and
large lakes may be of some use for the purification of the atmosphere,
and the putrid matter contained in water may be imbibed by aquatic
plants, or be deposited in some other manner."
When a confined volume of common air is placed in contact with a mixture
of iron filings and sulphur made into a paste with water, a certain
portion of the air is imbibed by the paste. This fact was first observed
by Hales. Priestley repeated the observation and found that about a
fifth or rather more of the volume of the air was thus absorbed. He
noted that the residual "air" was rather lighter than common air, it had
no action on lime-water and was exceedingly noxious to animals, by which
is meant that it could not be breathed by them. Priestley had thus
prepared nitrogen, but he failed to recognise the individuality of this
gas.
In his _Statical Essays_ Hales makes mention of an experiment in which
common air and air generated from pyrites by spirit of nitre made a
turbid red mixture, and in which part of the common air was absorbed.
This phenomenon "particularly struck" Priestley, who, acting upon
Cavendish's hint that the red appearance was probably dependent "upon
the spirit of nitre only" and that the metals might answer as well as
pyrites, proceeded to investigate the action of nitric acid upon a
number of the metals, and as the result of his inquiries he succeeded in
isolating the gas we now know as _nitric oxide_, but which he termed
_nitrous air_.
"Though," he says, "I cannot say that I altogether like the term,
neither myself nor any of my friends, to whom I have applied for the
purpose, have been able to hit upon a better."
This paper exhibits Priestley at his best. In it he describes all the
main properties of nitric oxide.
"One of the most conspicuous properties of this kind of air," he says,
"is the great diminution of any quantity of common air with which it
is mixed, attended with a turbid red, or deep orange colour, and a
considerable heat.... The diminution of a mixture of this and common
air is not an equal diminution of both the kinds, which is all that Dr
Hales could observe, but of about one fifth of the common air, and as
much of the nitrous air as is necessary to produce that effect; which,
as I have found by many trials, is about one half as much as the
original quantity of common air.
"I hardly know any experiment that is more adapted to amaze and
surprise than this is, which exhibits a quantity of air which, as it
were, devours a quantity of another kind of air half as large as
itself, and yet is so far from gaining any addition to its bulk that
it is considerably diminished by it....
"It is exceedingly remarkable that this effervescence and diminution,
occasioned by the mixture of nitrous air, is peculiar to common air,
or _air fit for respiration_, and, as far as I can judge from a great
number of observations, is at least very nearly, if not exactly, in
proportion to its fitness for this purpose; so that by this means the
goodness of air may be distinguished much more accurately than it can
be done by putting mice or any other animals to breathe in it.
"This was a most agreeable discovery to me, as I hope it may be a
useful one to the public; especially as from this time I had no
occasion for so large a stock of mice as I had been used to keep for
the purpose of these experiments."
Priestley here suggests the basis of a method of _Eudiometry_, or method
of measuring the goodness of air, which in his hands, but more
especially in those of Cavendish, led to most important results. The
quantitative analysis of the air may be said to have taken its rise from
the publication of Priestley's paper.
In the course of subsequent work on nitrous air Priestley had occasion
to study its action on iron, whereby he says:--
"A most remarkable and most unexpected change was made in the nitrous
air," the iron "makes it not only to admit a candle to burn in it, but
enables it to burn with an _enlarged flame_.... Sometimes I have
perceived the flame of the candle, in these circumstances, to be twice
as large as it is naturally, and sometimes not less than five or six
times larger; and yet without anything like an _explosion_, as in the
firing of the weakest inflammable air."
Priestley in this manner obtained _nitrous oxide_, the properties of
which he subsequently studied in some detail.
In the paper which follows, _viz._, "On Air infected with the Fumes of
Burning Charcoal," he incidentally gains further insight into the nature
of atmospheric air. By what he called throwing the focus of a burning
mirror on charcoal suspended in air contained in a glass tube standing
over water or mercury--a favourite method of his when he had occasion to
heat a substance in a gas--he could observe the phenomena with great
precision. He noticed the formation of the fixed air and determined the
degree of diminution when the burning took place over water or over
lime-water.
"In this manner," he says, "I diminished a given quantity of air
one-fifth. Air thus diminished by the fumes of burning charcoal not
only extinguishes flame, but is in the highest degree noxious to
animals; it makes no effervescence with nitrous air, and is incapable
of being diminished any farther by the fumes of more charcoal.... All
my observations show that air which has once been fully diminished ...
is not only incapable of any further diminution ... but that it has
likewise acquired _new properties_, most remarkably different from
those which it had before...."
By heating pieces of lead and tin in air by means of a burning glass he
observed the formation of a metallic calx, the volume of air was
diminished, and it also "was in the highest degree noxious and made no
effervescence with nitrous air."
The real significance of these phenomena was, however, wholly
unperceived by Priestley, and phlogiston, as usual, led him astray. He
had, of course, in all these experiments prepared _nitrogen_, and in a
state of sensible purity. He imagined, however, that he had simply
"phlogisticated" the air, the phlogiston coming from the charcoal and
the metals, and that this phlogisticated air was imbibed by the water.
An experiment described by Cavendish led Priestley to study the action
of "Spirit of Salt" (hydrochloric acid) upon copper. As Cavendish had
already stated, the gas so evolved "lost its electricity by coming into
contact with water." By collecting the gas over mercury Priestley was
able to study its properties more exactly. From certain anomalies in the
experiments he says:--
"I concluded that this subtle air did not arise from the copper, but
from the spirit of salt; and presently making the experiment with the
acid only, without any copper, or metal of any kind, this air was
immediately produced in as great plenty as before; so that this
remarkable kind of air is, in fact, nothing more than the vapour, or
fumes of spirit of salt, which appear to be of such a nature that they
are not liable to be condensed by cold, like the vapour of water and
other fluids, and therefore may be very properly called an _acid air_,
or more restrictively the _marine acid air_."
The new gas discovered by Priestley we now call hydrogen chloride.
Ordinary hydrochloric acid is simply an aqueous solution of it.
"Water impregnated with it makes the strongest spirit of salt that I
have seen, dissolving iron with the most rapidity.... Iron filings,
being admitted to this air, were dissolved by it pretty fast, half of
the air disappearing and the other half becoming inflammable air, not
absorbed by water. Putting chalk to it, fixed air was produced."
He subsequently found that the marine acid air was more conveniently
made by the action of oil of vitriol upon common salt.
From the "miscellaneous observations" with which this section of the
volume concludes, there can be little doubt that Priestley, without
knowing it, had prepared oxygen gas from nitre as far back as 1771. The
accounts he gives of the behaviour of the gas obtained by heating nitre
in a gun-barrel plainly indicate this fact.
"A candle," he says, "not only burned but the flame was increased, and
something was heard like a hissing similar to the decrepitation of
nitre in an open fire." He also noted the effect of nitrous air upon
it and concludes that "this series of facts relating to air extracted
from nitre appear to me to be very extraordinary and important, and in
able hands may lead to considerable discoveries."
The second section of the volume deals with experiments and observations
made in 1773 and the beginning of 1774, and opens with an account of the
discovery of _ammonia gas_.
"After I had made the discovery of the _marine acid air_, which the
vapour of spirit of salt may properly enough be called ... it occurred
to me that by a process similar to that by which this _acid_ air is
expelled from the spirit of salt an _alkaline_ air might be expelled
from substances containing volatile alkali.
"Accordingly I procured some volatile spirit of sal ammoniac, and
having put it into a thin phial, and heated it with the flame of a
candle, I presently found that a great quantity of vapour was
discharged from it; and being received in a vessel of quicksilver,
standing in a basin of quicksilver, it continued in the form of a
transparent and permanent air, not at all condensed by cold; so that I
had the same opportunity of making experiments upon it as I had before
on the acid air, being in the same favourable circumstances....
Wanting, however, to procure this air in greater quantities, and this
method being rather expensive, it occurred to me that alkaline air
might probably be procured, with the most ease and convenience, from
the original materials, mixed in the same proportions that chemists
had found by experience to answer the best for the production of the
volatile spirit of sal ammoniac. Accordingly I mixed one-fourth of
pounded sal ammoniac with three-fourths of slaked lime; and filling a
phial with the mixture, I presently found it completely answered my
purpose. The heat of a candle expelled from this mixture a prodigious
quantity of alkaline air; and the same materials ... would serve me a
considerable time without changing...."
He next studied the properties of the alkaline air. He found, of course,
it was readily soluble in water.
"Having satisfied myself with respect to the relation that alkaline
air bears to water, I was impatient to find what would be the
consequence of mixing this new air with the other kinds with which I
was acquainted before, and especially with _acid_ air; having a notion
that these two airs, being of opposite natures, might compose a
_neutral air_, and perhaps the very same thing with common air. But
the moment that these two kinds of air came into contact a beautiful
white cloud was formed, and presently filled the whole vessel in which
they were contained.... When the cloud was subsided there appeared to
be formed a solid _white salt_, which was found to be the common _sal
ammoniac_, or the marine acid united to the volatile alkali....
"_Fixed air_ admitted to alkaline air formed oblong and slender
crystals.... These crystals must be the same thing with the volatile
alkalis which chemists get in a solid form by the distillation of sal
ammoniac with fixed alkaline salts....
"Alkaline air, I was surprised to find, is slightly inflammable....
"That alkaline air is lighter than acid air is evident from the
appearances that attend the mixture, which are indeed very beautiful.
When acid air is introduced into a vessel containing alkaline air, the
white cloud which they form appears at the bottom only and ascends
gradually. But when the alkaline air is put to the acid the whole
becomes immediately cloudy quite to the top of the vessel."
Up to now Priestley had mainly confined himself to the narration of the
new _facts_ which he had discovered, barely mentioning any _hypotheses_
that occurred to him.
"The reason why I was so much upon my guard in this respect was lest,
in consequence of attaching myself to any hypothesis too soon, the
success of my future inquiries might be obstructed. But subsequent
experiments having thrown great light upon the preceding ones, and
having confirmed the few conjectures I then advanced, I may now
venture to speak of my hypotheses with a little less diffidence.
Still, however, I shall be ready to relinquish any notions I may now
entertain if new facts should hereafter appear not to favour them."
In a paper on "Common Air Diminished and made Noxious by Various
Processes" he attempts to apply the current doctrine of phlogiston to
account for the various phenomena he has observed, and with what success
may be inferred from his conclusion
"that in the precipitation of lime by breathing into lime-water, the
fixed air, which incorporates with lime, comes not from the lungs but
from the common air, decomposed by the phlogiston exhaled from them,
and discharged, after having been taken in with the aliment, and
having performed its function in the animal system."
Priestley's attempts at theorising brought little satisfaction to him or
to his readers. Indeed he says:--
"I begin to be apprehensive lest, after being considered as a _dry
experimenter_, I should pass into the opposite character of a
_visionary theorist_.... In extenuation of my offence let it, however,
be considered that _theory and experiments_ necessarily go
hand-in-hand, every process being intended to ascertain some
particular _hypothesis_, which, in fact, is only a conjecture
concerning the circumstances or the cause of some natural operation;
consequently that the boldest and most original experimenters are
those who, giving free scope to their imaginations, admit the
combination of the most distant ideas; and that though many of these
associations of ideas will be wild and chimerical, yet that others
will have the chance of giving rise to the greatest and most capital
discoveries, such as very cautious, timid, sober and slow-thinking
people would never have come at.
"Sir Isaac Newton himself, notwithstanding the great advantage which
he derived from a habit of _patient thinking_, indulged bold and
eccentric thoughts, of which his queries at the end of his book of
Optics are a sufficient evidence. And a quick conception of distant
analogies, which is the great key to unlock the secrets of Nature, is
by no means incompatible with the spirit of _perseverance_ in
investigations calculated to ascertain and pursue those analogies."
After this _apologia_, Priestley gives the reins to his imagination, or
rather he allows phlogiston to drive the halting, ambling thing for him,
with the result that he utterly loses his way and is eventually landed
into an impassable quagmire. It is not too much to say that not one of
the "Queries, Speculations and Hints" with which the volume closes has
stood the test of time.
The second volume, which made its appearance towards the end of 1775, is
dedicated to Sir John Pringle, at that time President of the Royal
Society. It opens, as usual, with a somewhat prolix but characteristic
preface. But to his biographer Priestley's prefaces are not the least
interesting or valuable of his literary productions.
"In a preface," he says, "authors have always claimed a right of
saying whatever they pleased concerning themselves, and not to lose
this right it must now and then be exercised."
In this respect Priestley has championed the prerogatives of authors for
all time. This particular preface begins with an expression of
self-laudation for the little delay the writer made in putting the first
volume to the press.
"In consequence of this considerable discoveries have been made by
people of distant nations; and this branch of science, of which
nothing, in a manner, was known till very lately, indeed now bids fair
to be farther advanced than any other in the whole compass of natural
philosophy.... And it will not now be thought very assuming to say
that by working in a tub of water or a basin of quicksilver we may
perhaps discover principles of more extensive influence than even that
of _gravity_ itself, the discovery of which, in its full extent,
contributed so much to immortalise the name of Newton.
"Having been the means of bringing so many champions into the field, I
shall, with peculiar pleasure, attend to all their achievements, in
order to prepare myself, as I promised in the preface to my last
volume, for writing the _history_ of the campaign."
After a delightfully _naïve_ compliment to his own ability as an
accumulator of facts, and to his merits as an "instrument in the hands
of Divine Providence ... concerning which I threw out some further hints
in my former preface, which the excellent French translator was not
permitted to insert in his version," he advances this testimony to his
impartiality as an historian:--
"I even think that I may flatter myself so much, if it be any
flattery, as to say that there is not, in the whole compass of
philosophical writing, a history of experiments so truly _ingenuous_
as mine, and especially the section on the discovery of
dephlogisticated air, which I will venture to exhibit as a model of
the kind. I am not conscious to myself of having concealed the least
hint that was suggested to me by any person whatever, any kind of
assistance that has been given me, or any views or hypotheses by which
the experiments were directed, whether they were verified by the
result or not."
There is much else in the preface that might be quoted as illustrative
of the character and mental attributes of its author. Priestley, the
natural philosopher, never forgot that he was a minister of religion,
and that to him theology was the greatest and most important of all the
sciences, and he cannot forbear even, in what he intended to be a
scientific disquisition on purely natural phenomena, from inculcating
his belief in the divine origin of Christianity and his opinion
concerning the doctrine of purgatory and the worship of the dead.
The first chapter is concerned with the discovery of what its author
called _Vitriolic Acid Air_, but which we now know as sulphur dioxide.
Priestley imagined that as the liquid marine acid--that is hydrochloric
acid--readily yielded an "air" on heating it might be that vitriolic
acid, or oil of vitriol, would also afford a characteristic "air" when
treated in a similar manner. Acting upon a suggestion of Mr Lane he
heated oil of vitriol with olive oil, when he readily obtained a new
species of air, which he collected over mercury as he "had been used to
do it with the marine acid air; and the whole process was as pleasing
and as elegant." Priestley at once surmised that the olive oil worked by
transferring its phlogiston to the vitriolic acid, and he naturally
concluded that any substance rich in phlogiston would bring about the
same result. He next tried charcoal.
"I put some bits of charcoal into my phial instead of the oil or other
inflammable matter which I had used before, and applying the flame of
a candle I presently found that the vitriolic acid air was produced as
well as in the former process, and in several respects more
conveniently, the production of air being equable, whereby the
disagreeable effect of a sudden explosion is avoided.... Finding that
a great variety of substances containing phlogiston enabled the oil of
vitriol to throw out a permanent acid air, I had some suspicion that
mere _heat_ might do the same, but I did not find that there was any
foundation for that suspicion.... But though I got no air from the oil
of vitriol by this process, air was produced at the same time in a
manner that I little expected, and I paid pretty dearly for the
discovery it occasioned. Despairing to get any air from the longer
application of my candles, I withdrew them, but before I could
disengage the phial from the vessel of quicksilver a little of it
passed through the tube into the hot acid, when instantly it was all
filled with dense white fumes, a prodigious quantity of air was
generated, the tube through which it was transmitted was broken into
many pieces, and part of the hot acid being spilled upon my hand
burned it terribly, so that the effect of it is visible to this day.
The inside of the phial was coated with a white saline substance, and
the smell that issued from it was extremely suffocating.
"This accident taught me what I am surprised I should not have
suspected before, viz., that some _metals_ will part with their
phlogiston to hot oil of vitriol, and thereby convert it into a
permanent elastic air, producing the very same effect with oil,
charcoal, or any other inflammable substance.
"Not discouraged by the disagreeable accident above mentioned, the
next day I put a little _quicksilver_ into the phial with the ground
stopple and tube, along with the oil of vitriol, when, long before it
was boiling hot, air issued plentifully from it, and being received in
a vessel of quicksilver appeared to be genuine vitriolic acid air,
exactly like that which I had procured before, being readily imbibed
by water and extinguishing a candle in the same manner as the other
had done....
"After this I repeated the experiment with several other metals....
Copper treated in the same manner yielded air very freely, with about
the same degree of heat that quicksilver had required, and the air
continued to be generated with very little application of more heat."
The theory apart, this paper is as important as these on ammonia and the
marine acid air, and exhibits Priestley at his best. The observations he
makes concerning the main properties of the new gas and its solubility
in water, its inability to burn and to support flame, its heaviness, its
power to unite with ammonia, to be absorbed by charcoal and to liquefy
camphor, are all accurate.
"Having hit upon a method of exhibiting some of the acids in the form
of air, nothing could be easier than to extend this process to the
rest."
Accordingly he attempted to procure what he called the _vegetable acid
air_ by heating "exceedingly strong concentrated acid of vinegar," and
states that he succeeded in obtaining an air which extinguished the
flame of a candle and was soluble in water. The paper is very short and
is full of contradictions. In reality, as he subsequently found, he was
dealing with vinegar largely adulterated with oil of vitriol. The
"vegetable acid air" had no real existence.
The next paper in the series is the most important of the whole, and the
one of all others that has contributed most largely to Priestley's
reputation. It is entitled "Of Dephlogisticated Air, and of the
Constitution of the Atmosphere," and deals with the discovery of oxygen.
It begins in the following characteristic fashion:--
"The contents of this section will furnish a very striking
illustration of the truth of a remark which I have more than once made
in my philosophical writings, and which can hardly be too often
repeated, as it tends greatly to encourage philosophical
investigations, viz., that more is owing to what we call
_chance_--that is, philosophically speaking, to the observation of
_events arising from unknown causes_ than to any proper _design_ or
preconceived _theory_ in this business. This does not appear in the
works of those who write _synthetically_ upon these subjects, but
would, I doubt not, appear very strikingly in those who are the most
celebrated for their philosophical acumen did they write
_analytically_ and ingenuously.
"For my own part, I will frankly acknowledge that at the commencement
of the experiments recited in this section I was so far from having
formed any hypothesis that led to the discoveries I made in pursuing
them that they would have appeared very improbable to me had I been
told of them; and when the decisive facts did at length obtrude
themselves upon my notice it was very slowly, and with great
hesitation, that I yielded to the evidence of my senses. And yet, when
I reconsider the matter, and compare my last discoveries relating to
the constitution of the atmosphere with the first, I see the closest
and the easiest connection in the world between them, so as to wonder
that I should not have been led immediately from the one to the other.
That this was not the case I attribute to the force of prejudice
which, unknown to ourselves, biases not only our _judgments_, properly
so called, but even the perceptions of our senses; for 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."
He then points out that there are few maxims in philosophy that have
laid firmer hold upon the mind than that air, meaning atmospherical air
... is a _simple elementary substance_, indestructible and unalterable,
at least as much so as water was supposed to be. Priestley, in the
course of his inquiries, was soon satisfied that atmospherical air was
not an unalterable thing; that bodies burning in it, and animals
breathing it and various other chemical processes, so far alter and
deprive it as to render it altogether unfit for the purposes to which it
is subservient; and he had discovered methods, particularly the process
of vegetation, which tended to restore it to its original purity.
"But," he says, "I own I had no idea of the possibility of going any
further in this way and thereby procuring air purer than the best
common air."
As this paper is one of the classics of chemistry, as well as the chief
corner-stone in the monument which Priestley erected to himself, it is
necessary to examine it, as well as certain other papers which grew
immediately out of it, in some degree of detail.
After a reference to a hypothesis of the origin and constitution of the
atmosphere which occurs among the "Queries, Speculations and Hints"
above referred to, and which is on a par with much in Priestley's
speculations, he proceeds to relate the circumstances which more
immediately led to the most important of all his discoveries. It was the
accident of possessing a burning lens of "considerable force," for want
of which he could not possibly make many of the experiments that he had
projected.
"But having afterwards procured a lens of twelve inches diameter and
twenty inches focal distance, I proceeded with great alacrity to
examine, by the help of it, what kind of air a great variety of
substances, natural and factitious, would yield, putting them into
vessels [short, wide, round-bottomed phials], which I filled with
quicksilver and kept inverted in a basin of the same. Mr Warltire, a
good chemist, and lecturer in Natural Philosophy, happening to be at
that time in Calne, I explained my views to him, and was furnished by
him with many substances, which I could not otherwise have procured.
"With this apparatus, after a variety of other experiments, an account
of which will be found in its proper place on the 1st August 1774, I
endeavoured to extract air from _mercurius calcinatus per se_;[17] and
I presently found that, by means of this lens, air was expelled from
it very readily. Having got about three or four times as much as the
bulk of my materials, I admitted water to it, and found that it was
not imbibed by it. But what surprised me more than I can well express
was that a candle burned in this air with a remarkably vigorous flame,
very much like that enlarged flame with which a candle burns in
nitrous air exposed to iron or liver of sulphur,[18] 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.
"At the same time that I made the above-mentioned experiment I
extracted a quantity of air with the very same property from the
common _red precipitate_[19] which, being produced by a solution of
mercury in spirit of nitre (nitric acid), made me conclude that this
peculiar property, being similar to that of the modification of
nitrous air above mentioned, depended upon something being
communicated to it by the nitrous acid; and since the _mercurius
calcinatus_ is produced by exposing mercury to a certain degree of
heat, where common air has access to it, I likewise concluded that
this substance had collected something of _nitre_, in that state of
heat, from the atmosphere.
"This, however, appearing to me much more extraordinary than it ought
to have done, I entertained some suspicion that the mercurius
calcinatus on which I had made my experiments, being bought at a
common apothecary's, might, in fact, be nothing more than red
precipitate; though, had I been anything of a practical chemist, I
could not have entertained any such suspicion. However, mentioning
this suspicion to Mr Warltire, he furnished me with some that he had
kept for a specimen of the preparation, and which, he told me, he
could warrant to be genuine. This being treated in the same manner as
the former, only by a longer continuance of heat, I extracted much
more air from it than from the other.
"This experiment might have satisfied any moderate sceptic; but,
however, being at Paris in the October following, and knowing that
there were several very eminent chemists in that place, I did not omit
the opportunity, by means of my friend Mr Magellan, to get an ounce of
mercurius calcinatus prepared by Mr Cadet, of the genuineness of which
there could not possibly be any suspicion; and at the same time I
frequently mentioned my surprise at the kind of air which I had got
from this preparation to Mr Lavoisier, Mr le Roy, and several other
philosophers, who honoured me with their notice in that city, and who,
I daresay, cannot fail to recollect the circumstance."
This last remark is significant in reference to a claim which was
subsequently put forward that the real discoverer of oxygen was
Lavoisier, and that he obtained it by heating mercuric oxide.[20]
Priestley also obtained the same air from _red lead_, which, he says,
"confirmed me more in my suspicion that the _mercurius calcinatus_
must get the property of yielding this kind of air from the
atmosphere, the process by which that preparation and this of red lead
is made being similar. As I never make the least secret of anything
that I observe, I mentioned this experiment also, as well as those
with the mercurius calcinatus and the red precipitate, to all my
philosophical acquaintance at Paris and elsewhere, having no idea, at
that time, to what these remarkable facts would lead." [Nitrous
oxide.]
Priestley, on his return to England, made an experiment with Cadet's
preparation, which he found to behave precisely like that he had
procured from Warltire. He observed that the new gas was only sparingly
soluble in water and that its power of causing a candle to burn with a
strong flame was in nowise diminished by agitation with water--facts
which he said convinced him
"that there must be a very material difference between the
constitution of the air from mercurius calcinatus and that of
phlogisticated nitrous air, [nitrous oxide] notwithstanding their
resemblance in some particulars."
It was not, however, until the following March (1775) (he having
meanwhile been intent upon his experiments on the vitriolic air [sulphur
dioxide]), that he ascertained the real nature of the new air, and was
led "though very gradually ... to the complete discovery of the
constitution of the air we breathe." By trials with the nitrous air and
with mice he found that the new gas was eminently fit for respiration:
nitrous air reduced its volume to a greater extent than in the case of
common air, and a mouse lived longer in it than it would in the same
volume of common air.
"Thinking of this extraordinary fact upon my pillow, the next morning
I put another measure of nitrous air to the same mixture, and to my
utter astonishment found that it was farther diminished to almost
one-half of its original quantity."
Priestley now utterly missed his way for a time. He sought to get the
new air from the various oxides of lead, but the fetish of phlogiston
again led him wrong, and eventually by a train of reasoning which is
fully set forth in the paper, but which need not here be repeated, there
remained, he says, no doubt in his mind
"but that _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."
Priestley's "complete discovery of the constitution of the air we
breathe" was thus wholly erroneous: he was very far indeed from having a
clear conception of its real nature.
Priestley's description of the main properties of oxygen is however
accurate, and lecturers in chemistry are indebted to him for some
striking experimental illustrations of them.
"I easily conjectured," he says, "that inflammable air would explode
with more violence and a louder report by the help of dephlogisticated
than of common air; but the effect far exceeded my expectations, and
it has never failed to surprise every person before whom I have made
the experiment.... The dipping of a lighted candle into a jar filled
with dephlogisticated air is alone a very beautiful experiment. The
strength and vivacity of the flame is striking, and the heat produced
by the flame in these circumstances is also remarkably great....
Nothing would be easier than to augment the force of fire to a
prodigious degree by blowing it with dephlogisticated air instead of
common air.... Possibly _platina_ might be melted by means of it.
"From the greater strength and vivacity of the flame of a candle, in
this pure air, it may be conjectured that it might be peculiarly
salutary to the lungs in certain morbid cases.... But perhaps we may
also infer from these experiments that though pure dephlogisticated
air might be very useful as a _medicine_, it might not be so proper
for us in the usual healthy state of the body: for, as a candle burns
out much faster in dephlogisticated than in common air, so we might,
as may be said, _live out too fast_, and the animal powers be too soon
exhausted in this pure kind of air. A moralist, at least, may say that
the air which Nature has provided for us is as good as we deserve....
Who can tell but that, in time, this pure air may become a fashionable
article in luxury. Hitherto only two mice and myself have had the
privilege of breathing it."
An experiment which Priestley says "I had the pleasure to see at Paris,
in the laboratory of Mr Lavoisier, my excellent fellow-labourer in these
inquiries, and to whom, in a variety of respects, the philosophical part
of the world has very great obligations," led him into a train of
inquiry upon the action of nitric acid upon a wide range of organic
substances, from which however no general results followed, in spite of
much experimenting. He had at one time the idea that a fundamental
difference existed in the behaviour of animal and vegetable matter with
respect to nitric acid, but the observations were contradictory, and
although it is readily possible to interpret the phenomena in the light
of our present knowledge, they led Priestley to no definite conclusions.
Of more importance is the work on the "Fluor Acid Air"--a substance
discovered by "Mr Scheele, a Swede; from which circumstance the acid is
often distinguished by the name of the _Swedish acid_." Priestley sought
to make the air by heating Derbyshire spar (fluor spar) with oil of
vitriol in glass vessels,
"as in the process of making spirit of nitre from saltpetre; and the
most remarkable facts that have been observed concerning it are, that
the vessels in which the distillation is made are apt to be corroded;
so that holes will be made quite through them; and that when there is
water in the recipient, the surface of it will be covered with a crust
of a friable stony matter."
What Priestley actually produced by this method of experimenting was
more or less pure _silicon fluoride_, which he proceeded to collect, in
his usual fashion, over quicksilver.
"I had no sooner produced this new kind of air but I was eager to see
the effect it would have on _water_, and to produce the stony crust
formed by their union, as described by Mr Scheele; and I was not
disappointed in my expectations. The moment the water came into
contact with this air the surface of it became white and opaque by a
_stony film_.... Few philosophical experiments exhibit a more pleasing
appearance than this, which can only be made by first producing the
air confined by quicksilver, and then admitting a large body of water
to it. Most persons to whom I have shown the experiment have been
exceedingly struck with it.... The union of this acid air and water
may also be exhibited in another manner, which to some persons makes a
still more striking experiment, _viz._, by admitting the air, as fast
as it is generated, to a large body of water resting on
quicksilver.... It is, then, very pleasing to observe that the moment
any bubble of air, after passing through the quicksilver, reaches the
water, it is instantly, as it were, converted into a stone; but
continuing hollow for a short space of time, generally rises to the
top of the water.... I have met with few persons who are soon weary of
looking at it; and some could sit by it almost a whole hour, and be
agreeably amused all the time."
Priestley's attempts to explain the real nature of the _fluor acid air_
were, as may be expected, not very happy.
"These appearances I explain by supposing that the vitriolic acid, in
uniting with the spar, is in part volatilised by means of some
phlogiston contained in it, so as to form a vitriolic acid air; and
there is also combined with this air a portion of the solid earthy
part of the spar, which continues in a state of solution till, coming
into contact with the water, the fluid unites with the acid, and the
earth is precipitated."
The third volume of the work was published in the early part of 1777,
with a dedication to Lord Stanhope. It opens, as usual, with the
characteristically discursive preface, extending to thirty pages, in
which the author apologises for the character of much in the volume. He
is constrained to admit that numerous as his _facts_ are, "few of them
will appear so brilliant in the eye of the _general scholar_" as in
either of the two former volumes, although he trusts they will "be
thought no less valuable by philosophers and chemists." Priestley, it
would seem, was conscious that he was beginning, as the phrase goes, "to
write himself out."
"Lest my readers should be alarmed at this addition of one volume
after another on the same subject, I do assure them that I shall now
certainly give them and myself some respite, and deliver the torch to
anyone who may be disposed to carry it, foreseeing that my attention
will be sufficiently engaged by speculations of a very different
nature.... It will be a great satisfaction to me, after the part that
I have taken in this business, to be a _spectator_ of its future
progress, when I see the work in so many and so good hands, and
everything in so rapid and so promising a way.