Chapter XI (2)

"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.

"On taking leave of this subject I would entreat the candour and
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),