“FIXED AIR” AND “MEPHITIC AIR”--THEIR DISCOVERY BY BLACK AND
BY RUTHERFORD
Before relating the history of the discoveries of Black, Rutherford,
and Priestley, it will be appropriate to give an account of a theory
which professed to explain the phenomena of combustion, and with it the
conversion of metals into calces, and the reduction of these calces to
the reguline or metallic state. Like other theories, it was slow in
developing. Its germ is to be traced to the writings of Johann Baptist
van Helmont of Brabant, Seigneur of Merode, Royenboch, Oorshot, and
Pellines, who was born in Brussels in 1577. He adopted a fantastical
creation of Paracelsus, the _archaeus_, a kind of demon which, by means
of fermentation, draws together all the particles of matter. Believing
that water was the true principle and origin of everything (for he had
succeeded in producing a willow tree, weighing 164 lbs., from water
alone, the earth in which it grew having neither gained nor lost
appreciably in weight), he conceived that it was acted on by a
_ferment_ or principle pre-existing in the seed developed by it, and
exhaling an odour by which the archaeus was attracted. Water undergoing
the action of this ferment developed a vapour, to which van Helmont
gave the name of “gas.” A “gas” was a substance intermediate between
spirit and matter, and the word was probably derived from _Geist_, the
common German word for spirit. Another word introduced by him to denote
the life-principle of the stars was _Blas_, connected probably with
_blasen_, to blow, and our English word _blast_.
It is curious to notice how the idea of an _archaeus_ survived down to
later times under the name of a “life-principle”--a conception that all
organic substances must necessarily owe their origin to life itself,
and not to the usual chemical and physical transformations.
Van Helmont was acquainted with various kinds of gases, as appears
from his treatise “De Flatibus.” His _gas sylvestre_ was evolved from
fermenting liquors, and he knew that it was formed during the
combustion of charcoal, and also that it was present in the Grotto del
Cane near Naples. He was likewise acquainted with combustible gases,
which he named _gas pingue_, _gas siccum_, or _gas fuliginosum_.
These principles of van Helmont’s apparently suggested to his
successors, Becher and Stahl, the notion of a principle inherent in
every combustible substance, which was lost during combustion. The
development of this--the phlogistic--theory is almost wholly due to the
latter chemist, and indeed it is difficult to trace Becher’s share in
it.
George Ernest Stahl was born at Anspach in 1660; he studied and
graduated in medicine at Halle, and in 1694 he was appointed second
professor of medicine at that University, where he continued
to teach for twenty-two years. His most important work was his
_Fundamenta chymiae dogmaticae et experimentale_. His theoretical
views are contained in the last part of this work. He there treats
of _zymotechnia_, or fermentation; _halotechnia_, or the production
of salts; and _pyrotechnia_, or the doctrine of combustion. It is
the last of these sections which gives an account of the doctrine of
_phlogiston_.
The fundamental conception of this doctrine is that all combustible
bodies are compounds. During combustion one of these constituents,
common to all, was dissipated and escaped, while the other, sometimes
an acid, sometimes an earthy powder or calx, remained behind. Thus
sulphur and phosphorus, when burnt, give acids; and the metals form
_calces_. Non-combustible substances, such as lime, were imagined to
be _calces_, and it was supposed that if phlogiston were restored
to them, they too would be converted into metals. This combustible
principle was thought to be inherent in all combustible bodies
whatsoever; it corresponds in kind with the “sulphur” of more ancient
writers, but differs from the latter inasmuch as no very precise ideas
were entertained of the identity of the “sulphur” which conferred on
the substances containing it as a constituent, or possessing it as a
property, their power of combustion. It was also made more definite by
Stahl that substances capable of burning or conversion into calces are
compounds containing phlogiston in combination with other substances.
Stahl can hardly be credited with more than the invention of the term
“phlogiston,” and with bringing the subject in a clear and definite
form before his contemporaries. For Stahl wrote in 1720; and we find
Mayow, in 1674, entering into an elaborate argument to prove that
sulphuric acid is not contained in sulphur, but that it is produced
by the union of the sulphur with his fire-air particles. But Stahl
amplified the doctrine which Mayow had controverted, in pointing
out that if such substances as phosphorus, sulphur, or metals are
heated, they burn, and are changed into phosphoric acid, sulphuric
acid, or “calces”; and reciprocally, if phosphoric acid, sulphuric
acid, or a calx such as that of tin or lead, is heated with matter
rich in phlogiston, such as charcoal, pitcoal, sugar, flour, etc.,
phlogiston is restored to the burnt substance, and the original
material, phosphorus, sulphur, tin, or lead, is reproduced. The idea
at once captivated the minds of the chemists of that age, who received
it with approbation, and devised experiments designed to extend the
applications of the theory and to confirm its truth.
Substances were not supposed always to be completely deprived of
phlogiston by combustion. Indeed, if the phlogiston were removed
wholly, or nearly so, it was by no means easy to restore it. Thus
the calx of zinc, or of iron, which was regarded as nearly devoid of
phlogiston, is difficult to reduce to the metallic state by ignition
with substances rich in phlogiston, such as coal or charcoal. The
addition of phlogiston alters the appearance of the substance as
regards colour or metallic lustre, and these vary according to the
proportion of phlogiston present.
There existed no very definite idea regarding the appearance or
properties of phlogiston itself. Becher’s name for it was _terra
pinguis_, and it was represented by Becher and by Stahl as a dry
substance of an earthy nature, consisting of very fine particles, which
were capable of being set into violent motion; this idea was derived
partly from the fact that combustion is usually accompanied by flame,
which was supposed to be produced by the motion of the particles of the
body, communicated to it by the phlogiston.
It must not be forgotten that at this time it was perfectly well known
that metals gain weight on calcination. Jean Rey was quite aware of
this, and Boyle relates an experiment to show that tin gains weight
when converted into calx; and it will be remembered that Mayow made
experiments on the ignition of antimony by the aid of a burning-glass,
and rightly conjectured that the substance produced was the same as
that formed by treating it with nitric acid, and subsequent ignition.
Boyle’s view was that calx of tin was a compound of _tin_ and _heat_;
Mayow’s more correct view was that calx of antimony was a compound of
_antimony_ and _fire-air_. But in spite of these well-proved facts,
the adherents of the theory of phlogiston ignored them, and it does
not appear to have occurred to Becher or to Stahl that they were
inconsistent with their theories.
When this difficulty was stated, which was not until a much later
date, a lame explanation of a metaphysical nature, and in itself
contradictory, was all that could be offered. It was that phlogiston
is endowed with the contrary of gravity or weight, _i.e._ levity or
absolute lightness. This means, of course, that it is repelled by the
earth. But if repelled by matter, how comes it that it enters into
combination with matter? For it could not remain united if its property
were to repel and not to attract. Notwithstanding this, however, the
idea satisfied some as to the gain in weight which metals undergo in
changing into calces.
It is indeed astonishing that men of such great ability and acumen as
Black and Cavendish should have so long lain under the yoke of this
absurd theory. It is probable that, in the case of these two great
chemists, they stated their results in terms of the theory, partly
because they were content to express the facts to which they wished
to call attention in this manner, partly because they were not in
a position to replace the theory by a more rational one. It is not
easy to revolutionise a language, even though its vocabulary be a
restricted one. The object of writing is to convey thoughts to others;
and it is certainly more convenient to make use of terms understood by
others, even if they only imperfectly convey the meaning which it is
desired to express, than to attempt a revolution which will probably
be unsuccessful, and even if successful, will at all events take time.
It is not so difficult to understand Priestley’s attitude, which
we shall have to consider later; for Priestley was first of all an
experimentalist, and was captivated more by the acquisition of a new
fact than by assigning to that fact its proper place in the cosmogony
of nature.
The influence of the phlogistic theory on the knowledge of the nature
of air was of such a kind as to retard its progress. For how could
that knowledge be furthered, when the most active constituent of air
was represented by a negation? It may be said that it is easy to be
wise after the event,--in this case the discovery of oxygen; but here
was a theory which was in contradiction to many known facts, and which
furnished but a lame explanation of phenomena, and which had been
anticipated by another theory, subsequently proved to be correct. Its
sole support was the authority of its inventors or adapters, and the
deeply-ingrained notions of centuries. We may read from it a lesson
that it is wiser to seek out facts which test and prove a theory rather
than those which support it, and we may learn for the hundredth time
the folly of relying on authority, however ancient and associated with
famous names it may be. This was happily expressed by Boyle when he
wrote:[3] “For I am wont to judge of opinions as of coins: I consider
much less in any one that I am to receive, whose inscription it bears,
than what metal ’tis made of. ’Tis indifferent enough to me whether
’twas stamped many years or ages since, or came but yesterday from the
mint. Nor do I regard how many or how few hands it has passed through,
provided I know by the touchstone whether or no it be genuine, and
does or does not deserve to have been current. For if, on due proof,
it appears to be good, its having been long, and by many, received for
such, will not tempt me to refuse it. But if I find it counterfeit,
neither the prince’s image nor superscription, nor the multitude of
hands it has passed through, will engage me to receive it. And one
disfavouring trial, well made, will much more discredit it with me
than all these spurious things I have named can recommend it.”
It has been necessary to enter at some length into the nature of the
phlogistic theory, because the discoveries of the time were expressed
in its language. The _fire-air_ or _vital air_ of Mayow was termed
dephlogisticated air; _i.e._ air wholly deprived of the power of
burning, or air more capable of supporting combustion than ordinary
air; while airs capable of burning were supposed to be more or less
highly charged with phlogiston; indeed, at one time, it was imagined
that hydrogen was phlogiston itself.
It is to Joseph Black that the discovery of carbon dioxide, that
constituent of air first to be definitely recognised, if we except
Mayow’s early work, is generally ascribed. But we must remember that
it had been prepared by Becher and by Hales, and had been doubtless
obtained in an impure state by many others. It will be seen that
Black’s work was so complete, and established the identity of this
gas in so definite a manner, that his right to be named as its true
discoverer can hardly be questioned.
Black was born near Bordeaux in 1728. His father, a wine-merchant,
was originally a native of Belfast, being descended from a Scottish
family which had been settled there for some time. When twelve years
of age. Black returned to Belfast, and received his education in the
local grammar-school, afterwards proceeding to the University of
Glasgow in 1746, at the age of eighteen. He was a pupil of Dr. Cullen,
then Lecturer on Chemistry at the College there, who is mentioned
by Professor Thomas Thomson, in his _History of Chemistry_, as an
excellent and instructive lecturer. Black intended to choose the career
of medicine, and he indeed practised occasionally as a medical man
during the greater part of his life.
[Illustration: JOSEPH BLACK.]
He began his medical studies in Edinburgh in the year 1751, and in
1755 he published, as his thesis for the degree of M.D., the work
which has rendered his name famous. It appears that as early as 1752
he had been occupied with investigations on quicklime, which was
then attracting attention as a remedy for urinary calculi. Opinion
was divided regarding its virtue. In a manuscript copy of notes of
Black’s lectures, which the author is so fortunate as to possess,
he mentions that his attention was directed to the subject through
the rival views of Drs. Alston and Whytt. It was not long before he
proved that, in opposition to the commonly received notion, quicklime
had gained nothing from the fire in which it was made, but that the
limestone used for its preparation had lost nearly half its weight in
becoming caustic. He also attempted successfully to trap the escaping
gas, and again placed it in presence of lime, confining it over water.
Instead of any escape of material when the lime became mild, “nothing
escapes--the cup rises considerably by absorbing air.” And in his
notes, a few pages farther on, he compares the loss of weight undergone
by limestone on being calcined, with its loss on being dissolved in
muriatic acid. These experiments appear from his journal to have been
made before November 1752.
His thesis was not published, however, until 1755. Immediately after,
in 1756, he succeeded Dr. Cullen as Professor in Glasgow, where he
remained until 1766. During these ten years he began and made great
progress with his well-known researches on the heat of fusion of ice,
and the heat of vaporisation of water, or, as he termed them, the
“latent heats” of water and of steam. In 1766, Dr. Cullen was appointed
Professor of Medicine in the University of Edinburgh, and Black again
succeeded him as Professor of Chemistry. There he lectured until 1797,
when he retired from public life; he died as peacefully as he had
lived, in 1799, in the seventy-first year of his age. Thomson, who
relates these particulars, was one of his last students; he writes:--“I
never listened to any lectures with so much pleasure as to his; and it
was the elegant simplicity of his manner, the perfect clearness of his
statements, and the vast quantity of information which he contrived in
this way to communicate, that delighted me.... His illustrations were
just sufficient to answer completely the object in view, and no more.”
Black’s original thesis for his degree was entitled _Experiments upon
Magnesia Alba, Quicklime, and other Alcaline Substances_. It was
published in 1755, and several times reprinted. It is now to be had in
a convenient form as one of the “Alembic Club Reprints.”
It was the custom in those days to administer alkalies as a remedy
for urinary calculi; and about the year 1750 lime-water was tried as
a substitute. Opinion was divided as regarded its efficacy; and it
was with the view of preparing a better remedy that Black undertook
researches on magnesia alba. Black prepared magnesia from “bittern,”
which remains in the pans after the crystallisation of salt from
sea-water, and also from Epsom salts, “which is evidently composed
of _magnesia_ and the vitriolic acid.” The magnesia is thrown down
from the sulphate as carbonate, by the addition of pearl ashes, at
the temperature of ebullition, the soluble product being “vitriolated
tartar,” or potassium sulphate. He describes how “magnesia is quickly
dissolved with effervescence or explosion of air, by the acids of
vitriol, nitre, and of common salt, and by distilled vinegar,” and
gives an account of the properties of the sulphate, nitrate, chloride,
and acetate. He subsequently heated this magnesia, and found that it
lost “a remarkable proportion of its weight in the fire,” and his
“attempts were directed to the investigation of this volatile part.”
The residue in the retort did not effervesce on the addition of acids;
hence the volatile part had been driven away by the heat. “Chemists
have often observed, in their distillations, that part of the body has
vanished from their senses, notwithstanding the utmost care to retain
it; and they have always found, upon further inquiry, that subtile part
to be air, which, having been imprisoned in the body, under a solid
form, was set free, and rendered fluid and elastic by the fire. We may
safely conclude that the volatile matter lost in the calcination of
_magnesia_ is mostly air; and hence the calcined _magnesia_ does not
emit air, or make an effervescence when mixed with acids.”
Magnesia, thus freed from “air” by ignition, was dissolved in “spirit
of vitriol” and thrown down with an alkali. Its weight was nearly equal
to that which it possessed before calcination, and it again effervesced
with acids. “The air seems to have been furnished by the alkali, from
which it was separated by the acid; for Dr. Hales has clearly proved
that alkaline salts contain a large quantity of fixed air, which they
emit in great abundance when joined to a pure acid. In the present
case, the alkali is really joined to an acid, but without any visible
emission of air: and yet the air is not retained in it; for the neutral
salt, into which it is converted, is the same in quantity, and in
every other respect, as if the acid employed had not been previously
saturated with magnesia, but offered to the alkali in its pure
state, and had driven the air out of it in their conflict. It seems,
therefore, evident that the air was forced from the alkali by the acid,
and lodged itself in the _magnesia_.”
After an account of some experiments showing that magnesia is not
identical with lime or with alumina, he proceeds:--“It is sufficiently
clear that the calcareous earths in their native state, and that the
alkalis and _magnesia_ in their ordinary condition, contain a large
quantity of fixed air; and this air certainly adheres to them with
considerable force, since a strong fire is necessary to separate it
from _magnesia_, and the strongest is not sufficient to expel it
entirely from fixed alkalis, or take away their power of effervescing
with acid salts.
“These considerations led me to conclude that the relation between
fixed air and alkaline substances was somewhat similar to the relation
between these and acids: that as the calcareous earths and alkalis
attract acids strongly, and can be saturated with them, so they also
attract fixed air, and are, in their ordinary state, saturated with it;
and when we mix an acid with an alkali, or with an absorbent earth,
that the air is then set at liberty, and breaks out with violence;
because the alkaline body attracts it more weakly than it does the
acid, and because the acid and air cannot both be joined to the same
body at the same time.... Crude lime was therefore considered as a
peculiar acrid earth, rendered mild by its union with fixed air; and
quicklime as the same earth, in which, by having separated the air, we
discover that acrimony or attraction for water, for animal, vegetable,
and for inflammable substances.”
The solubility of slaked lime in water is next discussed. If a solution
of lime “be exposed to the open air, the particles of quicklime which
are nearest the surface gradually attract the particles of fixed air
which float in the atmosphere.”
Black next points out that, on mixing _magnesia alba_ with lime-water,
the air leaves the magnesia and joins itself to the lime; and as both
magnesia and calcium carbonate are insoluble in water, the water is
left pure. Similarly quicklime deprives alkalies of their air and
renders them caustic. And it follows that if caustic alkali be added
to a salt of magnesia or of lime, it will separate the magnesia or the
calcareous earth from the acid, in a condition free from “air” but
combined with water.
In order to show that the “air” which exists in combination with lime
or alkalies is not the air which is contained in solution in water,
lime-water was placed under an air-pump, along with an equal quantity
of pure water; on making a vacuum, an approximately equal amount of
air was evolved from each. “Quicklime, therefore, does not attract air
when in its most ordinary form, but is capable of being joined to one
particular species only, which is dispersed through the atmosphere,
either in the shape of an exceedingly subtile powder, or more probably
in that of an elastic fluid. To this I have given the name of _fixed
air_, and perhaps very improperly; but I thought it better to use a
word already familiar in philosophy than to invent a new name, before
we be more fully acquainted with the nature and properties of this
substance, which will probably be the subject of my further inquiry.”
The next proceeding was to render “mild alkali” caustic by means of
lime, and to determine that nearly the same amount of acid is required
to saturate the caustic alkali as to saturate the mild alkali from
which the caustic alkali had been prepared. On exposure to air for
a fortnight, the caustic alkali again became mild, owing to its
absorption of fixed air. Careful experiments were made to prove that
such caustic alkali contains no lime, and does not therefore owe its
causticity and corrosive properties to the presence of that ingredient.
The volatile alkali (ammonium carbonate) was also rendered caustic, and
Black “obtained an exceedingly volatile and acrid spirit, which neither
effervesced with acids nor altered in the least the transparency of
lime-water; and although very strong was lighter than water, and
floated upon it like spirit of wine.”
After a description of some unsuccessful attempts to render mild
alkalies caustic by heat alone (_i.e._ to expel carbon dioxide from
potassium carbonate), Black examines the action of the “sedative salt”
or boracic acid on mild alkalies, by rubbing them together in presence
of some water. At first there is no effervescence, but on adding
successive quantities of boracic acid, brisk effervescence finally
takes place, borax being formed. “This phenomenon may be explained by
considering the fixed alkalis as not perfectly saturated with air ...
if they expel a small quantity of air from some of the salt, this air
is at the same time absorbed by such of the contiguous particles as are
destitute of it.” And on “exposing a small quantity of a pure vegetable
fixed alkali (carbonate of soda) to the air, in a broad and shallow
vessel, for the space of two months,” crystals were obtained, which
possessed a milder taste than that of ordinary salt of tartar, which
effervesced with acids more violently than usual, and which could not
be mixed with the smallest portion of boracic acid without emitting
a sensible quantity of air (hydrogen sodium carbonate). It therefore
follows that such alkaline substances have an attraction for fixed
air; and this was proved by mixing _magnesia alba_ in fine powder with
caustic alkali, and shaking for some time. The magnesia was converted
into the variety which did not effervesce with acids, and the alkali
was rendered mild, like a solution of salt of tartar. These are the
principal results of Black’s researches, and he concludes with a table
of affinity of acids for fixed alkali, calcareous earth, volatile
alkali, and magnesia, contrasting it with the affinity possessed by
fixed air for the same bases.
It was the habit of the Scottish students to pass down notes taken
during the lectures of their professors from one generation to another.
As the lectures were generally read, and not delivered _extempore_,
the process resulted in an almost verbatim report of the actual words
of the lecturer. One of these copies of lectures, bearing the date
1778, gives an account of the experiments which have been described,
in words almost identical with those used in the thesis of 1755. Black
appears to have shown his class this air, made, however, according to
Hales’ plan, by heating magnesium carbonate in a bent gun-barrel, and
collected over water in the usual way. He demonstrated its weight by
pouring it from one vessel to another, and showed that it extinguished
the flame of a candle. He mentions also that in 1752 he discovered that
this air is the same as choke-damp, and that it is fatal to animal
life. He speaks of the Grotto del Cane, and observes that fixed air
is produced by fermentation, and by the burning of charcoal, and showed
to his class experiments in which air from each source is shaken with
lime-water, giving a turbidity of carbonate. The well-known experiment
of inspiring air through lime-water, which, owing to the small amount
of carbonic anhydride it contains, does not produce a turbidity, and
expiring through lime-water, showing the formation of carbon dioxide in
the lungs, is described and performed. He next describes Cavendish’s
experiments on the solubility of fixed air and its density, and
researches by Dr. Brownrigg and Dr. Gahn of Sweden on its occurrence
in mineral waters. He also explains how calcareous petrifactions are
produced by the escape of fixed air from water, which then deposits its
dissolved calcium carbonate, present in solution as bicarbonate. The
deposit of iron from chalybeate waters is ascribed to the same cause,
and the explanation is attributed to Mr. Lane.
“Upon the whole,” these manuscript notes relate, “this sort of air
is quite distinct from common air, though it is commonly mixed with
it in small quantity.” “With regard to its origin, when treating of
inflammable substances and metals I shall consider this more
completely. I shall now only hint that it is a vital air, changed by
some matter, seemingly the principle of inflammability. This appears
from several phenomena when an animal or burning body is enclosed
with a certain quantity of this air, until it is changed as much as
possible.” The air is diminished in volume by the breathing of the
animal or by the burning of the candle. And Dr. Priestley has found
that “growing vegetables had the power of restoring this sort of air
to common or vital air again, which must be by their taking away some
matter which it had received from the burning body or animal.”
Black’s account of fixed air and its properties is the first example we
possess of a clear and well-reasoned series of experimental researches,
where nothing was taken on trust, but everything was made the subject
of careful quantitative measurement. It was not long since Hales had
pronounced air to be a chaotic mixture of effluvia. Black showed that
common air contains a small amount of fixed air, and that fixed air
must be considered as a fluid differing in many of its properties
from common air, especially in its being absorbed by quicklime and by
alkalies. It must be remembered that at that time carbon was not
recognised as an element; and hence, though Black knew that fixed air
was a product of the combustion of charcoal, he did not attribute it
to the union of carbon with oxygen, although the sentence quoted above
closely approaches to the truth.
The discovery of nitrogen was next in the order of time. It was made by
Daniel Rutherford, a pupil of Black’s, and at his instigation, and its
description formed a thesis for his degree of Doctor of Medicine.
Daniel Rutherford was born at Edinburgh on November 3rd, 1749. He was
the son of a medical man, Dr. John Rutherford, one of the founders
of the Medical School in that city. He was educated at Edinburgh
University, and after graduating in Arts, became a medical student,
taking his degree of M.D. in 1772. His diploma was obtained on 12th
September. He then travelled for three years in England, France, and
Italy, and in 1775 he returned to his native town, where he practised
his profession. In 1786 he succeeded Dr. John Hope in the Chair of
Botany in his University, but he did not on that account resign his
practice. He was president of the Royal College of Physicians of
Edinburgh from 1796 to 1798. During the greater part of his life he
suffered from gout; he died in 1819, at the age of seventy.
Rutherford does not seem to have pursued the study of chemistry
further: his duties led him into other fields. His genial, pleasant
face, seen in the portrait by Raeburn, shows him to have possessed a
happy disposition; and he is said to have maintained until his death
his friendship with Black, and his interest in the progress which
science was then rapidly making.
The title of Rutherford’s dissertation, of which I have been able to
find a copy only in the British Museum, is _Dissertatio Inauguralis
de aere fixo dicto, aut mephitico_. It was published at Edinburgh in
1772, seventeen years after Black’s memorable dissertation on Fixed
Air. As will be seen shortly, it precedes Priestley’s and Scheele’s
writings by a year or two. Evidently Black had noticed that a residue
was left after the combustion of carbonaceous bodies in air, and
absorption of the fixed air produced by the combustion, and had
suggested to Rutherford, then a student of his, the advantage of
further investigating the matter, and ascertaining the properties of
the residual gas.
[Illustration: DANIEL RUTHERFORD.]
Rutherford begins his essay with an apt quotation from Lucretius:--
Denique res omnes debent in corpore habere
Aëra, quandoquidem rara sunt corpora et aër
Omnibus est rebus circumdatus appositusque.
He next proceeds to define the atmosphere as a pellucid thin fluid,
in which clouds float and vapours rise. Its necessity for animal
and vegetable life is acknowledged by all. It possesses weight and
elasticity. It can be fixed by other bodies; but the air obtained from
them by distillation differs from ordinary vital, salubrious air, and
is often termed mephitic or poisonous.
After acknowledging his debt to his illustrious preceptor Black, he
proceeds to quote from the latter to the effect that mephitic or fixed
air is the air which proves fatal to animals and extinguishes fire;
which is easily absorbed by quicklime and by alkaline salts; which
occurs in the Grotto del Cane, and in mineral waters; and which is
produced during exhalation from the lungs, by combustion, and during
certain kinds of fermentation. Its density, compared with that of
ordinary air, is as 15½ or 16 to 9; hence it can be kept for some
time in an open glass, and a candle lowered into it is extinguished. It
has an agreeable taste and smell; and it changes the colour of syrup of
violets from blue to purple. It prevents putrefaction, but putrefied
bodies are not made fresh by it. It possesses the power of combining
with lime, which acquires new properties as the result of its action.
Rutherford then recalls Black’s experiments on lime and on magnesia,
pointing out how these bases absorb fixed air, and how it can be
recovered from them and from its compounds with alkalies, sometimes by
heat, and always by the action of acids.
Rutherford next describes experiments which show that a mouse, placed
in atmospheric air, and left till dead, diminishes the volume of the
air by one-tenth; and that the residual air, on treatment with alkali,
loses one-eleventh of its volume. The residue extinguishes the flame of
a candle; but tinder continues to smoulder in it for a short time. It
is thus proved that after the whole of the fixed air has been withdrawn
by alkalies, the residue is still incapable of supporting life and
combustion.
Some burning bodies deprive air of its “salubrity” more easily than
others. The phosphorus of urine continues to glow in air in which a
candle has ceased to burn, or in which charcoal has burned until it is
extinguished. Even after the absorption of all fixed air by alkalies,
phosphorus burns, emitting clouds of the dry acid of phosphorus, which
can be absorbed by lime-water.
“It therefore appears that pure air is not converted into mephitic air
by force of combustion, but that this air rather takes its rise or is
thrown out from the body thus resolved. And from this it is permissible
to draw the conclusion that that unwholesome air is composed of
atmospheric air in union with, and, so to say, saturated with,
phlogiston. And this conjecture is confirmed by the fact that air which
has served for the calcination of metals is similar, and has clearly
taken away from them their phlogiston.” Such air differs from the air
evolved from metals by the action of acids, which is more thoroughly
impregnated with phlogiston; and also from that from decaying flesh,
which is a mixture of mephitic air and combustible air.
He proceeds:--“I had intended to add something regarding the
composition of mephitic air, and to seek for a reason for its
unwholesome effects, but I have not been able to find out anything
with certainty. Certain experiments appear to show, however, that it
consists of atmospheric air in union with phlogistic material; for
it is never produced except from bodies which abound in inflammable
parts: the phlogiston appears to escape from such bodies when they
become converted into the calces of metals. I say from phlogistic
material, because, as already mentioned, pure phlogiston, in
combination with common air, can be seen to yield another kind of air
[viz. hydrogen].... I have lately heard that Priestley believes that
vegetables growing in mephitic air dispel its noxious ingredients, or,
as it were, extract them, and restore its original wholesomeness; and
that mephitic air, added to air from putrid flesh, partly mitigates its
unwholesome character. But I have been unable to try such experiments.”
We see, then, that Rutherford’s claims to the discovery of nitrogen
amount to this:--he removed the oxygen from ordinary air by
combustibles such as charcoal, phosphorus, or a candle; and having got
rid of the carbon dioxide, in those cases when it was formed, by alkali
or lime, he obtained a residue, now known as nitrogen. His view of the
nature of this gas, in the phlogistic language of the time, was that
the burning bodies had given up some of their “phlogistic material” to
the air, which was thus altered. Nitrogen was “phlogisticated air,”
even though incombustible; hydrogen, too, was phlogisticated air, but
air produced by the union of pure phlogiston with atmospheric air.
The step taken by Rutherford, under Black’s guidance, was an advance,
though not a great one, in the development of the theory of the true
nature of air; and he may be well credited with the discovery of
nitrogen.
[3] _A Free Inquiry into the Vulgar Notion of Nature_; Prefatory
remarks.