PHLOGISTONISM
Even before the appearance of _The Sceptical Chemist_ there was a
growing conviction that the old hypotheses as to the essential nature
of matter were inadequate and misleading. We have seen how the four
“elements” of the Peripatetics had become merged into the _tria
prima_—the “salt,” “sulphur,” and “mercury”—of the Paracelsians. As
the phenomena of chemical action became better known, the latter
iatro-chemists—or, rather, that section of them which recognised that
chemistry had wider aims than to minister merely to medicine—felt that
the conception of the _tria prima_, as understood by Paracelsus and
his followers, was incapable of being generalised into a theory of
chemistry. Becher, while clinging to the conception of three primordial
substances as making up all forms of matter, changed the qualities
hitherto associated with them. According to the new theory, all matter
was composed of a mercurial, a vitreous, and a combustible substance
or principle, in varying proportions, depending upon the nature of the
particular form of matter. When a body was burnt or a metal calcined,
the combustible substance—the _terra pinguis_ of Becher—escaped.
This attempt to connect the phenomena of combustion and calcination
with the general phenomena of chemistry was still further developed
by Stahl, and was eventually extended into a comprehensive theory of
chemistry, which was fairly satisfactory so long as no effort was made
to test its sufficiency by an appeal to the balance.
=George Ernest Stahl=, who developed Becher’s notion into the theory of
_phlogiston_ (φλογιοτός—burnt), and thereby created a generalisation
which first made chemistry a science, was born at Anspach in 1660,
became Professor of Medicine and Chemistry at Halle in 1693, physician
to the King of Prussia in 1716, and died in Berlin in 1734.
Stahl contributed little or nothing to practical chemistry; and no new
fact or discovery is associated with his name. His service to science
consists in the temporary success he achieved in grouping chemical
phenomena, and in explaining them consistently by a comprehensive
hypothesis.
The theory of phlogiston was originally broached as a theory of
combustion. According to this theory, bodies such as coal, charcoal,
wood, oil, fat, etc., burn because they contain a combustible
principle, which was assumed to be a material substance and uniform
in character. This substance was known as phlogiston. All combustible
bodies were to be regarded, therefore, as compounds, one of their
constituents being phlogiston: their different natures depended
partly upon the proportion of phlogiston they contain, and partly
upon the nature and amount of their other constituents. A body, when
burning, was parting with its phlogiston; and all the phenomena of
combustion—the flame, heat, and light—were caused by the violence
of the expulsion of that substance. Certain metals—as, for example,
zinc—could be caused to burn, and thereby to yield earthy substances,
sometimes white in colour, at other times variously coloured. These
earthy substances were called _calces_, from their general resemblance
to lime. Other metals, like lead and mercury, did not appear to burn;
but on heating them they gradually lost their metallic appearance, and
became converted into calces. This operation was known as calcination.
In the act of burning or of calcination phlogiston was expelled. Hence
metals were essentially compound: they consisted of phlogiston and a
calx, the nature of which determined the character of the metal. By
adding phlogiston to a calx the metal was regenerated. Thus, on heating
the calx of zinc or of lead with coal, or charcoal, or wood, metallic
zinc or lead was again formed. When a candle burns, its phlogiston
is transferred to the air; if burned in a limited supply of air,
combustion ceases, because the air becomes saturated with phlogiston.
Respiration is a kind of combustion whereby the temperature of the body
is maintained. It consists simply in the transference of the phlogiston
of the body to the air. If we attempt to breathe in a confined
space, the air becomes eventually saturated with the phlogiston, and
respiration stops. The various manifestations of chemical action, in
like manner, were attributed to this passing to and fro of phlogiston.
The colour of a substance is connected with the amount of phlogiston
it contains. Thus, when lead is heated, it yields a yellow substance
(litharge); when still further heated, it yields a red substance (red
lead). These differences in colour were supposed to depend upon the
varying amount of phlogiston expelled.
The doctrine of phlogiston was embraced by nearly all Stahl’s German
contemporaries, notably by Marggraf, Neumann, Eller, and Pott. It
spread into Sweden, and was accepted by Bergman and Scheele; into
France, where it was taught by Duhamel, Rouelle, and Macquer; and into
Great Britain, where its most influential supporters were Priestley and
Cavendish. It continued to be the orthodox faith until the last quarter
of the eighteenth century, when, after the discovery of oxygen, it was
overturned by Lavoisier.
During the sway of phlogiston chemistry made many notable advances—not
by its aid, but rather in spite of it. As a matter of fact, until
the time of Lavoisier few, if any, investigations were made with the
express intention of testing it, or of establishing its sufficiency.
When new phenomena were observed the attempt was no doubt made to
explain them by its aid, frequently with no satisfactory result.
Indeed, even in the time of Stahl, facts were known which it was
difficult or impossible to reconcile with his doctrine; but these
were either ignored, or their true import explained away. Although,
therefore, these advances were in no way connected with phlogiston,
it will be convenient to deal with the more important of them now,
inasmuch as they were made during the phlogistic period.
With the exception of Marggraf, Stahl’s German contemporaries
contributed few facts of first-rate importance to chemistry. =Pott=,
who was born at Halberstadt in 1692 and become Professor of Chemistry
in Berlin in 1737, is chiefly remembered by his work on porcelain,
the chemical nature and mode of origin of which he first elucidated.
=Marggraf=, born in Berlin in 1709, was one of the best analysts of his
age. He first clearly distinguished between lime and alumina, and was
one of the earliest to point out that the vegetable alkali (potash)
differed from the mineral alkali (soda). He also showed that gypsum,
heavy spar, and potassium sulphate were analogous in composition.
He clearly indicated the relation of phosphoric acid to phosphorus,
described a number of methods of preparing that acid, and explained the
origin of the phosphoric acid in urine.
Of the Swedish chemists of that period, the most notable was Scheele.
=Carl Wilhelm Scheele= was born in 1742 at Stralsund. When fourteen
years of age he was apprenticed to an apothecary at Gothenburg, and
began the study of experimental chemistry, which he continued to
prosecute as an apothecary at Malmö, Stockholm, Upsala, and eventually
at Köping on Lake Malar, where he died in 1786, in the forty-third year
of his age. During the comparatively short period of his scientific
activity Scheele made himself the greatest chemical discoverer of his
time.
[Illustration: CARL WILHELM SCHEELE.
From the statue by Börjeson at Stockholm.]
He first isolated chlorine, and determined the individuality of
manganese and baryta. He was an independent discoverer of oxygen,
ammonia, and hydrogen chloride. He discovered also hydrofluoric,
nitro-sulphonic, molybdic, tungstic, and arsenic, among the inorganic
acids; and lactic, gallic, pyrogallic, oxalic, citric, tartaric, malic,
mucic, and uric acids among the organic acids. He isolated glycerine
and milk-sugar; determined the nature of microcosmic salt, borax, and
Prussian blue, and prepared hydrocyanic acid. He demonstrated that
graphite is a form of carbon. He discovered the chemical nature of
sulphuretted hydrogen, arsenuretted hydrogen, and the green arsenical
pigment known by his name. He invented new processes for preparing
ether, powder of algaroth, phosphorus, calomel, and _magnesia alba_.
He first prepared ferrous ammonium sulphate, showed how iron may be
analytically separated from manganese; and described the method of
breaking up mineral silicates by fusion with alkaline carbonates.
Scheele’s contributions to chemical theory were slight and unimportant,
but as a discoverer he stands pre-eminent.
Of the French phlogistians we have space only to mention Duhamel and
Macquer.
=Henry Louis Duhamel du Monceau= was born at Paris in 1700. He was one
of the earliest to make experiments on ossification, and one of the
first to detect the difference between potash and soda.
=Peter Joseph Macquer= was born in 1718 at Paris. He investigated
the nature of Prussian blue (discovered by Diesbach, of Berlin, in
1710), worked on platinum, wrote one of the best text-books of his
time, published a dictionary of chemistry, and was an authority of the
chemistry of dyeing.
In addition to those already mentioned, the most notable names as
workers in chemistry in Great Britain during the eighteenth century are
Black, Priestley, and Cavendish.
=Joseph Black= was born in 1728 at Bordeaux, where his father was
engaged in the wine trade. A student of the University of Glasgow, he
became its Professor of Chemistry in 1756. In 1766 he was transferred
to the Chemical Chair of the University of Edinburgh, and died in 1799.
Black published only three papers, the most important of which is
entitled _Experiments upon Magnesia Alba, Quicklime, and Other Alkaline
Substances_. He proved that magnesia is a peculiar earth differing
in properties from lime. Lime is a pure earth, while limestone is
carbonate of lime. He showed that magnesia will also combine with
carbonic acid, and he explained that the difference between the mild
and caustic alkalis is that the former contain carbonic acid, whereas
the latter do not. He also explained how lime is able to convert the
mild alkalis into caustic alkalis. Simple and well known as these
facts are to-day, their discovery in 1755 excited great interest, and
marked an epoch in the history of chemistry. Black’s name is associated
with the discovery of latent and specific heat, and he made the first
determinations of the amount of heat required to convert ice into water.
[Illustration: JOSEPH PRIESTLEY.
From a mezzotint after Fuseli in the possession of the Royal Society.]
=Joseph Priestley=, the son of a clothdresser, was born in 1733 at
Fieldhead, near Leeds. When seven years of age, on the death of his
mother, he was taken charge of by his aunt, and was educated for
the Nonconformist ministry, eventually becoming a Unitarian. He was
first attracted to science by the study of electricity, of which he
compiled a history. At Leeds, where he had charge of the Mill Hill
congregation, he turned his attention to chemistry, mainly from the
circumstance that he lived near a brewery and had the opportunity
of procuring large quantities of carbonic acid, the properties of
which he carefully studied. He abandoned the ministry for a time to
become librarian and literary companion to Lord Shelburne, with whom
he remained seven years. During this time he industriously pursued
chemical inquiry, and discovered a large number of æriform bodies—viz.,
nitric oxide, hydrogen chloride, sulphur dioxide, silicon fluoride,
ammonia, nitrous oxide, and, most important of all from the point of
view of chemical theory, oxygen gas. Priestley’s work gave a remarkable
impetus to the study of pneumatic chemistry. It exercised great
influence on the extension of chemical science, and—in other hands
than his—on the development of chemical theory. The most important of
his contributions to science are contained in his _Experiments and
Observations on Different Kinds of Air_. This work not only gives an
account of the methods by which he isolated the gases he discovered,
but describes a great number of incidental observations, such as the
action of vegetation on respired air, showing that the green parts of
plants are able in sunlight to decompose carbonic acid and to restore
oxygen to the atmosphere. He was, in fact, one of the earliest to trace
the specific action of animals and plants on atmospheric air, and to
show how these specific actions maintained its purity and constancy of
composition. He initiated the art of eudiometry (gas analysis), and
was the first to establish that the air is not a simple substance,
as imagined by the ancients. Priestley is to be credited with the
invention of _soda-water_, which he prepared as a remedy for scurvy;
and his name is connected with the so-called _pneumatic trough_—a
simple enough piece of apparatus, but one which proved to be of the
greatest service to him in his inquiries.
After leaving Lord Shelburne, Priestley removed to Birmingham and
resumed his ministry. His religious and political opinions made him
obnoxious to the Church and State party; and during the riots of
1791 his house was wrecked, his books and apparatus destroyed, and
his life endangered. Eventually he emigrated to America, and settled
at Northumberland, where he died on February 6th, 1804, in the
seventy-first year of his age.
[Illustration: From a drawing by Alexander in the Print Room of the
British Museum.]
=Henry Cavendish= was born at Nice in 1731, and died in London in 1810.
He was a natural philosopher in the widest sense of that term, and
occupied himself in turn with nearly every branch of physical science.
He was a capable astronomer and an excellent mathematician, and he
was one of the earliest to work on the subject of specific heat, and
to improve the thermometer and the methods of making thermometric
observations. He also determined the mean density of the earth. He made
accurate observations on the properties of carbonic acid and hydrogen,
greatly improved the methods of eudiometry, and first established
the practical uniformity of the composition of atmospheric air. His
greatest discovery, however, was his determination of _the composition
of water_. He was the first to prove that water is not a simple or
elementary substance, as supposed by the ancients, but is a compound of
hydrogen and oxygen. In certain of his trials he found that the water
formed by the union of oxygen and hydrogen was acid to the taste; and
the search for the cause of this acidity led him to the discovery of
the _composition of nitric acid_. He was the first to make a fairly
accurate analysis of a natural water, and to explain what is known as
the _hardness of water_.
Phlogistonism may be said to have dominated chemistry during
three-fourths of the eighteenth century. Although radically false
as a conception and of little use in the true interpretation of
chemical phenomena, it cannot be said to have actually retarded the
pursuit of chemistry. Men went on working and accumulating chemical
facts uninspired and, for the most part, uninfluenced by it. Even
Priestley, perhaps one of the most conservative of the followers of
Stahl, regarded his dogma with a complacent tolerance; and as its
inconsistencies became apparent he was more than once on the point
of renouncing it. Of one thing he was quite convinced, and that was
that Stahl had greatly erred in his conception of the real nature of
phlogiston. Perhaps the most signal disservice which phlogiston did to
chemistry was to delay the general recognition of Boyle’s views of the
nature of the elements. The alchemists, it will be remembered, regarded
the metals as essentially compound. Boyle was disposed to believe that
they were simple. Becher and Stahl and their followers, until the last
quarter of the eighteenth century, also regarded them as compounds,
phlogiston being one of their constituents. On the other hand, what
we now know to be compounds—such as the calces, the acids, and water
itself—were held by the phlogistians to be simple substances.
The discovery, in 1774, of oxygen—the dephlogisticated air of
Priestley—and the recognition of the part it plays in the phenomena
which phlogiston was invoked to explain, mark the termination of one
era in chemical history and the beginning of another. Before entering
upon an account of the new era it is desirable to take stock of the
actual condition of chemical knowledge at the end of the phlogistic
period, and to show what advances had been made in pure and applied
chemistry during that time.
During the eighteenth century greater insight was gained into the
operations of the form of energy with which chemistry is mainly
concerned, and views concerning chemical affinity and its causes
began to assume more definite shape, chiefly owing to the labours of
Boerhaave, Bergman, Geoffroy, and Rouelle. It was clearly recognised
that the large group of substances comprised under the term “salts”
were compound, and made up of two contrasted and, in a sense,
antagonistic constituents, classed generically as acids and bases.
On the practical side chemistry made considerable progress. Analysis—a
term originally applied by Boyle—greatly advanced. It was, of course,
mainly qualitative; but, thanks to the labours of Boyle, Hoffmann,
Marggraf, Scheele, Bergman, Gahn, and Cronstedt, certain reactions
and reagents came to be systematically applied to the recognition of
chemical substances, and the precision with which these reagents were
used led to the detection of hitherto unknown elements. The beginnings
of a quantitative analysis were made even before the time of Boyle,
but its principles were greatly developed by him, and were further
extended by Homberg, Marggraf, and Bergman. Marggraf accurately
determined the amount of silver chloride formed by adding common salt
to a solution of a known weight of silver, and Bergman first pointed
out that estimations of substances might be conveniently made by
weighing them in the form of suitably prepared compounds, which, it
was implicitly assumed, were of uniform and constant composition. The
foundations of an accurate system of gaseous analysis were made by
Cavendish; and various forms of physical apparatus were applied to the
service of chemistry.
To the elements which were known prior to Boyle’s time, although not
recognised as such, there were added phosphorus (Brand, 1669), nitrogen
(Rutherford), chlorine (Scheele, 1774), manganese (Gahn, 1774), cobalt
(Brandt, 1742), nickel (Cronstedt, 1750), and platinum (Watson, 1750).
Baryta was discovered by Scheele, and strontia by Crawford. Phosphoric
acid was discovered by Boyle, and its true nature determined by
Marggraf; Cavendish first made known the composition of nitric acid.
As already stated, Scheele first isolated molybdic and tungstic acids
and determined the existence of a number of the organic acids (p. 75).
Other discoveries—such as the true nature of limestone and _magnesia
alba_ and their relations respectively to lime and magnesia by Black,
the many gaseous substances by Priestley, and the compound nature of
water by Cavendish—have already been referred to.
Technical chemistry also greatly developed during the eighteenth
century, thanks to the efforts of Gahn, Marggraf, Duhamel, Reaumur,
Macquer, Kunkel, and Hellot; and many important industrial
processes—such as the manufacture of sulphuric acid by Ward of
Richmond, and subsequently by Roebuck at Birmingham, and the Leblanc
process of conversion of common salt into alkali—had their origin
during this period.