CHLORINE.
The rivalry between the French and English chemists continued, but it
took a new departure. Gay Lussac and Thenard had stolen a march on
Davy by their discovery of a chemical method of making the metals of
the alkalis, whereby they were able to use these metals as chemical
reagents to greater advantage; but the tables were quickly turned. On
July 12th, 1810, Davy read to the Royal Society his memorable paper “On
the oxymuriatic Acid, its Nature and Combinations; and on the Elements
of the muriatic Acid; with some Experiments on Sulphur and Phosphorus,
made in the Laboratory of the Royal Institution.” This paper, in which
he first demonstrates the nature of chlorine, is very short--only some
twenty-six quarto pages--but it is unquestionably one of the most
brilliant, as it is one of the most forcible of his productions.
Davy is here seen at his best. He is bold and yet wary, and as
dexterous as trenchant; so confident is he in the strength of his
position that he casts aside every argument that might tell in his
favour, unless it is based on the most unimpeachable evidence. It is
difficult to know what to admire most--the clearness of perception, the
precision of the statement, the strictness of the logic, the aptness
of the illustration, or the argumentative skill with which the whole
is marshalled and presented. As a piece of induction, the memoir is a
model of its kind, and as an exercise in “the scientific use of the
imagination” it has few equals. Most scientific papers will stand a
considerable amount of winnowing, and there is no assay-master more
scrupulously strict than Time. “The more a science advances, the more
it becomes concentrated in little books,” says Leibnitz; but the most
fastidious of critics might read and re-read this work without wishing
to omit or amend a sentence.
Every chemical student to-day is told that the elementary nature of
chlorine was first _demonstrated_ by Davy, and if the student is
informed what Davy meant by the term “element,” the statement is not
incorrect. What, however, Davy actually did was to demonstrate that
the substance called oxymuriatic acid contained no oxygen; that it was
a peculiar substance which “has not as yet been decompounded,” and
therefore is “elementary as far as our knowledge extends.” The very
character of the name which he suggested indicates this cautious and
philosophical view. In making the suggestion, he says:--
“To call a body which is not known to contain oxygen and which
cannot contain muriatic acid, oxymuriatic acid, is contrary to
the principles of that nomenclature in which it is adopted;
and an alteration of it seems necessary to assist the progress
of discussion, and to diffuse just ideas on the subject. If the
great discoverer of this substance [Scheele, who first observed
it in 1774] had signified it by any simple name, it would have
been proper to have recurred to it; but, _dephlogisticated marine
acid_ is a term which can hardly be adopted in the present
advanced era of the science.
“After consulting some of the most eminent chemical philosophers
in this country, it has been judged most proper to suggest
a name founded upon one of its obvious and characteristic
properties--its colour, and to call it _chlorine_, or _chloric_
gas.[H]
“Should it hereafter be discovered to be compound, and even
to contain oxygen, this name can imply no error, and cannot
necessarily require a change.”
[H] [From χλωρος.]
As the actual facts and arguments on which Davy based his views
are seldom set forth in text-books, or presented to the student by
teachers, it may be desirable to give a detailed account of his famous
memoir. He begins by saying:--
“The illustrious discoverer of the oxymuriatic acid considered
it as muriatic acid freed from hydrogen; and the common muriatic
acid as a compound of hydrogen and oxymuriatic acid; and on this
theory he denominated oxymuriatic acid dephlogisticated muriatic
acid.
“M. Berthollet, a few years after the discovery of Scheele, made
a number of important and curious experiments on this body; from
which he concluded that it was composed of muriatic acid and
oxygen; and this idea for nearly twenty years has been almost
universally adopted.”
Having thus accurately stated the position, he proceeds to attack it.
In the first place, he points out that Henry, ten years before, had
shown that hydrogen could be produced from muriatic acid gas by the
agency of electricity; this hydrogen was assumed by Henry to be due to
water contained in the gas. Davy, in his Bakerian lecture of 1808, had
shown that muriatic acid gas gave hydrogen when treated with potassium,
and he had stated “that muriatic acid can in no instance be procured
from oxymuriatic gas, or from dry muriates, unless water or its
elements be present.”
Gay Lussac and Thenard had concluded “that muriatic acid gas contains
about one-quarter of its weight of water; and that oxymuriatic acid is
not _decomposable_ by any substances but hydrogen, or such as can form
triple combinations with it.”
He then points out, what he had already stated in a former paper, that
charcoal freed from hydrogen and moisture by intense ignition _in
vacuo_ may be heated to whiteness by the voltaic battery in oxymuriatic
or muriatic acid gases without affecting any change in them.
It now occurred to him that if the liquor of Libavius (stannic
chloride) is a combination of muriatic acid and oxide of tin, as then
surmised, oxide of tin ought to be separated from it by means of
ammonia. On admitting ammonia gas to the tin chloride over mercury, the
substances combined with great heat, a white solid was obtained; “some
of it was heated to ascertain if it contained oxide of tin, but the
whole volatilised, producing dense pungent fumes.” The experiment was
repeated with every care, but no oxide of tin could be obtained.
He was next led to study the behaviour of ammonia with the substances
he had formerly obtained, by the action of oxymuriatic gas on
phosphorus (see p. 129). One of these is solid, and is now known as
phosphorus pentachloride; the other is liquid, and is termed phosphorus
trichloride.
“The first,” he says, “on the generally received theory of the
nature of oxymuriatic acid, must be considered as a compound of
muriatic acid and phosphoric acid. It occurred to me that if the
acids of phosphorus really existed in these combinations, it
would not be difficult to obtain them, and thus to gain proof of
the existence of oxygen in oxymuriatic acid.”
He therefore brought ammonia gas into contact with the solid compound
of oxymuriatic acid and phosphorus. Much heat was produced, and a white
opaque powder was formed.
“Supposing that this substance was composed of the dry muriate
and phosphate of ammonia; as muriate of ammonia is very volatile,
and as ammonia is driven off from phosphoric acid, by a heat
below redness I conceived that by igniting the product obtained
I should procure phosphoric acid ... but found to my great
surprise that it was not at all volatile nor decomposable at
this degree of heat, and that it gave off no gaseous matter. The
circumstance that a substance composed principally of oxymuriatic
acid and ammonia should resist decomposition or change at so
high a temperature induced me to pay particular attention to the
properties of this new body.”
What he actually obtained was mainly a mixture of the so-called
_phospham_ and _chlorophosphamide_, remarkably stable substances,
the characteristic properties of which he describes with accuracy.
He then examined the action of ammonia gas on sulphur chloride, “the
sulphuretted muriatic liquor of Dr. Thomson,” but as the compounds
formed
“did not present the same uniform and interesting properties
as that from the phosphoric sublimate, I did not examine them
minutely: I contented myself by ascertaining that no substance
known to contain oxygen could be procured from oxymuriatic acid
in this mode of operation.”
He then shows that ammonia and oxymuriatic acid, in condensing to sal
ammoniac with liberation of nitrogen, contrary to the general belief,
form no water. According to Cruickshank, who appears to have been the
first to make the observation, “hydrogenous gas” required rather more
than its own volume of oxygenated muriatic acid to saturate it when a
mixture of the two was exploded by means of the electric spark, “the
products being water and muriatic acid.” Gay Lussac and Thenard had
stated that no water was thus formed.
“I have attempted,” says Davy, “to make the experiment still
more refined by drying the oxymuriatic acid and the hydrogen
by introducing them into vessels containing muriate of lime
[calcium chloride] and by suffering them to combine at common
temperatures; but I have never been able to avoid a slight
condensation; though in proportion as the gases were free from
oxygen or water, this condensation diminished.[I]
“MM. Gay Lussac and Thenard have proved by a copious collection
of instances, that in the usual cases where oxygen is procured
from oxymuriatic acid, water is always present, and muriatic acid
gas is formed; now as it is shewn that oxymuriatic acid gas is
converted into muriatic acid gas by combining with hydrogen, it
is scarcely possible to avoid the conclusion, that the oxygen is
derived from the decomposition of the water, and consequently
that the idea of the existence of water in muriatic acid gas, is
hypothetical, depending upon an assumption which has not yet been
proved--the existence of oxygen in oxymuriatic acid gas.
“MM. Gay Lussac and Thenard indeed have stated an experiment,
which they consider as proving that muriatic acid gas contains
one-quarter of its weight of combined water. They passed this gas
over litharge, and obtained so much water; but it is obvious,
that in this case, they formed the same compound as that produced
by the action of oxymuriatic acid on lead; and in this process
the muriatic acid must lose its hydrogen and the lead its oxygen;
which of course would form water; these able chemists, indeed,
from the conclusion of their memoir, seem aware, that such an
explanation may be given, for they say, that the oxymuriatic acid
_may_ be considered as a simple body.”
[I] Theoretically, there should be no contraction. One volume
of chlorine combines with one volume of hydrogen to form
two volumes of hydrogen chloride [muriatic acid gas].
Dalton’s law of gaseous volumes had been established by Gay
Lussac before 1810.
He then repeats the experiments which first led him to suspect the
existence of combined water in muriatic acid.
“When mercury is made to act upon 1 volume of muriatic acid gas,
by voltaic electricity, all the acid disappears, calomel is
formed, and about ·5 of hydrogen evolved.”
The same result is obtained by the use of potassium.
“And in some experiments made very carefully by my brother, Mr.
John Davy, on the decomposition of muriatic acid gas, by heated
tin and zinc, hydrogen, equal to about half its volume, was
disengaged, and metallic muriates, the same as those produced by
the combustion of tin and zinc in oxymuriatic gas, resulted.”
“It is evident from this series of observations, that Scheele’s
view (though obscured by terms derived from a vague and unfounded
general theory) of the nature of the oxymuriatic and muriatic
acids, may be considered as an expression of facts; whilst the
view adopted by the French school of chemistry, and which, till
it is minutely examined, appears so beautiful and satisfactory
rests in the present state of our knowledge upon hypothetical
grounds.”
He then proceeds to explain the action of water upon the chlorides of
tin, and phosphorus; and shows that it is by the decomposition of the
water that the hydrogen is furnished to the oxymuriatic acid, and the
oxygen to the tin and phosphorus.
“The vivid combustion of bodies in oxymuriatic acid gas, at first
view, appears a reason why oxygen should be admitted in it;
but heat and light are merely results of the intense agency of
combination. Sulphur and metals, alkaline earths and acids become
ignited during their mutual agency; and such an effect might be
expected in an operation so rapid as that of oxymuriatic acid
upon metals and inflammable bodies.”
“That the quantity of hydrogen evolved during the decomposition
of muriatic acid gas by metals, is the same that would be
produced during the decomposition of water by the same bodies,
appears, at first view, an evidence in favour of the existence
of water in muriatic acid gas; but as there is only one known
combination of hydrogen with oxymuriatic acid, one quantity must
always be separated. Hydrogen is disengaged from its oxymuriatic
combination by a metal, in the same manner as one metal is
disengaged by another from similar combinations.”
He once more shows that by the strongest analytical power he can
command oxymuriatic acid fails to yield any substance differing from
itself:
“I have caused strong explosions from an electrical jar, to pass
through oxymuriatic gas, by means of points of platina, for
several hours in succession; but it seemed not to undergo the
slightest change.”
Such, then, are the reasons which induced Davy to consider that
oxymuriatic acid contains no oxygen; that it had hitherto been
“undecompounded,” and that, therefore, by the strict logic of
chemistry, it was to be regarded as an elementary body. Had his paper
concluded at this point, his position would have been unassailable,
even in the light of nearly ninety years of subsequent work. But
he could not stop here. Berthollet, the author of the prevailing
theory, had discovered a salt then known as _hyper-oxymuriate_
of potash, presumably capable of furnishing an acid termed by
Chenevix _hyper-oxygenised muriatic acid_. This salt is now termed
potassium chlorate, after the acid which Davy subsequently succeeded
in isolating, and which, when the chlorine theory was generally
accepted, was called chloric acid by Gay Lussac. The existence of the
hyper-oxymuriate of potash was for a time a stumbling-block, and Davy
sought to explain it on the assumption that it was nothing more than a
triple compound of oxymuriatic acid, potassium, and oxygen.
“We have no right to assume the existence of any peculiar acid
in it, or of a considerable portion of combined water; and it is
perhaps more conformable to the analogy of chemistry to suppose
the large quantity of oxygen combined with the potassium, which
we know has an intense affinity for oxygen, and which from some
experiments, I am inclined to believe, is capable of combining
directly with more oxygen than exists in potash, than with the
oxymuriatic acid which, as far as is known, has no affinity for
that substance.”
It is perfectly true, as Davy surmised, that potassium can combine with
more oxygen than is contained in potash, but it is no less true, as he
himself proved by his discovery of the so-called _euchlorine_, that
chlorine can combine with oxygen. Although he made several attempts
to isolate Mr. Chenevix’s hyper-oxygenised muriatic acid, he was
not successful at the time, and was evidently disposed to doubt its
separate existence.
The remaining portion of the paper, although of interest as
exemplifying Davy’s power of dealing with the broad issues which
his views raise, need not detain us now. He seizes the opportunity,
however, to correct his statements with regard to the presumed compound
nature of sulphur and phosphorus, and gives details of observations,
some of which, as in other of his papers, have been “discovered” by
subsequent observers. Thus he states:--
“I have never been able to burn sulphur in oxygen without forming
sulphuric acid in small quantities; but in several experiments
I have obtained from 92 to 98 parts of sulphurous acid from 100
of oxygen in volume; from which I am inclined to believe that
sulphurous acid consists of sulphur dissolved in an equal volume
of oxygen.”
It was hardly to be expected that views so entirely opposed to
the convictions of chemists at the time should pass unchallenged.
Berzelius, the countryman of Scheele, warmly defended the doctrine
of the French School, and yet another Scotch professor sought to show
that Davy was still “vera troublesome.” The controversy, in which Davy
himself took little part, occasioned considerable stir at the period,
and was even of interest outside philosophical circles. The discussion
was not without its uses, inasmuch as it led to fresh discoveries.
The noise of it all, however, is now forgotten. Berzelius eventually
enjoined his cook to speak no longer of oxymuriatic acid: “Thou must
call it _chlorine_, Anna; that is better.” Dr. Murray, with the
pertinacity of his race, still clung to the old doctrine, and defended
it with no little dialectical subtlety, but he alone was faithful among
the faithless. It is true there has been an occasional flutter in the
dovecots since these times, and the faith of chemists in the validity
of Davy’s teaching has been once or twice assailed, but as yet it has
survived all assaults.
The Royal Institution possesses a book which no lover of science
can regard with other than reverential interest. It is a small,
well-bound quarto of some 386 manuscript pages, of notes taken by
Michael Faraday, when a bookbinder’s apprentice, of the last of Davy’s
lectures at the Institution. A Mr. Dance--his name deserves to be held
in remembrance--had given the youth a ticket for the lectures, and
Faraday, perched in the gallery over the clock, had zealously followed
the expositions of the brilliant lecturer, and had subsequently, when
asking for an engagement at the Institution, sent in these notes,
neatly written out and embellished with drawings of the apparatus, to
the Professor as evidence of the applicant’s “knowledge, diligence and
order.” Among the lectures is one on chlorine, given on March 14th,
1812, the notes of which are as characteristic of the auditor as of the
lecturer. We read:--
“Accustomed for years to consider the chemical principles of the
French School of Physical Sciences as correct, I had adopted
them and put faith in them until they became prejudices, and I
even felt unwilling to give them up when my judgment was fully
convinced by experiment that they were erroneous. I know that
this is the case in some degree with almost every person; he is
unwilling to believe that he is wrong and therefore feels averse
to adopt what is right when it opposes his principles.”
Then follows an account of various experiments showing the properties
of chlorine, and the proofs that it contains no oxygen:--
“Oxygen does combine with chlorine. I have ventured to name
the compound _euchlorine_; it is of a very bright yellow-green
colour. Names should represent things not opinions for in the
last case they often tend to misrepresent and mislead.
“Had Mr. Berthollet obtained oxygen from chlorine there would
have been no error in his theory, but by not attending to the
minute circumstances of his experiment, by not ascertaining that
the water present acted no part and was not decomposed he fell
into an error, and of course all the conclusions he drew were
false and erroneous. Nothing should be allowed but what can be
proved by experiment, and nothing should be taken for granted
upon analogy or supposition.”
Faraday concludes as follows:--
“Mr. Davy now proceeded to comment and make observations on the
former theory of chlorine gas. Here I was unable to follow him.
The plan which I pursue in taking of notes is convenient and
self-sufficient with respect to the theoretical and also the
practical part of the lecture, but for the embellishments and
ornaments of it it will not answer. Mr. Davy’s language at those
times is so superior (and indeed throughout the whole course
of the lecture) that then I am infinitely below him, and am
incapable of following him even in an humble style. Therefore I
shall not attempt it; it will be sufficient to give a kind of
contents of it. He said that hypotheses should not be considered
as facts and built upon accordingly. Nevertheless, if cautiously
pursued, they might lead to mature fruit. That nothing should
be taken for granted unless proved. By considering oxygen as
contained in chlorine the whole chemical world had been wrapped
in error respecting that body for more than one-third of a
century.
“He noticed that all the truly great scientific men were
possessed of great humility and diffidence of their own opinions
and powers. He spoke of Scheele, the discoverer of chlorine;
observed that he possessed a truly philosophical spirit, gave
up his opinions when he supposed them to be erroneous, and
without hesitation or reluctance adopted those of others which he
considered more correct; admired his spirit and recommended it to
all philosophers; compared it to corn, which looked but simple
and insignificant in blossom, and asked for little praise, yet
was the support of man.”
In his fifth Bakerian lecture, “On some of the Combinations of
Oxymuriatic Gas and Oxygene, and on the chemical Relations of these
Principles to inflammable Bodies,” read before the Royal Society on
November 15th, 1810, he still further developed his ideas respecting
the nature of chlorine. Gay Lussac and Thenard, who had convinced
themselves that potassium and sodium are not hydrates of potash and
soda, had made known the fact that potassium can combine with oxygen
in more than one proportion; and Davy had confirmed their conclusion,
seeing in it a further proof of his views concerning the constitution
of the hyper-oxymuriate of potash. He then studied the behaviour of
a large number of the metals and their oxides with chlorine, making
in many cases quantitative determinations, from which very fair
approximations to the combining proportions or atomic weights of the
substances may be deduced. Thus, he says “the number representing the
proportion in which mercury combines must be about 200,” and that “the
quantity of chlorine in corrosive sublimate is exactly double that in
calomel, and that the orange oxide contains twice as much oxygen as the
black, the mercury being considered the same in all.” The atomic weight
of silver deducible from the amount of chlorine taken up by that metal
during its conversion into horn-silver is almost exactly the value
obtained by the most rigorous analyses of modern times. It is, however,
noteworthy that in this paper Davy is brought into sharp conflict
with Dalton, and there is a characteristic exhibition of temper in
the way in which he protests against the manner in which Dalton had
sought to use certain of his numerical estimations in deducing the
weights of atoms. The comparative merits of Mr. Higgins and John
Dalton as the real authors of the explanation of the laws of chemical
combination have now been fully and finally assessed, but it was wholly
unnecessary for the purpose of Davy’s contention to underrate the
originality of the Manchester chemist. Dalton was no doubt wrong in the
assumption that 47 represented the weight of the atom of nitrogen, and
Davy was right in pointing out the invalidity of the basis on which
this assumption rested, and in his statement that 13·4 more nearly
represented the smallest proportion in which nitrogen is known to
combine. Davy says:--
“I shall enter no further at present into an examination of the
opinions, results, and conclusions of my learned friend; I am
however obliged to dissent from most of them, and to protest
against the interpretations that he has been pleased to make of
my experiments; and I trust to his judgment and candour for a
correction of his views.
“It is impossible not to admire the ingenuity and talent with
which Mr. Dalton has arranged, combined, weighed, measured, and
figured his atoms; but it is not, I conceive, on any speculations
upon the ultimate particles of matter, that the true theory of
definite proportions must ultimately rest. It has a surer basis
in the mutual decomposition of the neutral salts, observed by
Richter and Guyton de Morveau, in the mutual decompositions of
the compounds of hydrogen and nitrogen, of nitrogen and oxygen,
of water and the oxymuriatic compounds; in the multiples of
oxygen in the nitrous compounds; and those of acids in salts,
observed by Drs. Wollaston and Thomson; and above all, in the
decompositions by the Voltaic apparatus, where oxygen and
hydrogen, oxygen and inflammable bodies, acids and alkalies, &c.,
must separate in uniform ratios.”
It has been alleged that Davy in thus expressing himself offered a
kind of factious opposition to the views of Dalton. In so far as they
were _atomic_, this is possibly true, for Davy never brought himself
to regard the fact of chemical combination occurring in definite
proportions as admitting of the simple mechanical explanation of
Dalton, which he considered too speculative. That, however, he did
ample justice to Dalton’s merits ultimately will be seen from the terms
in which he speaks of them on the occasion of the award to Dalton
in 1826 of the first of the Royal medals. In one of his unfinished
Dialogues, written shortly before his death, “On the Powers which act
upon Matter and produce Chemical Changes,” he thus expresses himself:--
“The atomic doctrine, or theory, has been embraced by several
modern chemists; but the development of it is owing to Mr. Dalton
who seems to have been the first person to generalize the facts,
of chemistry relating to definite proportions.... Mr. W. Higgins
appears to have had only some loose idea of particles combining
with particles, without any profound views of the quantity being
unalterable; and there is good reason for thinking that these
ideas, as he expresses them, were gained from another source,
Dr. Bryan Higgins, who many years before supported the notion,
that chemical substances were formed of molecules, either simple
or compound, surrounded by an atmosphere of heat; and his views,
though not developed with precision, approached nearer to those
of Mr. Dalton, than those of his cousin. But neither of these
gentlemen attempted any statical expressions; and to Richter and
Dalton belongs the exclusive merit of having made the doctrine
practicable. As a theoretical view, other authors have a claim to
it, and the early followers of Newton, such as Kiel, Hartley, and
Marzucchi, all attempted a corpuscular chemistry, founded upon
figure, weight, and attractive power of the ultimate particles
of matter; but this chemistry was of no real use, and had no
other foundation than in the imagination. Indeed, in my opinion,
Mr. Dalton is too much of an _Atomic Philosopher_; and in making
atoms arrange themselves according to his own hypothesis, he has
often indulged in vain speculation; and the essential and truly
useful part of his doctrine, the expression of the quantities
in which bodies combine, is perfectly independent of any views
respecting the ultimate nature either of matter or its elements.”
He concludes the paper in which he so minutely studied the action of
chlorine upon oxides by asking, if it be said that the oxygen arises
from the decomposition of the oxymuriatic gas and not from the oxides,
why is it always the quantity contained in the oxide that is evolved?
And why in some cases, as those of the peroxides of potassium and
sodium, it bears no relation to the quantity of oxymuriatic gas?
“When potassium is burnt in oxymuriatic gas, a dry compound is
obtained. If potassium combined with oxygen is employed, the
whole of the oxygen is expelled, and the same compound formed. It
is contrary to sound logic to say, that this exact quantity of
oxygen is given off from a body not known to be compound, when we
are certain of its existence in another; and all the cases are
parallel.”
An argument in favour of the existence of oxygen in chlorine might be
derived from the circumstance of the formation of the latter gas by
the action of muriatic acid on peroxides. Davy found that, by heating
muriatic acid gas in contact with dry peroxide of manganese, water was
rapidly formed and oxymuriatic gas produced.
“Now as muriatic acid gas is known to consist of oxymuriatic
gas and hydrogen, there is no simple explanation of the result,
except by saying that the hydrogen of the muriatic acid combined
with oxygen from the peroxide to produce water.”
The bleaching power of chlorine had been explained by Scheele on the
supposition that it destroyed colours by combining with phlogiston.
Berthollet considered it to act by supplying oxygen. Davy then made
the well-known experiment proving that the dry gas “is incapable of
altering vegetable colours, and that its operation in bleaching depends
entirely upon its property of decomposing water and liberating its
oxygen.” It had been supposed that oxymuriatic acid gas was capable of
being condensed and crystallised at a low temperature. He shows that
it was only damp chlorine or its solution in water that yielded any
solid product. He exposed the pure gas, dried by muriate of lime, to a
temperature of -40° F., without observing any change. It is curious,
however, that liquid chlorine had actually been obtained by Northmore
five years before by heating the so-called hydrate of chlorine under
pressure. The phenomenon was misunderstood, and it was reserved for
Faraday, in 1823, to show that the product was actually the liquefied
gas.
Davy, who was not always happy in his suggestions as to chemical
nomenclature, proposed to denote the compounds of oxymuriatic gas by
the names of their bases with the termination _ane_.
“Thus, argentane may signify horn-silver; stannane Libavius’s
liquor; antimonane, butter of antimony; sulphurane, Dr. Thomson’s
sulphuretted liquor, and so on for the rest.... In cases when
two or more proportions of inflammable matter combine with one
of gas; or two or more of gas with one of inflammable matter, it
may be convenient to signify the proportions by affixing vowels
before the name, when the inflammable matter predominates, and
after the name when the gas is in excess; and in the order of the
alphabet, _a_ signifying two, _e_, three, _i_, four and so on.”
Thus he called phosphorus pentachloride _phosphorana_, and the
trichloride _phosphorane_, because there was a larger percentage
proportion of phosphorus in the latter compound than in the former.
That Davy was not unaware of the difficulties and inconveniences of
such a system of nomenclature may be inferred from what he says in his
“Elements” concerning the names for the two chlorides of mercury, the
true composition of which he was the first to discover:--
“The names _mercurane_ and _mercurana_ which may be adopted to
signify the relations of their composition, are too similar to
each other to be safely used as familiar appellations for the two
substances, as corrosive sublimate is a powerful poison, calomel
an excellent medicine.”
In matters of chemical nomenclature Davy was a great latitudinarian.
All that he contended for was that names should be independent of all
speculative views, and should rather be derived from some simple and
invariable property. It is remarkable, however, that he who invented
the happy term “chlorine” should have objected to the word “cyanogen.”
At the close of the short paper “On the Prussic Basis and Acid,” in
which he first made known the existence of the cyanides of phosphorus
and of iodine, he said:--
“I wish M. Gay Lussac could be prevailed upon to give up the
inexpressive and difficult names of cyanogen and hydrocyanic
acid, and to adopt the simple ones of prussic gas and prussic
acid.”
By treating the potassium hyper-oxymuriate of Berthollet (potassium
chlorate) with hydrochloric acid, a greenish-yellow explosive gas is
obtained which Chenevix had referred to as “hyper-oxygenised muriatic
acid,” and as indicating the existence of a compound of oxymuriatic gas
and oxygen in a separate state. Davy, as we have seen, was at first
inclined to doubt the existence of this substance, and to consider the
gas as simply chlorine. But on comparing it with chlorine prepared in
other ways he perceived a difference; its solution in water was of
lemon yellow or orange colour; when treated with mercury it becomes of
a brilliant yellow green. It is, moreover, highly explosive, especially
when heated, even at the warmth of the hand, when it loses its vivid
colour, and is resolved into a mixture of oxygen and chlorine. Metals,
arsenic, phosphorus, charcoal, nitric oxide, act upon it in a manner
different from that of chlorine. Davy makes use of these differences as
a proof of the correctness of his views of the nature of chlorine.
“If the power of bodies to burn in oxymuriatic gas depended upon
the presence of oxygen, they all ought to burn with much more
energy in the new compound; but copper and antimony, and mercury
and arsenic and iron and sulphur have no action upon it, till
it is decomposed; and they act then according to their relative
attractions on the oxygen, or on the oxymuriatic gas. There is a
simple experiment which illustrates this idea. Let a glass vessel
containing brass foil be exhausted, and the new gas admitted, no
action will take place; throw in a little nitrous gas [nitric
oxide], a rapid decomposition occurs, and the metal burns with
great brilliancy.
“As the new compound in its purest form is possessed of a
bright yellow-green colour, it may be expedient to designate it
by a name expressive of this circumstance and its relation to
oxymuriatic gas. As I have named that elastic fluid Chlorine; so
I venture to propose for this substance the name Euchlorine, or
Euchloric gas from ευ and χλωρος. The point of nomenclature I am
not inclined to dwell upon. I shall be content to adopt any name
that may be considered as most appropriate by the able chemical
philosophers attached to this Society” [the Royal Society].
Euchlorine was subsequently discovered by Soubeiran to be a mixture of
chlorine and chlorine peroxide, a gas which Davy himself afterwards
isolated in a pure state. It is however obvious from the accounts he
gives that even in his first paper he must have been experimenting
with a fairly pure product, due probably to the circumstance that he
had collected the mixed gases over mercury, which retains the greater
part of the chlorine. Former experimenters had collected the gas over
water, which dissolves the chlorine peroxide more readily than the
chlorine. Madame de Staël once observed that an interesting book might
be written on the important consequences which have sprung from little
differences. It ought to be noted, however, that Davy had himself
doubts whether his euchlorine was not a mixture of chlorine and the gas
which he subsequently discovered, and to which he says: “I shall not
propose to give any name till it is determined whether euchlorine is a
mixture or a definite compound.”
It has been stated that Davy discovered the two chlorides of
phosphorus. In a paper read to the Royal Society on June 18th, 1812,
“On some Combinations of Phosphorus and Sulphur and on some other
Subjects of Chemical Inquiry,” he reverts to these substances, as
they “offer decided evidences in favour of an idea that has been for
some time prevalent among many enlightened chemists and which I have
defended in former papers published in the Philosophical Transactions;
namely that bodies unite in definite proportions, and that there is a
relation between the quantities in which the same element unites with
different elements.”
He first makes a determination, singularly accurate for the time, of
the amount of chlorine contained in the lower chloride, and finds
that 13·6 grains on decomposition with water afforded 43 grains of
horn-silver; theory requires 42·6 grains. By synthetical experiments
he came to the conclusion that the amount of chlorine absorbed by
phosphorus to form the higher chloride was exactly double that
contained in the lower chloride: he found that 3 grains of phosphorus
combined with 20 grains of chlorine: in reality it should require only
17¾ grains.
He shows that by treatment with water the lower chloride yields
_phosphorous acid_, the properties and mode of decomposition of which
by heat he accurately describes. He further concludes, as the logical
consequence of his view of the composition of the two chlorides,
and the mode of their decomposition by water, that phosphorous acid
contains half the amount of oxygen present in phosphoric acid, the
quantity of phosphorus being the same. It is noteworthy that in his
argument, as indeed on all subsequent occasions when he speaks of the
decomposition of water in definite proportions, he regards water as
composed of 2 combining proportions of hydrogen and 1 of oxygen, and
the number representing it as 17, oxygen being regarded as 15. Certain
of his statements considered in the light of subsequent work are
interesting. Thus he says:--
“A solid acid volatile at a moderate degree of heat, may be
produced by burning phosphorus in very rare air, and this seems
to be phosphorous acid free from water; but some phosphoric acid,
and some yellow oxide of phosphorus are always formed at the same
time.”
He also observes that unless the product of the combustion of
phosphorus is strongly heated in oxygen it contains phosphorous acid
as well as phosphoric acid. He further states that sulphurous acid
(sulphur dioxide) consists of equal weights of oxygen and sulphur,
which is almost strictly true, and that sulphuretted hydrogen is
composed of 1 combining proportion of sulphur and 2 of hydrogen,
although his values for the combining proportions of sulphur and
oxygen are incorrect. He repeats Dalton’s experiment of the formation
of “solid sulphuric acid” by the mutual action of sulphur dioxide
and nitric oxide, and shows that the substance is only produced in
presence of vapour of water; the two substances, he says, then “form
a solid crystalline hydrate; which when thrown into water gives off
nitrous gas and forms a solution of sulphuric acid.” This substance is
the so-called “leaden-chamber crystal,” or nitrosulphonic acid, the
existence of which was first made known by Scheele.
Davy’s conclusions concerning the composition of the oxides and
chlorides of phosphorus were subsequently contested by Berzelius and
Dulong, who showed that although the amount of chlorine in the lower
chloride was identical with that which he had found, the ratio of this
amount to that in the higher chloride was as 3 to 5, and not as 1 to 2,
and that the same ratio held good as regards the oxygen in phosphorous
oxide and phosphoric oxide. Davy, six years afterwards, repeated
his experiments, but without discovering the fallacy in his first
observations.
* * * * *
The other incidents in Davy’s scientific career may be most
conveniently dealt with in connection with his personal history.