OF THE ATOMIC THEORY.
I come now to the last improvement which chemistry has received--an
improvement which has given a degree of accuracy to chemical
experimenting almost approaching to mathematical precision, which has
simplified prodigiously our views respecting chemical combinations;
which has enabled manufacturers to introduce theoretical improvements
into their processes, and to regulate with almost perfect precision the
relative quantities of the various constituents necessary to produce
the intended effects. The consequence of this is, that nothing is
wasted, nothing is thrown away. Chemical products have become not only
better in quality, but more abundant and much cheaper. I allude to the
atomic theory still only in its infancy, but already productive of
the most important benefits. It is destined one day to produce still
more wonderful effects, and to render chemistry not only the most
delightful, but the most useful and indispensable, of all the sciences.
Like all other great improvements in science, the atomic theory
developed itself by degrees, and several of the older chemists
ascertained facts which might, had they been aware of their importance,
have led them to conclusions similar to those of the moderns. The
very attempt to analyze the salts was an acknowledgment that bodies
united with each other in definite proportions: and these definite
proportions, had they been followed out, would have led ultimately to
the doctrine of atoms. For how could it be, that six parts of potash
were always saturated by five parts of sulphuric acid and 6·75 parts
of nitric acid? How came it that five of sulphuric acid always went as
far in saturating potash as 6·75 of nitric acid? It was known, that
in chemical combinations it was the ultimate particles of matter that
combined. The simple explanation, therefore, would have been--that the
weight of an ultimate particle of sulphuric acid was only five, while
that of an ultimate particle of nitric acid was 6·75. Had such an
inference been drawn, it would have led directly to the atomic theory.
The atomic theory in chemistry has many points of resemblance to
the fluxionary calculus in mathematics. Both give us the ratios
of quantities; both reduce investigations that would be otherwise
extremely difficult, or almost impossible, to the utmost simplicity;
and what is still more curious, both have been subjected to the same
kind of ridicule by those who have not put themselves to the trouble of
studying them with such attention as to understand them completely. The
minute philosopher of Berkeley, _mutatis mutandis_, might be applied to
the atomic theory with as much justice as to the fluxionary calculus;
and I have heard more than one individual attempt to throw ridicule
upon the atomic theory by nearly the same kind of arguments.
The first chemists, then, who attempted to analyze the salts may be
considered as contributing towards laying the foundation of the atomic
theory, though they were not themselves aware of the importance of the
structure which might have been raised upon their experiments, had
they been made with the requisite precision.
Bergman was the first chemist who attempted regular analyses of salts.
It was he that first tried to establish regular formulas for the
analyses of mineral waters, stones, and ores, by the means of solution
and precipitation. Hence a knowledge of the constituents of the salts
was necessary, before his formulas could be applied to practice. It was
to supply this requisite information that he set about analyzing the
salts, and his results were long considered by chemists as exact, and
employed by them to determine the results of their analyses. We now
know that these analytical results of Bergman are far from accurate;
they have accordingly been laid aside as useless: but this knowledge
has been derived from the progress of the atomic theory.
The first accurate set of experiments to analyze the salts was made by
Wenzel, and published by him in 1777, in a small volume entitled "Lehre
von der Verwandschaft der Körper," or, "Theory of the Affinities of
Bodies." These analyses of Wenzel are infinitely more accurate than
those of Bergman, and indeed in many cases are equally precise with
the best which we have even at the present day. Yet the book fell
almost dead-born from the press; Wenzel's results never obtained the
confidence of chemists, nor is his name ever quoted as an authority.
Wenzel was struck with a phenomenon, which had indeed been noticed
by preceding chemists; but they had not drawn the advantages from it
which it was capable of affording. There are several saline solutions
which, when mixed with each other, completely decompose each other, so
that two new salts are produced. Thus, if we mix together solutions
of nitrate of lead and sulphate of soda in the requisite proportions,
the sulphuric acid of the latter salt will combine with the oxide of
lead of the former, and will form with it sulphate of lead, which will
precipitate to the bottom in the state of an insoluble powder, while
the nitric acid formerly united to the oxide of lead, will combine with
the soda formerly in union with the sulphuric acid, and form nitrate of
soda, which being soluble, will remain in solution in the liquid. Thus,
instead of the two old salts,
Sulphate of soda
Nitrate of lead,
we obtain the two new salts,
Sulphate of lead
Nitrate of soda.
If we mix the two salts in the requisite proportions, the decomposition
will be complete; but if there be an excess of one of the salts, that
excess will still remain in solution without affecting the result. If
we suppose the two salts anhydrous, then the proportions necessary for
complete decomposition are,
Sulphate of soda 9
Nitrate of lead 20·75
------
29·75
and the quantities of the new salts formed will be
Sulphate of lead 19
Nitrate of soda 10·75
-----
29·75
We see that the absolute weights of the two sets of salts are the
same: all that has happened is, that both the acids and both the bases
have exchanged situations. Now if, instead of mixing these two salts
together in the preceding proportions, we employ
Sulphate of soda 9
Nitrate of lead 25·75
That is to say, if we employ 5 parts of nitrate of lead more than
is sufficient for the purpose; we shall have exactly the same
decompositions as before; but the 5 of excess of nitrate of lead will
remain in solution, mixed with the nitrate of soda. There will be
precipitated as before,
Sulphate of lead 19
and there will remain in solution a mixture of
Nitrate of soda 10·75
Nitrate of lead 5
The phenomena are precisely the same as before; the additional 5 of
nitrate of lead have occasioned no alteration; the decomposition has
gone on just as if they had not been present.
Now the phenomena which drew the particular attention of Wenzel is,
that if the salts were neutral before being mixed, the neutrality
was not affected by the decomposition which took place on their
mixture.[7] A salt is said to be neutral when it neither possesses the
characters of an acid or an alkali. Acids _redden_ vegetable _blues_,
while alkalies render them _green_. A neutral salt produces no effect
whatever upon vegetable blues. This observation of Wenzel is very
important: it is obvious that the salts, after their decomposition,
could not have remained neutral unless the elements of the two salts
had been such that the bases in each just saturated the acids in either
of the salts.
[7] This observation is not without exception. It does not hold when
one of the salts is a phosphate or an arseniate, and this is the cause
of the difficulty attending the analysis of these genera of salts.
The constituents of the two salts are as follows:
{ 5 sulphuric acid
9 sulphate of soda { 4 soda,
{ 6·75 nitric acid
20·75 nitrate of lead {14 oxide of lead.
Now it is clear, that unless 5 sulphuric acid were just saturated by
4 soda and by 14 oxide of lead; and 6·75 of nitric acid by the same 4
soda and 14 oxide of lead, the salts, after their decomposition, could
not have preserved their neutrality. Had 4 soda required only 5·75 of
nitric acid, or had 14 oxide of lead required only 4 sulphuric acid, to
saturate them, the liquid, after decomposition, would have contained
an excess of acid. As no such excess exists, it is clear that in
saturating an acid, 4 soda goes exactly as far as 14 oxide of lead; and
that, in saturating a base, 5 sulphuric acid goes just as far as 6·75
nitric acid.
Nothing can exhibit in a more striking point of view, the almost
despotic power of fashion and authority over the minds even of men
of science, and the small number of them that venture to think for
themselves, than the fact, that this most important and luminous
explanation of Wenzel, confirmed by much more accurate experiments than
any which chemistry had yet seen, is scarcely noticed by any of his
contemporaries, and seems not to have attracted the smallest attention.
In science, it is as unfortunate for a man to get before the age in
which he lives, as to continue behind it. The admirable explanation of
combustion by Hooke, and the important experiments on combustion and
respiration by Mayow, were lost upon their contemporaries, and procured
them little or no reputation whatever; while the same theory, and
the same experiments, advanced by Lavoisier and Priestley, a century
later, when the minds of men of science were prepared to receive them,
raised them to the very first rank among philosophers, and produced a
revolution in chemistry. So much concern has fortune, not merely in the
success of kings and conquerors, but in the reputation acquired by men
of science.
In the year 1792 another labourer, in the same department of chemistry,
appeared: this was Jeremiah Benjamin Richter, a Prussian chemist, of
whose history I know nothing more than that his publications were
printed and published in Breslau, from which I infer that he was a
native of, or at least resided in, Silesia. He calls himself Assessor
of the Royal Prussian Mines and Smeltinghouses, and Arcanist of the
Commission of Berlin Porcelain Manufacture. He died in the prime of
life, on the 4th of May, 1807. In the year 1792 he published a work
entitled "Anfansgründe der Stochyometrie; oder, Messkunst Chymischer
Elemente" (Elements of Stochiometry; or, the Mathematics of the
Chemical Elements). A second and third volume of this work appeared in
1793, and a fourth volume in 1794. The object of this book was a rigid
analysis of the different salts, founded on the fact just mentioned,
that when two salts decompose each other, the salts newly formed
are neutral as well as those which have been decomposed. He took up
the subject nearly in the same way as Wenzel had done, but carried
the subject much further; and endeavoured to determine the capacity
of saturation of each acid and base, and to attach numbers to each,
indicating the weights which mutually saturate each other. He gave the
whole subject a mathematical dress, and endeavoured to show that the
same relation existed, between the numbers representing the capacity of
saturation of these bodies, as does between certain classes of figurate
numbers. When we strip the subject of the mystical form under which he
presented it, the labours of Richter may be exhibited under the two
following tables, which represent the capacity of saturation of the
acids and bases, according to his experiments.
1. ACIDS.
Fluoric acid 427
Carbonic 577
Sebacic 706
Muriatic 712
Oxalic 755
Phosphoric 979
Formic 988
Sulphuric 1000
Succinic 1209
Nitric 1405
Acetic 1480
Citric 1683
Tartaric 1694
2. BASES.
Alumina 525
Magnesia 615
Ammonia 672
Lime 793
Soda 859
Strontian 1329
Potash 1605
Barytes 2222
To understand this table, it is only necessary to observe, that if we
take the quantity of any of the acids placed after it in the table,
that quantity will be exactly saturated by the weight of each base put
after it in the second column: thus, 1000 of sulphuric acid will be
just saturated by 525 alumina, 615 magnesia, 672 ammonia, 793 lime, and
so on. On the other hand, the quantity of any base placed after its
name in the second column, will be just saturated by the weight of each
acid placed after its name in the first column: thus, 793 parts of lime
will be just saturated by 427 of fluoric acid, 577 of carbonic acid,
706 of sebacic acid, and so on.
This work of Richter was followed by a periodical work entitled "Ueber
die neuern Gegenstande der Chymie" (On the New Objects of Chemistry).
This work was begun in the year 1792, and continued in twelve different
numbers, or volumes, to the time of his death in 1807.[8]
[8] I have only seen eleven parts of this work, the last of which
appeared in 1802; but I believe that a twelfth part was published
afterwards.
Richter's labours in this important field produced as little attention
as those of Wenzel. Gehlen wrote a short panegyric upon him at his
death, praising his views and pointing out their importance; but I
am not aware of any individual, either in Germany or elsewhere, who
adopted Richter's opinions during his lifetime, or even seemed aware
of their importance, unless we are to except Berthollet, who mentions
them with approbation in his Chemical Statics. This inattention was
partly owing to the great want of accuracy which it is impossible
not be sensible of in Richter's experiments. He operated upon too
large quantities of matter, which indeed was the common defect of the
times, and was first checked by Dr. Wollaston. The dispute between the
phlogistians and the antiphlogistians, which was not fully settled in
Richter's time, drew the attention of chemists to another branch of
the subject. Richter in some measure went before the age in which he
lived, and had his labours not been recalled to our recollection by the
introduction of atomic theory, he would probably have been forgotten,
like Hooke and Mayow, and only brought again under review after the
new discoveries in the science had put it in the power of chemists in
general to appreciate the value of his labours.
It is to Mr. Dalton that we are indebted for the happy and simple idea
from which the atomic theory originated.
John Dalton, to whose lot it has fallen to produce such an alteration
and improvement in chemistry, was born in Westmorland, and belongs
to that small and virtuous sect known in this country by the name of
Quakers. When very young he lived with Mr. Gough of Kendal, a blind
philosopher, to whom he read, and whom he assisted in his philosophical
investigations. It was here, probably, that he acquired a considerable
part of his education, particularly his taste for mathematics. For
Mr. Gough was remarkably fond of mathematical investigations, and has
published several mathematical papers that do him credit. From Kendal
Mr. Dalton went to Manchester, about the beginning of the present
century, and commenced teaching elementary mathematics to such young
men as felt inclined to acquire some knowledge of that important
subject. In this way, together with a few courses of lectures on
chemistry, which he has occasionally given at the Royal Institution
in London, at the Institution in Birmingham, in Manchester, and once
in Edinburgh and in Glasgow, he has contrived to support himself for
more than thirty years, if not in affluence, at least in perfect
independence. And as his desires have always been of the most moderate
kind, his income has always been equal to his wants. In a country
like this, where so much wealth abounds, and where so handsome a
yearly income was subscribed to enable Dr. Priestley to prosecute
his investigations undisturbed and undistracted by the necessity of
providing for the daily wants of his family, there is little doubt
that Mr. Dalton, had he so chosen it, might, in point of pecuniary
circumstances, have exhibited a much more brilliant figure. But he has
displayed a much nobler mind by the career which he has chosen--equally
regardless of riches as the most celebrated sages of antiquity, and as
much respected and beloved by his friends, even in the rich commercial
town of Manchester, as if he were one of the greatest and most
influential men in the country. Towards the end of the last century, a
literary and scientific society had been established in Manchester, of
which Mr. Thomas Henry, the translator of Lavoisier's Essays, and who
distinguished himself so much in promoting the introduction of the new
mode of bleaching into Lancashire, was long president. Mr. Dalton, who
had already distinguished himself by his meteorological observations,
and particularly by his account of the Aurora Borealis, soon became a
member of the society; and in the fifth volume of their Memoirs, part
II., published in 1802, six papers of his were inserted, which laid the
foundation of his future celebrity. These papers were chiefly connected
with meteorological subjects; but by far the most important of them all
was the one entitled "Experimental Essays on the Constitution of mixed
Gases; on the Force of Steam or Vapour from water and other liquids in
different temperatures, both in a torricellian vacuum and in air; on
Evaporation; and on the Expansion of Gases by Heat."
From a careful examination of all the circumstances, he considered
himself as entitled to infer, that when two elastic fluids or gases,
A and B, are mixed together, there is no mutual repulsion among their
particles; that is, the particles of A do not repel those of B, as they
do one another. Consequently, the pressure or whole weight upon any
one particle arises solely from those of its own kind. This doctrine
is of so startling a nature and so contrary to the opinions previously
received, that chemists have not been much disposed to admit it. But at
the same time it must be confessed, that no one has hitherto been able
completely to refute it. The consequences of admitting it are obvious:
we should be able to account for a fact which has been long known,
though no very satisfactory reason for it had been assigned; namely,
that if two gases be placed in two separate vessels, communicating
by a narrow orifice, and left at perfect rest in a place where the
temperature never varies, if we examine them after a certain interval
of time we shall find both equally diffused through both vessels. If we
fill a glass phial with hydrogen gas and another phial with common air
or carbonic acid gas and unite the two phials by a narrow glass tube
two feet long, filled with common air, and place the phial containing
the hydrogen gas uppermost, and the other perpendicularly below it, the
hydrogen, though lightest, will not remain in the upper phial, nor the
carbonic acid, though heaviest, in the undermost phial; but we shall
find both gases equally diffused through both phials.
But the second of these essays is by far the most important. In it he
establishes, by the most unexceptionable evidence, that water, when
it evaporates, is always converted into an elastic fluid, similar in
its properties to air. But that the distance between the particles is
greater the lower the temperature is at which the water evaporates.
The elasticity of this vapour increases as the temperature increases.
At 32° it is capable of balancing a column of mercury about half an
inch in height, and at 212° it balances a column thirty inches high,
or it is then equal to the pressure of the atmosphere. He determined
the elasticity of vapour at all temperatures from 32° to 212°, pointed
out the method of determining the quantity of vapour that at any time
exists in the atmosphere, the effect which it has upon the volume of
air, and the mode of determining its quantity. Finally, he determined,
experimentally, the rate of evaporation from the surface of water at
all temperatures from 32° to 212°. These investigations have been of
infinite use to chemists in all their investigations respecting the
specific gravity of gases, and have enabled them to resolve various
interesting problems, both respecting specific gravity, evaporation,
rain and respiration, which, had it not been for the principles laid
down in this essay, would have eluded their grasp.
In the last essay contained in this paper he has shown that all elastic
fluids expand the same quantity by the same addition of heat, and this
expansion is very nearly 1-480th part for every degree of Fahrenheit's
thermometer. In this last branch of the subject Mr. Dalton was followed
by Gay-Lussac, who, about half a year after the appearance of his
Essays, published a paper in the Annales de Chimie, showing that the
expansion of all elastic fluids, when equally heated, is the same. Mr.
Dalton concluded that the expansion of all elastic fluids by heat is
equable. And this opinion has been since confirmed by the important
experiments of Dulong and Petit, which have thrown much additional
light on the subject.
In the year 1804, on the 26th of August, I spent a day or two at
Manchester, and was much with Mr. Dalton. At that time he explained to
me his notions respecting the composition of bodies. I wrote down at
the time the opinions which he offered, and the following account is
taken literally from my journal of that date:
The ultimate particles of all simple bodies are _atoms_ incapable
of further division. These atoms (at least viewed along with their
atmospheres of heat) are all spheres, and are each of them possessed of
particular weights, which may be denoted by numbers. For the greater
clearness he represented the atoms of the simple bodies by symbols. The
following are his symbols for four simple bodies, together with the
numbers attached to them by him in 1804:
Relative
weights.
[oxygen] Oxygen 6·5
[hydrogen] Hydrogen 1
[carbon] Carbon 5
[azote] Azote 5
The following symbols represent the way in which he thought these atoms
were combined to form certain binary compounds, with the weight of an
integrant particle of each compound:
Weights.
[oxygen][hydrogen] Water 7·5
[oxygen][azote] Nitrous gas 11·5
[carbon][hydrogen] Olefiant gas 6
[azote][hydrogen] Ammonia 6
[oxygen][carbon] Carbonic oxide 11·5
The following were the symbols by which he represented the composition
of certain tertiary compounds:
Weights.
[oxygen][carbon][oxygen] Carbonic acid 18
[oxygen][azote][oxygen] Nitrous oxide 16·5
[carbon][hydrogen][carbon] Ether 11
[hydrogen][carbon][hydrogen] Carburetted hydrogen 7
[oxygen][azote][oxygen] Nitric acid 18
A quaternary compound:
[oxygen][azote][oxygen] Oxynitric acid 24·5
[oxygen]
A quinquenary compound:
[oxygen]
[azote] [azote][oxygen] Nitrous acid 29·5
[oxygen]
A sextenary compound:
[carbon][oxygen][carbon] Alcohol 23·5
[hydrogen][carbon][hydrogen]
These symbols are sufficient to give the reader an idea of the notions
entertained by Dalton respecting the nature of compounds. Water is
a compound of one atom oxygen and one atom hydrogen as is obvious
from the symbol [oxygen][hydrogen]. Its weight 7·5 is that of an atom
of oxygen and an atom of hydrogen united together. In the same way
carbonic oxide is a compound of one atom oxygen and one atom carbon.
Its symbol is [oxygen][carbon], and its weight 11·5 is equal to an
atom of oxygen and an atom of carbon added together. Carbonic acid is
a tertiary compound, or it consists of three atoms united together;
namely, two atoms of oxygen and one atom of carbon. Its symbol is
[oxygen][carbon][oxygen], and its weight 18. A bare inspection of the
symbols and weights will make Mr. Dalton's notions respecting the
constitution of every body in the table evident to every reader.
It was this happy idea of representing the atoms and constitution of
bodies by symbols that gave Mr. Dalton's opinions so much clearness.
I was delighted with the new light which immediately struck my
mind, and saw at a glance the immense importance of such a theory,
when fully developed. Mr. Dalton informed me that the atomic theory
first occurred to him during his investigations of olefiant gas and
carburetted hydrogen gases, at that time imperfectly understood, and
the constitution of which was first fully developed by Mr. Dalton
himself. It was obvious from the experiments which he made upon them,
that the constituents of both were carbon and hydrogen, and nothing
else. He found further, that if we reckon the carbon in each the same,
then carburetted hydrogen gas contains exactly twice as much hydrogen
as olefiant gas does. This determined him to state the ratios of these
constituents in numbers, and to consider the olefiant gas as a compound
of one atom of carbon and one atom of hydrogen; and carburetted
hydrogen of one atom of carbon and two atoms of hydrogen. The idea
thus conceived was applied to carbonic oxide, water ammonia, &c.; and
numbers representing the atomic weights of oxygen, azote, &c., deduced
from the best analytical experiments which chemistry then possessed.
Let not the reader suppose that this was an easy task. Chemistry at
that time did not possess a single analysis which could be considered
as even approaching to accuracy. A vast number of facts had been
ascertained, and a fine foundation laid for future investigation; but
nothing, as far as weight and measure were concerned, deserving the
least confidence, existed. We need not be surprised, then, that Mr.
Dalton's first numbers were not exact. It required infinite sagacity,
and not a little labour, to come so near the truth as he did. How could
accurate analyses of gases be made when there was not a single gas
whose specific gravity was known, with even an approach to accuracy;
the preceding investigations of Dalton himself paved the way for
accuracy in this indispensable department; but still accurate results
had not yet been obtained.
In the third edition of my System of Chemistry, published in 1807, I
introduced a short sketch of Mr. Dalton's theory, and thus made it
known to the chemical world. The same year a paper of mine on _oxalic
acid_ was published in the Philosophical Transactions, in which I
showed that oxalic acid unites in two proportions with strontian,
forming an _oxalate_ and _binoxalate_; and that, supposing the
strontian in both salts to be the same, the oxalic acid in the latter
is exactly twice as much as in the former. About the same time, Dr.
Wollaston showed that bicarbonate of potash contains just twice the
quantity of carbonic acid that exists in carbonate of potash; and that
there are three oxalates of potash; viz., _oxalate_, _binoxalate_, and
_quadroxalate_; the weight of acids in each of which are as the numbers
1, 2, 4. These facts gradually drew the attention of chemists to Mr.
Dalton's views. There were, however, some of our most eminent chemists
who were very hostile to the atomic theory. The most conspicuous
of these was Sir Humphry Davy. In the autumn of 1807 I had a long
conversation with him at the Royal Institution, but could not convince
him that there was any truth in the hypothesis. A few days after I
dined with him at the Royal Society Club, at the Crown and Anchor,
in the Strand. Dr. Wollaston was present at the dinner. After dinner
every member of the club left the tavern, except Dr. Wollaston, Mr.
Davy, and myself, who staid behind and had tea. We sat about an hour
and a half together, and our whole conversation was about the atomic
theory. Dr. Wollaston was a convert as well as myself; and we tried to
convince Davy of the inaccuracy of his opinions; but, so far from being
convinced, he went away, if possible, more prejudiced against it than
ever. Soon after, Davy met Mr. Davis Gilbert, the late distinguished
president of the Royal Society; and he amused him with a caricature
description of the atomic theory, which he exhibited in so ridiculous a
light, that Mr. Gilbert was astonished how any man of sense or science
could be taken in with such a tissue of absurdities. Mr. Gilbert
called on Dr. Wollaston (probably to discover what could have induced
a man of Dr. Wollaston's sagacity and caution to adopt such opinions),
and was not sparing in laying the absurdities of the theory, such as
they had been represented to him by Davy, in the broadest point of
view. Dr. Wollaston begged Mr. Gilbert to sit down, and listen to
a few facts which he would state to him. He then went over all the
principal facts at that time known respecting the salts; mentioned the
alkaline carbonates and bicarbonates, the oxalate, binoxalate, and
quadroxalate of potash, carbonic oxide and carbonic acid, olefiant gas,
and carburetted hydrogen; and doubtless many other similar compounds,
in which the proportion of one of the constituents increases in a
regular ratio. Mr. Gilbert went away a convert to the truth of the
atomic theory; and he had the merit of convincing Davy that his former
opinions on the subject were wrong. What arguments he employed I do
not know; but they must have been convincing ones, for Davy ever after
became a strenuous supporter of the atomic theory. The only alteration
which he made was to substitute _proportion_ for Dalton's word, _atom_.
Dr. Wollaston substituted for it the term _equivalent_. The object of
these substitutions was to avoid all theoretical annunciations. But, in
fact, these terms, _proportion_, _equivalent_, are neither of them so
convenient as the term _atom_: and, unless we adopt the hypothesis with
which Dalton set out, namely, that the ultimate particles of bodies are
_atoms_ incapable of further division, and that chemical combination
consists in the union of these atoms with each other, we lose all the
new light which the atomic theory throws upon chemistry, and bring our
notions back to the obscurity of the days of Bergman and of Berthollet.
In the year 1808 Mr. Dalton published the first volume of his New
System of Chemical Philosophy. This volume consists chiefly of two
chapters: the first, on _heat_, occupies 140 pages. In it he treats of
all the effects of heat, and shows the same sagacity and originality
which characterize all his writings. Even when his opinions on a
subject are not correct, his reasoning is so ingenious and original,
and the new facts which he contrives to bring forward so important,
that we are always pleased and always instructed. The second chapter,
on the _constitution of bodies_, occupies 70 pages. The chief object
of it is to combat the peculiar notions respecting elastic fluids,
which had been advanced by Berthollet, and supported by Dr. Murray,
of Edinburgh. In the third chapter, on _chemical synthesis_, which
occupies only a few pages, he gives us the outlines of the atomic
theory, such as he had conceived it. In a plate at the end of the
volume he exhibits the symbols and atomic weights of thirty-seven
bodies, twenty of which were then considered as simple, and the other
seventeen as compound. The following table shows the atomic weight of
the simple bodies, as he at that time had determined them from the best
analytical experiments that had been made:
Weight of
atom.
Hydrogen 1
Azote 5
Carbon 5
Oxygen 7
Phosphorus 9
Sulphur 13
Magnesia 20
Lime 23
Soda 28
Potash 42
Strontian 46
Barytes 68
Iron 38
Zinc 56
Copper 56
Lead 95
Silver 100
Platinum 100
Gold 140
Mercury 167
He had made choice of hydrogen for unity, because it is the lightest
of all bodies. He was of opinion that the atomic weights of all other
bodies are multiples of hydrogen; and, accordingly, they are all
expressed in whole numbers. He had raised the atomic weight of oxygen
from 6·5 to 7, from a more careful examination of the experiments
on the component parts of water. Davy, from a more accurate set of
experiments, soon after raised the number for oxygen to 7·5: and
Dr. Prout, from a still more careful investigation of the relative
specific gravities of oxygen and hydrogen, showed that if the atom of
hydrogen be 1, that of oxygen must be 8. Every thing conspires to prove
that this is the true ratio between the atomic weights of oxygen and
hydrogen.
In 1810 appeared the second volume of Mr. Dalton's New System of
Chemical Philosophy. In it he examines the elementary principles,
or simple bodies, namely, oxygen, hydrogen, azote, carbon, sulphur,
phosphorus, and the metals; and the compounds consisting of two
elements, namely, the compounds of oxygen with hydrogen, azote,
carbon, sulphur, phosphorus; of hydrogen with azote, carbon, sulphur,
phosphorus. Finally he treats of the fixed alkalies and earths. All
these combinations are treated of with infinite sagacity; and he
endeavours to determine the atomic weights of the different elementary
substances. Nothing can exceed the ingenuity of his reasoning. But
unfortunately at that time very few accurate chemical analyses existed;
and in chemistry no reasoning, however ingenious, can compensate for
this indispensable datum. Accordingly his table of atomic weights at
the end this second volume, though much more complete than that at the
end of the first volume, is still exceedingly defective; indeed no one
number can be considered as perfectly correct.
The third volume of the New System of Chemical Philosophy was only
published in 1827; but the greatest part of it had been printed nearly
ten years before. It treats of the metallic oxides, the sulphurets,
phosphurets, carburets, and alloys. Doubtless many of the facts
contained in it were new when the sheets were put to the press; but
during the interval between the printing and publication, almost the
whole of them had not merely been anticipated, but the subject carried
much further. By far the most important part of the volume is the
Appendix, consisting of about ninety pages, in which he discusses,
with his usual sagacity, various important points connected with heat
and vapour. In page 352 he gives a new table of the atomic weights of
bodies, much more copious than those contained in the two preceding
volumes; and into which he has introduced the corrections necessary
from the numerous correct analyses which had been made in the interval.
He still adheres to the ratio 1:7 as the correct difference between the
weights of the atoms of hydrogen and oxygen. This shows very clearly
that he has not attended to the new facts which have been brought
forward on the subject. No person who has attended to the experiments
made on the specific gravity of these two gases during the last twelve
years, could admit that these specific gravities are to each other as 1
to 14. If 1 to 16 be not the exact ratio, it will surely be admitted on
all hands that it is infinitely near it.
Mr. Dalton represented the weight of an atom of hydrogen by 1, because
it is the lightest of bodies. In this he has been followed by the
chemists of the Royal Institution, by Mr. Philips, Dr. Henry, and
Dr. Turner, and perhaps some others whose names I do not at present
recollect. Dr. Wollaston, in his paper on Chemical Equivalents,
represented the atomic weight of oxygen by 1, because it enters into
a greater number of combinations than any other substance; and this
plan has been adopted by Berzelius, by myself, and by the greater
number, if not the whole, of the chemists on the continent. Perhaps the
advantage which Dr. Wollaston assigned for making the atom of oxygen
unity will ultimately disappear: for there is no reason for believing
that the other supporters of combustion are not capable of entering
into as many compounds as oxygen. But, from the constitution of the
atmosphere, it is obvious that the compounds into which oxygen enters
will always be of more importance to us than any others; and in this
point of view it may be attended with considerable convenience to have
oxygen represented by 1. In the present state of the atomic theory
there is another reason for making the atom of oxygen unity, which I
think of considerable importance. Chemists are not yet agreed about the
atom of hydrogen. Some consider water a compound of 1 atom of oxygen
and 2 atoms of hydrogen; others, of 1 atom of oxygen and 1 atom of
hydrogen. According to the first view, the atom of hydrogen is only
1-16th of the weight of an atom of oxygen; according to the second, it
is 1-8th. If, therefore, we were to represent the atom of hydrogen by
1, the consequence would be, that two tables of atomic weights would be
requisite--all the atoms in one being double the weight of the atoms in
the other: whereas, if we make the atom of oxygen unity, it will be the
atom of hydrogen only that will differ in the two tables. In the one
table it will be 0·125, in the other it will be 0·0625: or, reckoning
with Berzelius the atom of oxygen = 100, we have that of hydrogen =
12·5 or 6·25, according as we view water to be a compound of 1 atom of
oxygen with 1 or 2 atoms of hydrogen.
In the year 1809 Gay-Lussac published in the second volume of the
Mémoires d'Arcueil a paper on the union of the gaseous substances with
each other. In this paper he shows that the proportions in which the
gases unite with each other are of the simplest kind. One volume of one
gas either combining with one volume of another, or with two volumes,
or with half a volume. The atomic theory of Dalton had been opposed
with considerable keenness by Berthollet in his Introduction to the
French translation of my System of Chemistry. Nor was this opposition
to be wondered at; because its admission would of course overturn all
the opinions which Berthollet had laboured to establish in his Chemical
Statics. The object of Gay-Lussac's paper was to confirm and establish
the new atomic theory, by exhibiting it in a new point of view. Nothing
can be more ingenious than his mode of treating the subject, or more
complete than the proofs which he brings forward in support of it. It
had been already established that water is formed by the union of one
volume of oxygen and two volumes of hydrogen gas. Gay-Lussac found by
experiment, that one volume of muriatic acid gas is just saturated by
one volume of ammoniacal gas: the product is sal ammoniac. Fluoboric
acid gas unites in two proportions with ammoniacal gas: the first
compound consists of one volume of fluoboric gas, and one volume of
ammoniacal; the second, of one volume of the acid gas, and two volumes
of the alkaline. The first forms a neutral salt, the second an alkaline
salt. He showed likewise, that carbonic acid and ammoniacal gas could
combine also in two proportions; namely, one volume of the acid gas
with one or two volumes of the alkaline gas.
M. Amédée Berthollet had proved that ammonia is a compound of one
volume of azotic, and three volumes of hydrogen gas. Gay-Lussac himself
had shown that sulphuric acid is composed of one volume sulphurous
acid gas, and a half-volume of oxygen gas. He showed further, that the
compounds of azote and oxygen were composed as follows:
Azote. Oxygen.
Protoxide of azote 1 volume + ½ volume
Deutoxide of azote 1 " + 1
Nitrous acid 1 " + 2
He showed also, that when the two gases after combining remained in the
gaseous state, the diminution of volume was either 0, or ⅓, or ½.
The constancy of these proportions left no doubt that the combinations
of all gaseous bodies were definite. The theory of Dalton applied to
them with great facility. We have only to consider a volume of gas
to represent an atom, and then we see that in gases one atom of one
gas combines either with one, two, or three atoms of another gas, and
never with more. There is, indeed, a difficulty occasioned by the way
in which we view the composition of water. If water be composed of
one atom of oxygen and one atom of hydrogen, then it follows that a
volume of oxygen contains twice as many atoms as a volume of hydrogen.
Consequently, if a volume of hydrogen gas represent an atom, half a
volume of oxygen gas must represent an atom.
Dr. Prout soon after showed that there is an intimate connexion between
the atomic weight of a gas and its specific gravity. This indeed is
obvious at once. I afterwards showed that the specific gravity of a
gas is either equal to its atomic weight multiplied by 1·111[.1] (the
specific gravity of oxygen gas), or by 0·555[.5] (half the specific
gravity of oxygen gas), or by O·277[.7] (1-4th of the specific
gravity of oxygen gas), these differences depending upon the relative
condensation which the gases undergo when their elements unite. The
following table exhibits the atoms and specific gravity of these three
sets of gases:
I. Sp. Gr. = Atomic Weight × 1·1111
Atomic Sp.
weight. gravity.
Oxygen gas 1 1·1111
Fluosilicic acid 3·25 3·6111
II. Sp. Gr. = Atomic Weight × 0·555[.5].
Atomic weight. Sp. gravity.
Hydrogen 0·125 0·069[.4]
Azotic 1·75 0·072[.2]
Chlorine 4·5 2·5
Carbon vapour 0·75 0·416[.6]
Phosphorus vapour 2 1·111[.1]
Sulphur vapour 2 1·111[.1]
Tellurium vapour 4 2·222[.2]
Arsenic vapour 4·75 2·638[.8]
Selenium vapour 5 2·777[.7]
Bromine vapour 10 5·555[.5]
Iodine vapour 15·75 8·75
Steam 1·125 0·625
Carbonic oxide gas 1·75 0·972[.2]
Carbonic acid 2·75 1·527[.7]
Protoxide of azote 2·75 1·527[.7]
Nitric acid vapour 6·75 3·75
Sulphurous acid 4 2.222[.2]
Sulphuric acid vapour 5 2·777[.7]
Cyanogen 3·25 1·805[.5]
Fluoboric acid 4·25 2·361[.1]
Bisulphuret of carbon 4·75 2·638[.8]
Chloro-carbonic acid 6·25 3·472[.2]
III. Sp. Gr. = Atomic Weight × 0·277[.7].
Atomic weight. Sp. gravity.
Ammoniacal gas 2·125 0·5902[.7]
Hydrocyanic acid 3·375 0·9375
Deutoxide of azote 3·75 1·041[.6]
Muriatic acid 4·625 1·2847[.2]
Hydrobromic acid 10·125 2·8125
Hydriodic acid 15·875 4·40973
[Transcriber's Note: The numbers within [] thus [.2] represent numbers
with a dot above them in the original.]
When Professor Berzelius, of Stockholm, thought of writing his
Elementary Treatise on Chemistry, the first volume of which was
published in the year 1808, he prepared himself for the task by reading
several chemical works which do not commonly fall under the eye of
those who compose elementary treatises. Among other books he read the
Stochiometry of Richter, and was much struck with the explanations
there given of the composition of salts, and the precipitation of
metals by each other. It followed from the researches of Richter, that
if we were in possession of good analyses of certain salts, we might
by means of them calculate with accuracy the composition of all the
rest. Berzelius formed immediately the project of analyzing a series
of salts with the most minute attention to accuracy. While employed in
putting this project in execution, Davy discovered the constituents
of the alkalies and earths, Mr. Dalton gave to the world his notions
respecting the atomic theory, and Gay-Lussac made known his theory of
volumes. This greatly enlarged his views as he proceeded, and induced
him to embrace a much wider field than he had originally contemplated.
His first analyses were unsatisfactory; but by repeating them and
varying the methods, he detected errors, improved his processes, and
finally obtained results, which agreed exceedingly well with the
theoretical calculations. These laborious investigations occupied him
several years. The first outline of his experiments appeared in the
77th volume of the Annales de Chimie, in 1811, in a letter addressed
by Berzelius to Berthollet. In this letter he gives an account of
his methods of analyses together with the composition of forty-seven
compound bodies. He shows that when a metallic protosulphuret is
converted into a sulphate, the sulphate is neutral; that an atom of
sulphur is twice as heavy as an atom of oxygen; and that when sulphite
of barytes is converted into sulphate, the sulphate is neutral, there
being no excess either of acid or base. From these and many other
important facts he finally draws this conclusion: "In a compound formed
by the union of two oxides, the one which (when decomposed by the
galvanic battery) attaches itself to the positive pole (the _acid_ for
example) contains two, three, four, five, &c., times as much oxygen,
as the one which attaches itself to the negative pole (the alkali,
earth, or metallic oxide)." Berzelius's essay itself appeared in the
third volume of the Afhandlingar, in 1810. It was almost immediately
translated into German, and published by Gilbert in his Annalen der
Physik. But no English translation has ever appeared, the editors of
our periodical works being in general unacquainted with the German
and other northern languages. In 1815 Berzelius applied the atomic
theory to the mineral kingdom, and showed with infinite ingenuity that
minerals are chemical compounds in definite or atomic proportions, and
by far the greater number of them combinations of acids and bases. He
applied the theory also to the vegetable kingdom by analyzing several
of the vegetable acids, and showing their atomic constitution. But
here a difficulty occurs, which in the present state of our knowledge,
we are unable to surmount. There are two acids, the _acetic_ and
_succinic_, that are composed of exactly the same number, and same kind
of atoms, and whose atomic weight is 6·25. The constituents of these
two acids are
Atomic weight.
2 atoms hydrogen 0·25
4 " carbon 3
3 " oxygen 3
----
6·25
So that they consist of _nine_ atoms. Now as these two acids are
composed of the same number and the same kind of atoms, one would
expect that their properties should be the same; but this is not the
case: acetic acid has a strong and aromatic smell, succinic acid has
no smell whatever. Acetic acid is so soluble in water that it is
difficult to obtain it in crystals, and it cannot be procured in a
separate state free from water; for the crystals of acetic acid are
composed of one atom of acid and one atom of water united together; but
succinic acid is not only easily obtained free from water, but it is
not even very soluble in that liquid. The nature of the salts formed
by these two acids is quite different; the action of heat upon each
is quite different; the specific gravity of each differs. In short
all their properties exhibit a striking contrast. Now how are we to
account for this? Undoubtedly by the different ways in which the atoms
are arranged in each. If the electro-chemical theory of combination be
correct, we can only view atoms as combining two by two. A substance
then, containing nine atoms, such as acetic acid, must be of a very
complex nature. And it is obvious enough that these nine atoms might
arrange themselves in a great variety of binary compounds, and the way
in which these binary compounds unite may, and doubtless does, produce
a considerable effect upon the nature of the compound formed. Thus, if
we make use of Mr. Dalton's symbols to represent the atoms of hydrogen,
carbon and oxygen, we may suppose the nine atoms constituting acetic
and succinic acid to be arranged thus:
[hydrogen][carbon][hydrogen]
[oxygen][oxygen][oxygen]
[carbon][carbon][carbon]
Or thus:
[carbon][hydrogen][carbon]
[oxygen][oxygen][oxygen]
[carbon][hydrogen][carbon]
Now, undoubtedly these two arrangements would produce a great change in
the nature of the compound.
There is something in the vegetable acids quite different from the
acids of the inorganic kingdom, and which would lead to the suspicion
that the electro-chemical theory will not apply to them as it does to
the others. In the acids of carbon, sulphur, phosphorus, selenium, &c.,
we find one atom of a positive substance united to one, two, or three
of a negative substance: we are not surprised, therefore, to find the
acid formed negative also. But in acetic and succinic acids we find
every atom of oxygen united with two electro-positive atoms: the wonder
then is, that the acid should not only retain its electro-negative
properties, but that it should possess considerable power as an acid.
In benzoic acid, for every atom of oxygen, there are present no fewer
than seven electro-positive atoms.
Berzelius has returned to these analytical experiments repeatedly, so
that at last he has brought his results very near the truth indeed.
It is to his labours chiefly that the great progress which the atomic
theory has made is owing.
In the year 1814 there appeared in the Philosophical Transactions a
description of a Synoptical Scale of Chemical Equivalents, by Dr.
Wollaston. In this paper we have the equivalents or atomic weights
of seventy-three different bodies, deduced chiefly from a sagacious
comparison of the previous analytical experiments of others, and almost
all of them very near the truth. These numbers are laid down upon
a sliding rule, by means of a table of logarithms, and over against
them the names of the substances. By means of this rule a great many
important questions respecting the substances contained on the scale
may be solved. Hence the scale is of great advantage to the practical
chemist. It gives, by bare inspection, the constituents of all the
salts contained on it, the quantity of any other ingredient necessary
to decompose any salt, and the weights of the new constituents that
will be formed. The contrivance of this scale, therefore, may be
considered as an important addition to the atomic theory. It rendered
that theory every where familiar to all those who employed it. To
it chiefly we owe, I believe, the currency of that theory in Great
Britain; and the prevalence of the mode which Dr. Wollaston introduced,
namely, of representing the atom of oxygen by unity, or at least by
ten, which comes nearly to the same thing.
Perhaps the reader will excuse me if to the preceding historical
details I add a few words to make him acquainted with my own attempts
to render the atomic theory more accurate by new and careful analyses.
I shall not say any thing respecting the experiments which I undertook
to determine the specific gravity of the gases; though they were
performed with much care, and at a considerable expense, and though
I believe the results obtained approached accuracy as nearly as the
present state of chemical apparatus enables us to go. In the year
1819 I began a set of experiments to determine the exact composition
of the salts containing the different elementary bodies by means of
double decomposition, as was done by Wenzel, conceiving that in that
way the results would be very near the truth, while the experiments
would be more easily made. My mode was to dissolve, for example, a
certain weight of muriate of barytes in distilled water, and then to
ascertain by repeated trials what weight of sulphate of soda must be
added to precipitate the whole of the barytes without leaving any
surplus of sulphuric acid in the liquid. To determine this I put
into a watch-glass a few drops of the filtered liquor consisting of
the mixture of solutions of the two salts: to this I added a drop of
solution of sulphate of soda. If the liquid remained clear it was a
proof that it contained no sensible quantity of barytes. To another
portion of the liquid, also in a watch-glass, I added a drop of muriate
of barytes. If there was no precipitate it was a proof that the liquid
contained no sensible quantity of sulphuric acid. If there was a
precipitate, on the addition of either of these solutions, it showed
that there was an excess of one or other of the salts. I then mixed
the two salts in another proportion, and proceeded in this way till I
had found two quantities which when mixed exhibited no evidence of the
residual liquid containing any sulphuric acid or barytes. I considered
these two weights of the salts as the equivalent weights of the salt,
or as weights proportional to an integrant particle of each salt. I
made no attempt to collect the two new formed salts and to weigh them
separately.
I published the result of my numerous experiments in 1825, in a work
entitled "An Attempt to establish the First Principles of Chemistry by
Experiment." The most valuable part of this book is the account of the
salts; about three hundred of which I subjected to actual analysis. Of
these the worst executed are the phosphates; for with respect to them
I was sometimes misled by my method of double decomposition. I was not
aware at first, that, in certain cases, the proportion of acid in
these salts varies, and the phosphate of soda which I employed gave me
a wrong number for the atomic weight of phosphoric acid.