Modern Chemistry and the Molecular Revolution

[Dermezel]

Recently I have been reading about the history of the science of chemistry (in my firm opinion, I do not believe you really understand a science unless you know its history, since science is based on comparing hypothesis with respect to empirical data. )

Evidently the primary exponent of molecular theory was Avogadro, the man who brought the law of large numbers to chemical equations and heralded the advent of modern chemistry as we know it today.

But Avogadro was rejected for nearly forty-years by the scientific establishment, who firmly believed all elements were equally divisible into particles, i.e. they believed mercury was composed of the similar number of protons, neutrons and electrons as hydrogen.

It wasn't until forty years after Avogardo's proposed laws that another chemist, Stanislao Cannizzaro argued that many otherwise incoherent aspects of chemistry could be explained by acknowledging the existence of molecules. The chemistry establishment was so against this, that they even proposed a dualistic "equal time" theory, arguing chemists could choose what to believe- either molecular theory, or the then established "all chemicals are equivalent particles" theory.

Cannizzaro argued for molecular theory before a large, world wide collection of chemists in his famous SKETCH OF A COURSE OF
CHEMICAL PHILOSOPHY, presented before the Royal University.

It begins as thus:

Quote:

I BELIEVE that the progress of science made in
these last years has confirmed the hypothesis of
Avogadro, of Ampere, and of Dumas on the similar
constitution of substances in the gaseous state ; that
is, that equal volumes of these substances, whether
simple or compound, contain an equal number of
molecules : not however an equal number of atoms,
since the molecules of the different substances, or
those of the same substance in its different states,
may contain a different number of atoms, whether of
the same or of diverse nature.

In order to lead my students to the conviction which
I have reached myself, I wish to place them on the
same path as that by which I have arrived at it the
path, that is, of the historical examination of chemical
theories.

I commence, then, in the first lecture by showing
how, from the examination of the physical properties of gaseous bodies, and from the law of Gay-Lussac on
the volume relations between components and com-
pounds, there arose almost spontaneously the
hypothesis alluded to above, which was first of all
enunciated by Avogadro, and shortly afterwards by
Ampere. Analysing the conception of these two
physicists, I show that it contains nothing contra-
dictory to known facts, provided that we distinguish,
as they did, molecules from atoms ; provided that we
do not confuse the criteria by which the number and
the weight of the former are compared, with the
criteria which serve to deduce the weight of the
latter ; provided that, finally, we have not fixed in our
minds the prejudice that whilst the molecules of
compound substances may consist of different numbers
of atoms, the molecules of the various simple substances
must all contain either one atom, or at least an equal
number of atoms.

In the second lecture I set myself the task of
investigating the reasons why this hypothesis of
Avogadro and Ampere was not immediately accepted
by the majority of chemists. I therefore expound
rapidly the work and the ideas of those who examined
the relationships of the reacting quantities of substances
without concerning themselves with the volumes
which these substances occupy in the gaseous state ;
and I pause to explain the ideas of Berzelius, by the
influence of which the hypothesis above cited
appeared to chemists out of harmony with the
facts.

I examine the order of the ideas of Berzelius, and
show how on the one hand he developed and com-
pleted the dualistic theory of Lavoisier by his own
electro-chemical hypothesis, and how on the other
hand, influenced by the atomic theory of Dalton (which had been confirmed by the experiments of Wollaston),
he applied this theory and took it for his guide in his
later researches, bringing it into agreement with the
dualistic electro-chemical theory, whilst at the same
time he extended the laws of Richter and tried to
harmonise them with the results of Proust. I bring
out clearly the reason why he was led to assume that
the atoms, whilst separate in simple bodies, should
unite to form the atoms of a compound of the first
order, and these in turn, uniting in simple propor-
tions, should form composite atoms of the second
order, and why (since he could not admit that when two
substances give a single compound, a molecule of the
one and a molegule of the other, instead of uniting
to form a single molecule, should change into two
molecules of the same nature) he could not accept the
hypothesis of Avogadro and of Ampere, which in
many cases leads to the conclusion just indicated.

I then show how Berzelius, being unable to escape
from his own dualistic ideas, and yet wishing to
explain the simple relations discovered by Gay-Lussac
between the volumes of gaseous compounds and their
gaseous components, was led to formulate a hypothesis
very different from that of Avogadro and of Ampere,
namely, that equal volumes of simple substances in the
gaseous state contain the same number of atoms,
which in combination unite intact ; how, later, the
vapour densities of many simple substances having
been determined, he had to restrict this hypothesis by
saying that only simple substances which are
permanent gases obey this law ; how, not believing
that composite atoms even of the same order
could be equidistant in the gaseous state under the
same conditions, he was led to suppose that in the
molecules of hydrochloric, hydriodic, and hydrobromic acids, and in those of water and sulphuretted hydrogen,
there was contained the same quantity of hydrogen,
although the different behaviour of these compounds
confirmed the deductions from the hypothesis of
Avogadro and of Ampere.

I conclude this lecture by showing that we have
only to distinguish atoms from molecules in order
to reconcile all the experimental results known
to Berzelius, and have no need to assume any
difference in constitution between permanent and
coercible, or between simple and compound gases,
in contradiction to the physical properties of all
elastic fluids.

In the third lecture I pass in review the various
researches of physicists on gaseous bodies, and show
that all the new researches from Gay-Lussac to
Clausius confirm the hypothesis of Avogadro and of
Ampere that the distances between the molecules, so
long as they remain in the gaseous state, do not
depend on their nature, nor on their mass, nor on*
the number of atoms they contain, but only on their
temperature and on the pressure to which they are
subjected.

In the fourth lecture I pass under review the chemical
theories since Berzelius : I pause to examine how
Dumas, inclining to the idea of Ampere, had habituated
chemists who busied themselves with organic substances
to apply this idea in determining the molecular
weights of compounds ; and what were the reasons
which had stopped him half way in the application of
this theory. I then expound, in continuation of this r
two different methods the one due to Berzelius, the
other to Ampere and Dumas which were used to
determine formulae in inorganic and in organic
chemistry respectively until Laurent and Gerhardt sought to bring both parts of the science into harmony.
I explain clearly how the discoveries made by Gerhardt,
Williamson, Hofmann, Wurtz, Berthelot, Frankland,
and others, on the constitution of organic compounds
confirm the hypothesis of Avogadro and Ampere, and
how that part of Gerhardt's theory which corresponds
best with the facts and best explains their connection,
is nothing but the extension of Ampere's theory,
that is, its complete application, already begun by
Dumas.

I draw attention, however, to the fact that Gerhardt
did not always consistently follow the theory which
had given him such fertile results ; since he assumed
that equal volumes of gaseous bodies contain the same
number of molecules, only in the majority of cases,
but not always.

I show how he was constrained by a prejudice, the
reverse of that of Berzelius, frequently to distort the
facts. Whilst Berzelius, on the one hand, did not
admit that the molecules of simple substances could
be divided in the act of combination, Gerhardt
supposes that all the molecules of simple substances
are divisible in chemical action. This prejudice
forces him to suppose that the molecule of mercury
and of all the metals consists of two atoms, like that
of hydrogen, and therefore that the compounds of all
the metals are of the same type as those of hydrogen.
This error even yet persists in the minds of chemists,
and has prevented them from discovering amongst the
metals the existence of biatomic radicals perfectly
analogous to those lately discovered by Wurtz in
organic chemistry.

Despite these tireless arguments, most practicing chemists were unwilling to accept Avogadro's ideas concerning the existence of molecules, and in the end it was ruled that chemists could choose to believe either Avogadro's theories which had a massive amount of evidence behind them, or the Berzelian theory that molecules didn't exist, and all chemicals were equally divisible.

I find this fascinating as a prototypical example of scientific revolution, where today we see parallels between evolutionary psychology and modern behavioral-cognitive theory, the former arguing that evolved instincts effect human nature, the latter arguing that we need merely presume stimulus and response. Likewise with Marxist theory vs. Marginalism, the former including the labor theory of value to explain otherwise inexplicable anomalies in economics, the latter arguing that all can be reduced to supply and demand. And last, with Jared Diamond's argument for scientific, geographically determined history, vs. the established historians who wish to narrow all historical study to ancient writings.

It seems as if modern day science is undergoing another revolutionary stage, at least with respect to popular culture, where established thought argues that so-called "soft" sciences (psychology, sociology, economics) are an art based on feelings and intuition and philosophical assumptions, and the emerging scientific school arguing that these studies can be systematized into tested hypothesis verified or falsified by empirical data.

This thread is meant to discuss scientific revolutions in general, or, more specifically, the debate surrounding the molecular revolution in chemistry and what it means for science today.