1 The Birth of Science - MSRI

144 5. Medicine and Other Empirical Sciences
Hellenistic notion of oncos (). 87 Oncos is conceived as the ultimate
component of substances, but, unlike atoms (of which it seems to be made),
it is capable of transformation, through reorganization of its parts, thus accounting
for qualitative changes in substances. 88
In the Leyden and Stockholm papyri, when amounts of ingredients are
indicated (which is often not the case), the information is given in parts.
The occasional use of weight units indicates that the parts are to be measured
with a balance. There is evidence from several sources that this use
of the balance led to the principle of conservation of mass 89 attributed to
Lavoisier and regarded as one of the greatest achievements of eighteenthcentury
chemistry. In Lucretius the principle of conservation of mass is not
only clearly stated (in the poetical form that enabled the work to survive,
of course), 90 but even justified on the grounds that atoms are indestructible.
Another attestation is in Lucian’s Life of Demonax: when someone page 188
asks Demonax “how many minae of smoke do you get burning a thousand
minae of wood?”, he gets the answer “weigh the ashes; the remainder
is smoke”. 90a Obviously it matters little that from our vantage point the
proposed method is incorrect (because atmospheric oxygen also takes part
in the combustion). More interesting than the answer is the question. Why
on earth would one ask about the weight of a certain amount of smoke?
The only sense the question makes is as an attempt to ridicule an existing
scientific theory to the effect that all objects have a “mass” (or weight)
and that the mass is preserved. Though such a theory has been regarded
by some as absent in Antiquity, 91 it is implicitly used by Heron of Alex-
87
Sextus Empiricus (Adversus dogmaticos, IV, 318) attributes the use of this concept to Heraclides
of Pontus (fourth century B.C.) and to Asclepiades of Prusa, a Greek physician who worked in
Rome in the first century B.C. and maintained, for instance, that fevers can propagate through the
emissions of corpuscles from the body (compare Sextus Empiricus, Adversus mathematicos, III, 5).
88
Compare Sextus Empiricus (Adversus dogmaticos, IV, 42–44). The original meaning of oncos is
volume, mass, bulk. It would be interesting to investigate in detail what contribution the memory
of the ancient concept of oncos made to the formation of the modern concept of molecules. Here
we just make two observations in this direction. First, the term oncos in scientific texts was systematically
translated in Latin as moles (bulk, large mass), even when the meaning is that of volume,
as is clear from the Latin translations of Archimedes’ On floating bodies made by William of Moerbeke,
I. Barrow and G. Torelli. Second, the passage in which Robert Boyle introduces the modern
idea (Chymista scepticus, London, 1661, chapter 1, prop. 2) is reminiscent of the Sextus Empiricus
passage, and in fact the whole work is pointedly set against the backdrop of ancient Skepticism, a
doctrine for which Sextus is almost our only source.
89
An important prescientific precursor of this principle was already present in the statement of
conservation of matter made by Empedocles (fr. 4 in [Empedocles/Gallavotti] = frs. 17, 14, 13, 17,
22, 20 in [Diels: FV], vol. I, pp. 314–321; see particularly verses 30–32 of fr. 17).
90
Lucretius, De rerum natura, II, 294–296.
90a
Lucian, Vita Demonactis, 39, 2–6.
91
Max Jammer, after quoting Demonax’s answer, writes: “Such ideas, however, remained isolated
statements.. . . And never did such ideas give rise to the formation of the concept of ‘quantity
of matter’ in a technical sense” ([Jammer: CM], p. 27).
Revision: 1.9 Date: 2002/09/14 19:12:01