DESCARTES AS BRICOLEUR

Claus Zittel

I.

Can hot water freeze faster than cold water? In 1963 Ernesto Mpemba, a pupil from Tanzania, made an astonishing discovery while fabricat- ing ice-cream at school: he found out that he could achieve his goal in a better time when he put either hot milk in the ice-box or previ- ously heated milk that had cooled down. Therefore, when confronted with Newton’s law of cooling by his physics teacher, he persisted in contradicting him and was laughed at by his class mates: he was doing “Mpemba-physics”, they sneered. Mpemba, however, continued his experiments quite unmoved and even observed that his method was a common practice with the ice-cream makers of his town. The matter eventually reached a member of the school board, who repeated the experiments successfully and, together with Mpemba, published the results in an article.1 The phenomenon has ever since been accepted and is known as the ‘Mpemba-effect’.2 What Ernesto Mpemba could

1 Mpemba E.B. – Osborne D.C., “Cool?”, Physics Education 4 (1969) 172–175. 2 Cf. the article by Jeng M. (1998), “Can hot water freeze faster than cold water?”, http://math.ucr.edu/home/baez/physics/General/hot_water.html and “Hot water can freeze faster than cold?!?” (2005), http://arxiv.org/PS_cache/physics/ pdf/0512/0512262.pdf (with indications as to secondary bibliographical references), as well as Auerbach D., “Supercooling and the Mpemba effect: When hot water freezes quicker than cold”, American Journal of Physics (1995) 63, 10, 882–885. It must be noted that Descartes did precisely not chose the most contra-intuitive case, which is that hot water freezes faster, but made his experiments with water that had cooled down again. Nevertheless, he could have covered and explained this effect with his theory, since he assumes that the particles of water have undergone a transformation in consequence of the heating. Modern explanations principally take the same direction as Descartes’, cf. Chaplin M., “Water Structure and Behavior”, http://www.lsbu.ac.uk/water/explan4. html#mpemba. As regards the appreciation and observation of the Mpemba-effect in history of science, the research on Descartes supplies us, as far as I can see, with no studies whatsoever. With the exception of the above-mentioned reference in Thomas Kuhn’s text, there is only a very scant discussion by letter between Gallear and Deeson as to whether this effect was part of general knowledge in the 17. century: See: Gal- lear R., “The Bacon -Descartes-Mpemba phenomenon”, Physics Education 9 (1974), 490 and Deeson E., “Bacon-Descartes-Mpemba”, Phys. Educ. 10 (1975) 124–125. On the 338 claus zittel not know: The phenomenon had already been observed by Aristotle, and later Francis Bacon. 3 But it is Descartes who has furnished us with the most detailed account of the experiment in his text Les Météores (AT VI 238) and in his letters. In a letter to Mersenne dated 1. March 1638, for instance, he writes: I wonder at your mentioning your purpose to point out the mistakes you will fi nd in my book with regard to my experiments; for I will venture to assure you that there is nothing wrong in any of them, as I have carried them out myself, particularly the one where you can observe that warm water freezes sooner than cold one. I have not spoken of warm and cold water, but have stated that water that has been held over a fi re for a long time freezes sooner than other water. For to rightfully carry out this experiment, the water has to cool down after boiling, until it has reached the same temperature as that from a well. Then one has to take water from this well and fi ll both waters into identical jars. There are, however, few people able to carry out experiments correctly, and when they are wrongly done, one often fi nds the exact opposite of what one ought to fi nd.4 Descartes was right with his contra-intuitive example: It is possible that, under the same conditions, even water of 90°C freezes faster than water of 18°C. Science later regarded the descriptions of this phenomenon as incompatible with Newton’s law of cooling and for a long time dismissed them as mere myth. They were eventually rehabilitated – signifi cantly enough from outside the scientifi c community. This is an excellent example of how the formulation of a natural law can determine empirical per- ception and discredit a certain type of observation. Descartes’ physics were in many fi elds “Mpemba-physics”, and the text quoted above is quite representative of his methods of research: All his life Descartes observed and experimented, without hastily formulating principles. This, however, is neither consistent with the usual image of Descartes, nor with the customary representations of the history of science, accord- ing to which the turn to empiricism and to the experiment in Baconian

other hand, Scott’s judgement of Descartes’ Mpemba-experiment is remarkable for its ignorance: ‘This statement, which the simplest of experiments could have refuted, was repeated with elaborate details in a letter to Mersenne, and it emphasises Descartes’ readiness to rely on a priori conclusions.’ Scott J.F., The scientifi c work of René Descartes (1596–1650) (London: 1976) 67. 3 Aristoteles , Meteorology 348 b32; Francis Bacon , Novum Organum (1620), in Spedding J. – Ellis R.L. – Heath D.D. (eds.), The Works of Francis Bacon (New York: 1869) vol. VIII, 179–203, 235, 337 (this example Thomas Kuhn would likewise not miss, in: Die Struktur wissenschaftlicher Revolutionen (Frankfurt am Main: 1991) 31. 4 AT II 29.