Dispute About Objects that Float in Water

A fortnight into June Galileo returned to Florence. Only a few days later he got embroiled, during the worst of the Tuscan summer heat, in a discussion about ice.

Galileo was staying with his good friend, the wealthy Salviati, at the Villa delle Selve - "the house by the woods". It was beautifully situated on a prominence near the little town of Signa, roughly half way between Florence and Pisa. From the noble house with its simple, firm Renaissance style, orchards of olive trees and vineyards stretched all the way down to the Arno. Salviati would retire here with his friends when the summer heat became too oppressive in Florence.

Filippo Salviati was deeply interested in science. Visiting his villa at the same time were two professors from Pisa, and for some reason they began to discuss the nature of ice.

Aristotle said that when things cooled they condensed. Clearly, ice was cooled water - and thus condensed, according to the two professors. Ergo ice was heavier than water.

But - Galileo objected - ice actually floated in water. If one believed Archimedes, would this not mean it was lighter?

Nonsense, the pair persisted. Heaviness and lightness had nothing to do with the characteristics of floating, because Aristotle had never said anything about it. (It was true that the master had hardly touched on the matter - he had in all only written one and a half pages on bodies that float and sink.) Shape was what was decisive. Ice floats on water because it is flat and thin.

Galileo knew he was right. In principle shape was irrelevant - it was specific gravity that determined if something floated or not, and he expressed himself in no uncertain terms to the two professors.

Perhaps the little argument in Salviati's villa might have ended there. But a few days later the disconcerted Aristotelians from Pisa were encouraged in the strongest terms to defend themselves and their scientific tradition against the arrogant Galileo. The previously mentioned local philosopher, Ludovico delle Colombe, had joined the contretemps.

About the same age as Galileo, delle Colombe clearly had some old grudge against him. He had written a short discourse on the 1604 nova, which had been torn to shreds by a certain "Alimberto Mauri", and he believed that Galileo was behind it. He had read Horky's attack on the telescope with great glee.

And delle Colombe had followed Horky's example and attacked Galileo in writing. But he went one decisive step further and shifted the battle about science and prestige into a new arena: that of theology. Without actually naming Galileo, he tried to hit him in the most dangerous place: where astronomy and Bible study converge.

In Against the Motion of the Earth delle Colombe wrote:

"Could these poor fellows [namely, the promoters of the Copernican theory] perhaps have recourse to an interpretation of the Scripture different than the literal sense? Definitely not, because all theologians, without exception, say that when Scripture can be understood literally, it ought never be interpreted differently."27

Earlier that summer, when Galileo had still been in Rome, delle Colombe had tried to enlist several like-minded people in a campaign against the Grand Duke's mathematician. He wrote to Christopher Clavius and complained of Galileo's observations of the Moon's uneven surface and the implications this might have. His aim was clearly to provoke the Jesuit Clavius into a theological and scientific discussion with Galileo, but Clavius had seen the Moon for himself and was convinced. He did not even bother to reply to the Florentine philosopher.

Ludovico delle Colombe now heard about the ice argument. He immediately contacted the professors at Pisa and not only told them that, of course, Aristotle and they were right - but more to the point, he could prove that Galileo was mistaken. And he could defeat this jumped-up grand ducal Mathematician on his home ground - he could prove it with the aid of an experiment!

What ensued was as much a battle for status as for scientific truth. Cigoli saw it plainly when he wrote from Rome, making a reference to delle Colombe's odd surname:

"Those ugly birds want to make a name for themselves not through their own value but through the choice of adversary."28

What delle Colombe was doing, was in effect to challenge Galileo to a duel. Not with actual weapons, but through the agency of a public experiment that would prove which of them was right.

The experiment was remarkably simple and anyone could understand it. No one doubted that ivory had a greater specific gravity than water. Normally, therefore, a piece of ivory would sink. But if one took a small splinter of the material and placed it carefully on the surface - look for yourself!lt floats! Ergo there was the proof that shape had an effect on the ability to float.

Neither Galileo nor anyone else had the least inkling that this concerned a phenomenon called surface tension, and therefore had little to do with the general ability to float. It seemed indisputable that delle Colombe had a good argument.

The discussion therefore became one about how the experiment was to be conducted. Either Galileo wanted the splinter to be wetted before it was placedon thesurface of thewater, orhewantedtoturnthe experiment round so that it would demonstrate what floated up from the bottom of a vessel -a splinter of ivory, no matter how small, would, of course, stay at he bottom. Galileo's strength lay in the fact that he could think up lots of experiments that would prove him right, whereas his opponent was dependent on his single one, which was simple and appeared convincing.

The result was just about a draw. The Grand Duke was not pleased that his mathematician was in public dispute with delle Colombe, the more so since the latter had found support from a somewhat dubious Medici relative, Cosimo I's illegitimate son Giovanni. Galileo therefore withdrew from the entire proceeding, and instead wrote a dissertation in which he interpreted the whole problem in his own way: Discorso alle cose che stanno in su l'acqua o che in quella si muovono or Discourse on floating bodies in water or those which move in it.

Of its title there is this to say: firstly, this dissertation was written in Italian, not Latin. From now on it was the well informed general public that Galileo was addressing. His writings were to be accessible and comprehensible to courtiers and the bourgeoisie, and not just to scientists - indeed, he was excluding foreign colleagues like Kepler, who normally could not read Italian, or at least would have to struggle through the text with the help of their knowledge of Latin.

Secondly, typically enough he does not give way one iota to delle Colombe. Galileo had to be right - on all points.

He will explain how something can both float on water, and sink. In his dissertation Galileo attempts an astute explanation for the uncomfortable fact that tiny pieces of heavy materials do actually float. He refers to an instance he regards as analogous, namely that an empty clay pot floats, even though fired clay is heavier than water. But one must also include the volume within the pot as well, or as he puts it "the sum of the air and the material".

This - which is obviously quite correct - he transfers to floating fragments. He believes that they sink a tiny bit below the surface, without breaking it, thus forming an "air pocket" above them. The volume of this air must be included, and that is how the ivory floats.

This reasoning was brilliantly conceived - but erroneous. It cannot be denied that, on his own premises, delle Colombe was right in a way. And in addition to his cock-suredness, it was Galileo's innate tendency to search for the simplest and most rational explanations that caused problems for him.

He could not accept that the surface of water could have properties in any way different to water in general.

The Aristotelians were not to be convinced, and they soon launched a counter-attack.

Regardless of this, Discourse on floating bodies was a marvellous work, that linked the general comments Galileo had made on motion, to an investigation of things that move through water. In particular he emphasised that there is no "lightness" that can lift objects - in opposition to a "heaviness" that makes them fall. This is a fundamental assumption for Aristotle, linked to his doctrine of the elements: fire and air move upwards, water and earth downwards. In wishing to get rid of "lightness", he was aiming at the very foundation of Aristotelian physics.

Before Discourse on floating bodies came out, Galileo took part in a debate. It was not a public affair in the city, but rather on his home ground, at the Grand Duke's court. His opponent was no delle Colombe of modest social status, either, but the Aristotelian Papazzoni, the newly appointed professor at Pisa. The discussion was pure show, an intellectual entertainment Cosimo II had arranged for two very eminent guests after a splendid banquet.

Both guests were cardinals, and they threw themselves wholeheartedly into the discussion, fired just a little, perhaps, by the food and certainly the wine. Cardinal Gonzaga sided with Papazzoni, while the other supported Galileo, who indubitably came out of the skirmish best.

This other cardinal was Galileo's friend and admirer from his stay in Rome, Maffeo Barberini, whose star was continually on the ascendant in the clerical firmament. He was fascinated by Galileo's intellectual audacity, and was not too worried if Aristotelian doctrine was brought to its knees. Such a supporter in the Vatican could be a good person to have, because now his enemies were rallying for the attack.

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