When water does not boil at the boiling point Hasok Chang Department of Science and Technology Studies, University College London, Gower Street, London WC1E 6BT, UK Every schoolchild learns that, under standard pressure, pure water always boils at 100 -C. Except that it does not. By the late 18th century, pioneering scientists had already discovered great variations in the boiling temperature of water under fixed pressure. So, why have most of us been taught that the boiling point of water is constant? And, if it is not constant, how can it be used as a ‘fixed point’ for the calibration of thermometers? History of science has the answers. The mirage of ‘true ebullition’ One of the 18th-century pioneers of boiling was Jean-Andre ´ De Luc (1727–1817) – a Swiss meteorologist, physicist, geologist, mountaineer, theologian and businessman (Figure 1). De Luc must have seemed like a madman as he strolled down the streets of Geneva shaking a flask of water with a thermometer sealed into it. He was in the last stages of completing his long-awaited masterpiece, Investi- gations on the Modifications of the Atmosphere, which was published in 1772 (Figure 2). De Luc was exploiting a kinetic effect that is familiar to anyone who has made the mistake of shaking a can of fizzy drink too vigorously. His aim was to extract all the dissolved air from the fluid. This was part of De Luc’s quest for the elusive phenomenon that he called ‘true ebullition’. Most people had thought distilled water was completely pure, but De Luc pointed out that it contained plenty of dissolved air and he found that the air facilitated what seemed like premature boiling. So, he reasoned, the air needed to be removed: This operation lasted four weeks, during which I hardly ever put down my flask, except to sleep, to do business in town, and to do things that required both hands. I ate, I read, I wrote, I saw my friends, I took my walks, all the while shaking my water [1]. Four mad weeks of shaking had its rewards. De Luc reported that the de-gassed water exhibited very strange behaviour  it would not boil at all at the normal boiling point; instead, it became ‘superheated’ to 112 8C and then exploded. This superheating was consistent with what De Luc had seen in an earlier series of investigations, in which he realized that boiling, as usually performed, was quite a crude operation. When one puts a pot full of water on an open flame, the container and the ‘first’ layer of water directly in contact with the container are hotter than the rest of the water. Boiling occurs when bubbles of vapour form in that first layer, but the ‘boiling temperature’ is taken with the thermometer placed in the main body of the water. This was not a coherent experiment. Because it was impossible to put a thermometer into the first, extremely thin layer of water, he sought instead to bring the whole body of water to the same temperature. De Luc thought he could achieve this by slow heating with a gentle heat- source, while minimizing heat-loss from the water. To this end, he employed a round flask with a long, thin neck, which he plunged into a hot bath of nut oil. When he did this, he encountered a surprising phenomenon, which later came to be called ‘bumping’. The water in this arrangement often boiled in an irregular way by producing large, occasional bubbles of vapour; sometimes the bubbles were explosive enough to throw some of the water out of the flask. During bumping, the temperature of the water fluctuated between 100 8C and somewhere over 103 8C [2]. And all this was before De Luc even shook the dissolved air out of the water. The investigation of boiling took De Luc many months and revealed more and more complexities, until he recog- nized six distinct phenomena, all of which might in some sense qualify as ‘boiling’. The 15-chapter supplement on the variations of the temperature of boiling water, which De Luc added to his Investigations, is testimony to the complexity of his findings [3]. At the end of his long search for ‘true ebullition’, he ended up not knowing what boiling was at all or at what temperature one could say it hap- pened. He issued the following words of caution about the fixed points of thermometers [4]: Today people believe that they are in secure possession of these points, and pay little attention to the uncertainties that even the most famous men had regarding this matter, nor to the kind of anarchy that resulted from such uncertainties, from which we still have not emerged at all. The Royal Society committee and the steam point Five years later De Luc was in London, serving on an illustrious seven-man committee appointed by the Royal Society to make definitive recommendations regarding the fixed points of thermometers. His business had collapsed shortly after the publication of his book, and he immigrated to England, where he was installed in Windsor as ‘Reader’ to Queen Charlotte, the consort of George III. The Royal Society committee, chaired by the enigmatic aristocrat Henry Cavendish (1731–1810), investigated many suspected causes of variation in the temperature Review Endeavour Vol.31 No.1 Corresponding author: Chang, H. (h.chang@ucl.ac.uk). Available online 2 March 2007. www.sciencedirect.com 0160-9327/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.endeavour.2007.01.005