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No. 15

THE HEAT OF ICE

Ice has an intriguing property, which fascinated scientists studying the nature of heat in the eighteenth century. As well as being intrinsically interesting, these studies had practical implications; it was just at the time steam power was beginning to be harnessed to drive the Industrial Revolution. The curious property is that when ice at the freezing point (0 ºC, or, in the units used in Britain then, 32 ºF) is heated, its temperature stays the same until all the ice has melted into water. Only then does the temperature of the water increase as more heat is applied. The same sort of thing, of course, happens when other substances, such as metals, are melted, but ice is much easier to study.

Other people had thought that if a lump of ice at the melting point were heated by a tiny amount it would all melt. But the person who studied what was really going on in a careful series of experiments in the 1760s was a professor at Glasgow University, Joseph Black. Whenever Black did experiments, he measured everything that could be measured, as accurately as possible. He had made his name by studying the amount of gas produced or absorbed in chemical reactions. In one of his experiments, a carefully weighed amount of limestone was heated, to produce quicklime, which was then weighed. The quicklime weighed less, because the gas we now call carbon dioxide had been driven off. A weighed amount of water was added to the quicklime to produce slaked lime, which was weighed. Then, a weighed amount of a mild alkali was added to convert the slaked lime back into what weighing proved to be the same amount of limestone that he had started with. Along the way, the differences in weight told him how much gas had been lost or absorbed at each stage. This was quantitative science, as opposed to qualitative science, in which the changes in the character of the substances (their quality) was noted, but there were no measurements of how much they had changed (the quantity).

© Middle Temple Library/Science Photo Library

Joseph Black (1728–1799).

© Sheila Terry/Science Photo Library

Joseph Black giving a practical demonstration of latent heat to students of Glasgow University in the 1760s.

Black carried over this quantitative approach – a cornerstone of modern experimental science – into his studies of heat. He found that the amount of heat needed to melt a certain amount of ice at 32 ºF into water at the same temperature was enough to raise the temperature of the water from 32 ºF all the way to 140 ºF (or 60º C). He also studied the way water turns into steam, showing that when a mixture of water and water vapour at the boiling point (212 ºF, or 100 ºC) is heated, the temperature does not increase until all the water has been turned into vapour. And if a certain weight of water – say, a pound – at 32 ºF is added to the same quantity of water at 212 ºF, the resulting liquid has a temperature of 122 ºF (or 50º C), halfway between boiling and freezing. This led him to the idea of ‘specific heat’, which is the amount of heat required to raise the temperature of a certain amount of stuff by one degree (in modern units, the heat required to raise the temperature of 1 gram by 1 ºC). Black coined the term ‘specific heat’, and also gave the name ‘latent heat’ to the heat absorbed by a melting substance. And when a liquid such as water freezes, the same amount of latent heat is released as it does so; similarly, latent heat is released when vapour condenses into liquid. In Black’s own words: ‘I, therefore, set seriously about making experiments, conformable to the suspicion that I entertained concerning the boiling of fluids … I imagined that, during the boiling, heat is absorbed by the water, and enters into the composition of the vapour produced from it, in the same manner as it is absorbed by ice in melting, and enters into the composition of the produced water. And, as the ostensible effect of the heat, in this last case, consists, not in warming the surrounding bodies, but in rendering the ice fluid; so, in the case of boiling, the heat absorbed does not warm surrounding bodies, but converts the water into vapour. In both cases, considered as the cause of warmth, we do not perceive its presence: it is concealed, or latent, and I give it the name of LATENT HEAT.’⁷

These discoveries were noted by a certain James Watt, an instrument maker at the university, who built experimental apparatus for Black, and who went on to develop steam engines.