JOSEPH BLACK

The difference between temperature and heat is now well established even though some students may be confused by these two related terms. Temperature (the intensity of heat) can be measures quiet easily using a thermometer. In contrast, the measurement of heat (quantify of energy) is rather more complicated.

About 250 years ago Joseph Black carried out a series of very important experiments concerning the measurement of heat and relationship between heat and temperature. He showed that although there was no change in temperature, large quantities of heat were absorbed in melting ice and in turning boiling water to steam. He introduced the term latent heat for the heat required to bring about these changed of state.

Joseph Black also found that different amounts of heat were required to produce the same temperature rise in equal masses of different substances. For instance, about 1.7 times more heat is required to raise the temperature of water from 15’C to 25’C than to give the same increase in temperature to an equal mass of ethyl alcohol. To explain this variation between different substances, Joseph Black introduces the idea of specific heats. While carrying out this work he laid down the principles of heat measurement – calorimetry – which are still used today.

For the previous hundred years or more, advances in chemistry had been hampered by the Phlogiston Theory. However, as Joseph Black was suspicious of theories which were not supported by experimental evidence, he was able to make several valuable additions to chemical knowledge.

Until the middle of the eighteenth century very little was known about gases, in fact many people thought there was only one gas – air. About a century before (in 1640 to be precise), van Helmont had discovered the gas which is now known as carbon dioxide, but with the rise of the Phlogiston Theory the significance of this was lost.

Joseph Black discovered carbon dioxide in 1754 while experimenting with two mild alkalis – the carbonates of magnesium and calcium. He found that when these compounds were heated, each produce a stronger alkali while at the same time fixed air – carbon dioxide – was liberated. The weight of the stronger alkali was less than that of the mild alkali from which it was obtained.

Joseph Black was born in 1728 at Bordeaux in France. Both his parents were of Scots decent. After spending six years at school in Belfast, he entered the University of Glasgow in 1746 to study chemistry and medicine. In 1756 he became professor of anatomy and lecturer in chemistry in Glasgow. Ten years later he succeeded to chair of medicine and chemistry in the University of Edinburgh.

Joseph Black was very popular with his students because he took great pains in planning his courses and because his lectures were illustrated with many experiments.

As well as making a number of valuable contributions to both physics and chemistry, he also found time to practice as a physician. He died peacefully in his chair at the age of 71.

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