Science in the Scottish Enlightenment

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# 6.3.2 Heat of vaporisation

Black read a paper on these experiments to the Glasgow Literary Society in April 1762, and then turned to the investigation of vaporisation. For reasons he himself found difficult to explain, Black was initially reluctant to accept that there was a similar heat of vaporisation. This was in spite of the fact that he (and presumably many cooks) had observed that it takes far longer to boil off water than it takes to raise water to boiling point. In October 1762, he devised a very simple experiment to measure the heat of vaporisation. He took a flat-bottomed tinplated pan and heated small quantities of water in it, using a steady furnace. Knowing the initial temperature of the water (50°F), the time it took to reach boiling point (four minutes) and the extra time it took to boil off (twenty minutes), he could calculate the heat of vaporisation. The quantity he obtained was 810°F. (This is equivalent to 1890 KJ/Kg, rather less than the modern value of 2268 KJ/Kg.)

Almost exactly two years later, Black and his student William Irvine carried out the reverse experiment, namely the determination of the heat liberated when steam is condensed to water. Once again, Black displayed his penchant for the simplest apparatus. He used an ordinary laboratory still fitted with a condenser filled with water (at 52°F). The quantity of water condensed was measured and found to be at 132°F. The temperature of the water in the condenser was at 123°F. From this data, Black and Irvine calculated that the latent heat of steam was at least 774°F. This was obviously too low, but it was close enough to the 810°F Black obtained for the conversion of water to steam to show that the two processes were probably equal and opposite.

Black's work on the heat of vaporisation provides us with an early example of the interaction between science and technology, because Black and Robison were close friends of James Watt (1736–1819), the pioneer of steam power. Watt was born in Greenock, but he trained as a scientific instrument-maker in London. On his return to Glasgow in 1757, he was appointed instrument-maker to the university, probably through his friendship with Professor Robert Dick, who may have introduced Watt to Black. Black and Watt entered a partnership with Alexander Wilson, later professor of astronomy, in November 1758.

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