The Day the Universe Changed - James Burke [91]
In 1756 a Dissenter called Joseph Black, a Scottish doctor, was looking for a way to make better magnesia alba, recently introduced as a treatment for indigestion. In doing so he carried out the first ever detailed study of a chemical reaction. Using very accurate weighing instruments, he dissolved chalk in acid, noticed that it bubbled while dissolving, and weighed the result. There was a weight loss of 40 per cent. He burned the mixture and noticed that the weight regained was almost equal to that which had bubbled off as gas. There was, he concluded, some kind of air ‘fixed’ in the chalk. He called it ‘fixed air’. Black’s experiment spurred interest in gas and air, and furthered the work of investigators such as Joseph Priestley in Birmingham and Antoine-Laurent Lavoisier in France.
In 1764 Black was invited to solve a problem for a firm of Scottish distillers. Ever since the Union of England and Scotland in 1707, all earlier discrimination against Scots trade overseas had ceased. As a result Glasgow had become a major port of entry for sugar and tobacco. In the 1750s more than half of all the reexportation of tobacco to the Continent was through Glasgow. The extra money generated by this trade stimulated consumer demand in the region, particularly for textiles and whisky.
The distillers now had a potentially very profitable market, if only they were able to supply it. Their problem lay in finding out how to distil much greater quantities of whisky efficiently and cheaply. Distilling called for tremendous heat in order to turn large amounts of liquid into vapour. Equally large amounts of water were then needed to remove the heat from the vapours in order to condense out the whisky. Black was hired to investigate the problem of quantifying how much fuel and water would be needed.
He visited the distilleries, which were situated in the Highlands where the large quantities of water needed for cooling purposes were to be found. He noticed that even during a day of bright sunshine the ice and snow on the hills did not melt, as he would have expected. Unusual amounts of heat were obviously required to melt it.
Black began experimenting with melting ice. He placed equal amounts of ice and near-freezing water in the same ambient temperature, and left them for ten hours. He found that the water absorbed heat from the surrounding atmosphere and rose fairly quickly to room temperature at an hourly rate of 14°F. For the ice to do the same, that is to melt and rise to room temperature, took a full ten hours. At the general warming rate the ice had therefore absorbed ten hours’ worth of heat, i.e. 140°F. However, the thermometer in the ice had only registered a change from freezing to room temperature. When the ice changed to water it was absorbing a considerable amount of heat which did not show on the thermometer. Black called this ‘hidden heat’.
Black then moved on to the production of steam. On the same slow fire he put equal amounts of water, one of which he heated from cold to boiling. It absorbed heat at a rate of 40.5°F an hour. The water in the other pan was allowed to boil away. This took much longer and involved a total absorption of no less than 810°F of heat. Black concluded that the reason it took so long to boil liquid away, that steam scalded so much and that large quantities of cold water were needed to condense steam back into liquid, was in each case due to the transfer of ‘hidden’ or latent heat involved in the operation. He then worked out that if it took one unit of heat to raise water to boiling point it would take five times as much heat to turn it into steam.
Apart from helping the Scots whisky trade, Black’s new theory also helped a colleague at Glasgow University. He was the son of a master carpenter, born in Greenock and working as an instrument-maker to the university science laboratories, where one of his jobs was to make instruments to show Black’s new theory about latent heat.
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