Black’s curious and puzzling thing
Stories from Physics for 11-14 14-16
Joseph Black, who became professor of medicine and chemistry at Glasgow University, was born in Bordeaux in 1728 to a family of wine merchants. Black was interested in changes of state and introduced the term latent heat to refer to energy that appeared hidden in the process of melting but would reappear in freezing. He contrasted latent heat with sensible heat, the energy that, when transferred, results in the changes in temperature of materials between changes of state. In a collection of lectures published in 1807, after his death, Black noted a curious effect that occurred when an equal of amount of ice and 80°C water were mixed together (the units have been converted from Fahrenheit in the original text):
…the result was that the fluid was no hotter than water just ready to freeze. Nay, if a little sea salt be added to the water and it be heated only to 74 or 77°C, we shall produce a fluid sensibly colder than the ice was in the beginning, which has appeared a curious and puzzling thing to those unacquainted with the general fact.
Black observed that different quantities of substances needed differing amounts of energy transferred to them to raise their temperatures by the same amount and coined the term specific heat. His research was instrumental in the separation of the concepts of ‘heat’ and ‘temperature’.
Black is also credited with discovering that, when heated, magnesium carbonate releases a gas, which he labelled ‘fixed air’ but was later renamed ‘carbon dioxide’ by Lavoisier.