6.3.3 Specific heats
Finally, we must consider Black's contribution to the discovery of specific heats, the fact that different substances take up heat at different rates. Two experiments on mercury and water had indicated the problem. Fahrenheit had found that mixing equal volumes of mercury and water produced a striking result. If the mercury was initially hotter than the water, the temperature of the mixture was less than the average, and the reverse was true if the water was originally hotter. Martine's experiment, which we have met in connection with the latent heat of ice, shed more light on this matter. When two glasses, one containing water and the other an equal volume of mercury, were placed in front of a steady fire, the temperature of the mercury rose twice as rapidly as the water.
Black was able to solve these riddles. Mercury clearly had a lower capacity for heat than water, and hence it heated up (and cooled down) more rapidly. As he never published his conclusions, we know very little about his thinking on this question, but he may have arrived at this solution because he regarded the absorption of heat as a chemical process, and hence a function of chemical composition, rather than density, or bulk (as Boerhaave had suggested).