How did Joseph Black’s experiments with ice lead to the idea of latent heat?

Joseph Black’s discovery of latent heat emerged from his investigation into why substances like ice do not melt instantly when they reach their freezing point, and why water does not boil away immediately at its boiling point.1

To explore this, Black conducted an experiment using two similar glass flasks containing equal amounts of water. He added a small amount of alcohol to one flask to prevent it from freezing, then placed both in a freezing mixture. Once one was frozen and the other remained liquid—despite both being at the same temperature—he allowed them to warm up naturally.1

Black observed that while the temperature of the liquid water rose steadily, the ice remained at its freezing point for a significant duration as it melted. By calculating that the heat absorbed by the ice over 10 hours would have raised the temperature of an equal quantity of water by 78 °C (140 °F), he demonstrated that heat was being absorbed without a change in temperature. This "hidden" energy, which he termed latent heat, is required to change a substance's physical state.21