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Drop ice into a glass of water and the thermometer sits at 0 °C until the last piece melts, even though heat keeps flowing in from the room. Where does that heat go? In the 1760s Joseph Black, a chemistry lecturer in Glasgow, asked that question and answered it with two glasses, a quiet hall and a lot of patience. This post covers who he was, how he found latent heat, how to rework his numbers, and what his method can teach you.
Who was Joseph Black?
The main early account of his life is the preface by his former student John Robison to Black's Lectures on the Elements of Chemistry, published after Black's death. Robison writes that Black was born in France, on the banks of the Garonne, in 1728. His father, from a Scottish family settled in Belfast, was a wine merchant in Bordeaux.
Black studied medicine and in 1756 became professor of anatomy and lecturer in chemistry at the University of Glasgow. His essay on magnesia and quicklime, about what he called "fixed air", is the work that links his name with carbon dioxide, as the title of John West's 2015 history chapter records. In 1766 he moved to the chemistry chair at Edinburgh. He died in 1799.
How did Black discover latent heat?
Robison says Black started from things everyone had seen. A sunny winter day does not clear the hills of snow at once, and a frosty night does not suddenly cover a pond with thick ice. During a thaw, a thermometer sinks when pushed into melting snow. So melting snow must be taking in heat that does not show on a thermometer.
He wanted to measure it. There was then no ice-house in Glasgow, so he waited for winter. In December 1761 he made what Robison calls "the decisive experiment", in a large hall next to his college rooms. He found that the heat taken in by ice as it melted would have raised the same water "somewhat more than 140 degrees" on the Fahrenheit scale.
Because the thermometer gave no sign of this heat, Black called it concealed or latent heat. He read his account to a literary society of the university on 23 April 1762. He went on to estimate the much larger heat hidden in steam. Robison adds that James Watt, then fitting up instruments for the university, followed these ideas closely while experimenting with a model steam engine.
Can you rework Black's ice experiment?
Black describes the experiment himself in the Lectures (around pages 116 and 117 of volume 1). Here are his numbers.
Two thin glass globes, 4 inches across and nearly the same weight, each held 5 ounces of water.
One was frozen into ice in a mixture of snow and salt. Both were hung in a still hall at 47 °F.
The water glass, starting at 33 °F, warmed by 7 degrees to 40 °F in half an hour.
The ice glass took ten and a half hours to melt and reach 40 °F.
Step 1: compare heat. Both glasses took in heat at nearly the same rate, because the room and glasses were the same. Ten and a half hours is 21 half-hours, so the ice received 21 times as much heat as the water glass did in its half-hour.
Step 2: convert to degrees. That heat could have warmed liquid water by 7 × 21 = 147 degrees. Only about 8 degrees showed up, warming the melted ice from 32 to 40 °F. The remaining 139 or 140 degrees, Black wrote, "had been absorbed by the melting ice".
Step 3: convert to SI. A Fahrenheit degree is 5/9 of a kelvin, so 140 F° = 140 × 5/9 = 77.8 K. With water's specific heat of 4.186 J/(g K), the latent heat is 77.8 × 4.186 ≈ 326 J/g.
The NCERT Class 11 thermal properties chapter gives the latent heat of fusion of ice as 3.33 × 10⁵ J/kg, or 333 J/g. Black was within about 2%.
His steam estimate was rougher. Robison reports that Black found it took about five times as long on the same fire to boil water away as to heat it from about 50 °F to boiling, and inferred that the steam carried off about 810 degrees. That is 810 × 5/9 × 4.186 ≈ 1,880 J/g, against NCERT's 22.6 × 10⁵ J/kg, or 2,260 J/g. Latent heat and calorimetry are both in the JEE (Main) 2026 syllabus.
Common mistakes:
Adding 32 to a temperature difference. Convert a difference in degrees with 5/9 alone. The +32 applies only to actual readings.
Forgetting the warming after melting. Black subtracted the 8 degrees that raised the melted water from 32 to 40 °F. In mixing problems, list every stage separately.
Assuming equal heating without checking. Black's method only works because both glasses gained heat at the same rate. Ask what makes that true before you use it.
For more on mixing and melting problems, see our guide to calorimetry and heat transfer.
What can a student learn from Black's way of working?
Start from what everyone has seen. Slow thaws and slow freezes were familiar to everyone. Black was the first, Robison claims, to ask what they implied.
Measure by comparison. He had no way to measure heat directly, so he compared one glass with another under the same conditions.
Wait for the right conditions. He held the idea through a whole summer until there was ice to test it.
Choose words carefully. "Latent" said exactly what he meant, and Robison notes that people who missed this thought latent heat was a different kind of heat.
If you get stuck on which stage of a heating curve a question is about, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can go back to the idea of a phase change before the next problem.
Try this tonight: rework Black's experiment assuming the ice had taken 12 hours to melt, and see what latent heat it would have given.
