Joseph Black

Whose Thermometer Would Not Rise · Chemist & Physician · 1728–1799

Who noticed that a thermometer stuck in melting ice does not move for hours while heat pours in — and concluded that heat and temperature are two different things, which is the foundation of the whole subject.

Who was Black?

Joseph Black was born at Bordeaux in 1728 to a Scots-Irish wine merchant, educated at Glasgow and Edinburgh, and held chairs of chemistry at Glasgow and then Edinburgh, where he was one of the great lecturers of the age. His medical thesis identified fixed air — carbon dioxide — and distinguished the mild from the caustic alkalis, work that founded quantitative chemistry. His work on heat came from taking a thermometer seriously in two situations where it behaves oddly. First, he found that equal masses of different substances placed over the same fire do not warm at the same rate, so that each substance has its own capacity for heat, and the quantity of heat in a body cannot be read off its thermometer. Second, and this took him years to accept, he found that a pound of ice standing in a warm room takes many hours to melt while the thermometer buried in it stands unmoving at the freezing point the whole time. Heat is entering continuously and the temperature does not rise; so the heat is being absorbed and concealed — latent, as he named it — and spent on changing the state rather than the temperature. The same holds at the boil: water at a rolling boil goes on taking heat and stays at the same temperature until the last of it has gone. James Watt was the instrument-maker at Glasgow and Black's friend, and it was this understanding of latent heat that showed him what the Newcomen engine was wasting and led to the separate condenser. Black explained heat as a subtle fluid, the caloric, which was the account of his century and is wrong.

Major achievements

• Distinguished the quantity of heat from the temperature — the foundation of thermal physics
• Discovered that different substances have different capacities for heat
• Discovered latent heat, of fusion and of vaporisation, and measured it
• Identified fixed air (carbon dioxide) and the difference between mild and caustic alkalis
• Developed the method of mixtures for measuring heat quantities

Life in brief

• 1728 — Born at Bordeaux: To a Scots-Irish wine merchant.
• 1754 — The thesis on fixed air: Quantitative chemistry begins.
• 1757 — Professor at Glasgow: And the beginning of the heat experiments.
• 1761 — Latent heat: The ice that melts while the thermometer stands still.
• 1765 — Watt's separate condenser: Black's doctrine put to work.
• 1766 — Professor at Edinburgh: Thirty years of lectures to packed rooms.
• 1799 — Dies at Edinburgh: The caloric he taught has fifty years left.

Explore the life of Black — five chapters

1. Fixed Air — A medical thesis that founded quantitative chemistry
2. Not All Substances Warm at the Same Rate — Heat capacity, and one word doing the work of two
3. The Ice That Would Not Warm Up — Latent heat, and a result he resisted for years
4. What Watt Did With It — The separate condenser, and the industrial world
5. A Fluid That Does Not Exist — Thirty years of lectures, and a theory with fifty years left

Read the five-chapter life of Joseph Black →

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