William Cullen and the Experiment Behind Artificial Refrigeration

Scottish science · Understanding heat and cold

William Cullen’s experiments showed how evaporation under reduced pressure could produce cooling. This was an important step in the history of artificial refrigeration, rather than the invention of a ready-to-use household refrigerator. The distinction lets us see what was remarkable about the experiment itself.

A refrigerator does not make a substance called “cold”. It moves heat away from the things inside. Cullen’s work belongs near the beginning of the scientific story behind that process: changing a liquid into a vapour requires energy, and that energy can be taken from the liquid’s surroundings.

Who was William Cullen?

Cullen was born in Hamilton in 1710 and trained in medicine. His career crossed the boundaries that now separate chemistry, physics and clinical teaching. At Glasgow he taught chemistry and medicine, holding the Chair of Medicine from 1751. He moved to Edinburgh in 1755 and became one of the influential medical teachers of his generation.

That setting matters. The early work was a laboratory investigation into heat and materials, not a proposal for a domestic appliance. Thermometers, air pumps and carefully observed changes allowed researchers to ask questions that an icehouse or a naturally cool cellar could not answer in the same controlled way.

When did Cullen demonstrate refrigeration?

The surviving institutional accounts do not give one consistent date. The University of Glasgow’s biography dates an early demonstration there to 1748. David Banks’s scholarly history of refrigeration and heat-pump pioneers describes a device in 1756, following Cullen’s move to Edinburgh.

A precise-sounding date is not an improvement if it hides a source disagreement. Cullen’s place in the development of refrigeration does not depend on choosing one unsupported anniversary.

How could evaporation make something colder?

When liquid becomes vapour, it needs energy to make that change of state. If the energy comes from the liquid and the objects around it, their temperature can fall. Reducing the pressure also allows a volatile liquid to boil at a lower temperature than it would at normal atmospheric pressure.

This helps answer a common puzzle: a liquid can be boiling without being hot enough to scald. Boiling temperature depends on pressure and on the substance. The familiar temperature of boiling water in an open kitchen pot is one particular set of conditions, not a universal rule for every liquid.

What was still missing before the household fridge?

A brief cooling effect is different from a machine that keeps removing heat hour after hour. Later engineers had to develop practical cycles, suitable working fluids, reliable equipment and ways to release the extracted heat somewhere else. A household refrigerator also needs an insulated cabinet and controls that respond to its contents and surroundings.

In a conventional vapour-compression refrigerator, refrigerant takes in heat as it evaporates on the cold side. The system then raises its pressure and transfers heat to the room as the refrigerant condenses. Cullen’s experiment illustrates an important physical principle in that cycle, but he did not build the whole modern system.

Nor does every modern cooler use evaporation. Thermoelectric devices can move heat using electricity and solid-state components. Research documented by the US Department of Energy explores both thermoelectric cooling and its combination with conventional refrigeration. The broad legacy is an understanding of heat transfer, not one unchanged mechanism inside every appliance.

A Scottish connection between heat and cold

Cullen makes a useful companion to James Watt and the separate condenser. One story concerns using a phase change to cool; the other concerns avoiding repeated heat loss inside an engine. Later, James Dewar’s vacuum flask tackled a different problem: slowing the arrival of unwanted heat.

Explore those connections in our Scottish inventions and discoveries guide. For a personal reminder of the places behind Scottish history, our Scottish location T-shirts offer a route from the story to a meaningful place. You can also search the store for your own surname.

Sources and further reading

• University of Glasgow: William Cullen — his Glasgow and Edinburgh career, and the university’s 1748 demonstration date.
• David Banks: Scottish pioneers of tools for low-temperature geothermal applications (2016) — scholarly account of Cullen’s experiment, the 1756 date and its thermodynamic context.
• US Department of Energy: Thermoelectric and vapour-compression refrigeration — why modern cooling cannot all be described as evaporation-based.