By Eamonn Ryan
In the mid-18th century, the concept of refrigeration was largely confined to natural methods – storing ice harvested in winter or utilising cool underground spaces. This is Part 2 of a two-part series.
When the vacuum pump was activated, it rapidly removed air (and crucially, ether vapor) from the sealed vessel. This reduction in pressure above the ether dramatically lowered its boiling point. The ether began to evaporate much more quickly than it would at atmospheric pressure. As the ether rapidly changed from a liquid to a gas, it absorbed a significant amount of latent heat from its surroundings – including the water placed nearby. This rapid absorption of heat caused the temperature of the water to drop precipitously, eventually leading it to freeze into ice.
The significance of a scientific first
Cullen’s experiment was a purely scientific demonstration, not a practical appliance for everyday use. The vacuum pumps of the era were cumbersome, and maintaining a deep vacuum was challenging. However, its scientific significance was immense:
- Proof of concept: It definitively proved that artificial cold, sufficient to freeze water, could be generated by manipulating the principles of evaporation and pressure.
- Foundation for vapour-compression: This evaporative cooling principle is the very foundation of modern vapor-compression refrigeration, which underpins virtually all refrigerators, freezers, and air conditioners today. While Cullen used an open system (the vapor was simply pumped away), his work inspired later inventors to develop closed-loop systems where the refrigerant could be re-condensed and reused.
- Understanding of heat transfer: It contributed significantly to the evolving understanding of heat transfer and thermodynamics, paving the way for the work of later scientists like Carnot, Joule and Kelvin.
Legacy and lasting impact
While it would take another century for practical, commercial refrigeration systems to emerge (with figures like Jacob Perkins, John Gorrie, and Carl von Linde building upon these principles), William Cullen’s 1756 demonstration remains a landmark. He showed that cold was not merely the absence of heat, but a state that could be actively engineered. His pioneering work in creating artificial ice opened the door to a future where temperature control would revolutionize food preservation, medicine, and countless industrial processes, fundamentally altering human society and global trade. His quiet experiment in Glasgow was, in essence, the very first chilling whisper of the modern cold chain.