What was the first refrigeration coolant used?

The first refrigeration coolant used was ether, specifically diethyl ether, in the early 19th century. It was followed by other substances like ammonia and carbon dioxide as refrigeration technology advanced. However, due to safety and efficiency concerns, these early coolants were gradually replaced by more effective and less hazardous refrigerants. The development of chlorofluorocarbons (CFCs) in the 1920s marked a significant advancement in refrigeration coolants, although they were later phased out due to environmental concerns.