A frigorific mixture is a pile of chemical substances that will always stabilize to the same temperature, so it makes for a good reference point. In this case, if you give ammonium chloride, water, and ice a good stir, they will end up at 0°F. If you mix just water and ice, it will be 32°F, and the far less frigorific mixture of human blood (while still inside a healthy human) was 96°F (since redefined to 98.6°F). While these were Fahrenheit’s original reference points, the modern Fahrenheit scale has since been adjusted and is now pinned to water freezing at 32°F and boiling at 212°F.
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