Pictet's experiment

Pictet's experiment is the demonstration of the reflection of heat and the apparent reflection of cold in a series of experiments[1] performed in 1790 (reported in English in 1791 in An Essay on Fire[2]) by Marc-Auguste Pictet—ten years before the discovery of infrared heating of the Earth by the Sun.

This was sometimes demonstrated with the explosion of a flammable mix of gasses in a blackened balloon, as described and depicted by John Tyndall in 1863.

This demonstration was important to Benjamin Thompson, Count Rumford who argued for the existence of "frigorific rays" conveying cold.

[9] The apparent reflection of cold if a cold object is placed in one focus surprised Pictet[10] and two scholars writing about the experiment in 1985 noted "most physicists, on seeing it demonstrated for the first time, find it surprising and even puzzling.

Physicists at Sofia University in Bulgaria reported on reproducing the experiment for high school students in 2017.