1892 dictionary definition of primum frigidum. [3]

In science, primum frigidum was a hypothetical elementary substance, conceived by Greek philosopher Parmenides, which was postulated to be the source of all cooling or cold in the world. [1]

In 1627, Francis Bacon produced thinking to write about primum frigidum. Likewise, Pierre Gassendi reasoned that nitre was primum frigidum.

The first exhaustive study of the legitimacy of primum frigidum seems to have been done in 1665 by Robert Boyle. [2] From his researches, Boyle concluded that neither air, nor earth, nor nitre, nor water could be the principle of cold and that a primum frigidum was an “unwarrantable conceit.” [1]

References
1. Shachtman, Tom. (1999). Absolute Zero and the Conquest of Cold (pg. 28-29). Mariner Books.
2. Boyle, Robert. (1665). New Experiments and Observations Touching Cold. Publisher.
3. Stanford, John. (1892). The Stanford Dictionary of Anglicised Words and Phrases (pg. 651). University Press.