From Benjamin Franklin to Benjamin Vaughan, 29 April 1784
To Benjamin Vaughan
Reprinted from William Temple Franklin, ed., Memoirs of the Life and Writings of Benjamin Franklin … (3 vols., 4to, London, 1817–18), III, 466.
Passy, April 29, 1784.
My dear Friend,
I received your kind letters of the 16th and 20th instant.7 I thank you for your philosophical news. We have none here. I see your philosophers are in the way of finding out at last what fire is. I have long been of opinion that it exists every where in the state of a subtile fluid.8 That too much of that fluid in our flesh gives us the sensation we call heat; too little, cold. Its vibrations, light. That all solid or fluid substances which are inflammable have been composed of it; their dissolution in returning to their original fluid state, we call fire. This subtile fluid is attracted by plants and animals in their growth, and consolidated. Is attracted by other substances, thermometers, &c. &c., variously; has a particular affinity with water, and will quit many other bodies to attach itself to water, and go off with it in evaporation. Adieu. Yours most sincerely,
B. F.
7. Not found. One of them probably enclosed the “observations on cold by Professor Wilson” that Percival and BF subsequently discussed; see their letters of May 11 and July 17. The astronomer Patrick Wilson (XIX, 228), curious about the temperature differential between snow and the air above it, had tested the cooling properties of various materials (snow, sand, charcoal, metal, etc.) when subjected to the extreme cold of the nights in December and January. His rambling paper, dated Feb. 14, 1784, was titled “Experiments and Observations upon a Remarkable Cold which accompanies the Separation of Hoar-Frost from a Clear Air.” It had not as yet found a publisher. The chemist Joseph Black (XXVII, 464), whose theories of heat underpinned Wilson’s work, forwarded the paper to the Royal Society of Edinburgh, where it was read on July 5. The society published it in its inaugural volume of transactions, which would not appear for another four years: Royal Society of Edinburgh Trans., I (1788), 146–77.
8. For BF’s earlier ideas about fire and heat see VII, 184–90; X, 49–50; see also XXXVIII, 127. He developed these ideas further in “Loose Thoughts on a Universal Fluid,” June 25, below.