"Darwin was also experimenting with gases and asked Boulton to make him a flask with a twisted neck. ...he hoped to elucidate the properties of air... In modern jargon, Darwin is saying that the density, ρ, of the air should be proportional to the pressure, p, and inversely proportional to the temperature T. In symbols, this gives ρ = constant ✕ p/T, which is precisely the , now written p/ρ = RT, where R is constant and T is the absolute temperature. This was in 1763, 30 years before the law’s official discovery, and is a good example of his insight in physical science. On this occasion it is thrown off in a letter, not a formal paper"