Note 54 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

Mallet (1881) determined the density at 30° and 100°, previous to which Gore (1869) had determined the vapour density at 100°, whilst Thorpe and Hambly (1888) made fourteen determinations between 26° and 88°, and showed that within this limit of temperature the density gradually diminishes, just like the vapour of acetic acid, nitrogen dioxide, and others. The tendency of HF to polymerise into H2F2 is probably connected with the property of many fluorides of forming acid salts—for example, KHF2 and H2SiF6. We saw above that HCl has the same property (forming, for instance, H2PtCl6, &c., p. 457), and hence this property of hydrofluoric acid does not stand isolated from the properties of the other halogens.