Note 46 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE
George Kamensky translation
According to Roscoe the specific gravity of perchloric acid = 1·782 and of the hydrate HClO4,H2O in a liquid state (50°) 1·811; hence a considerable contraction takes place in the combination of HClO4 with H2O.