[41] The formation of the compound SnCl_{4},3H_{2}O is accompanied by so great a contraction that these crystals, although they contain water, are heavier than the anhydrous chloride SnCl_{4}. The penta-hydrated crystallo-hydrate absorbs dry hydrochloric acid, and gives a liquid of specific gravity 1·971, which at 0° yields crystals of the compound SnCl_{4},2HCl,6H_{2}O (it corresponds with the similar platinum compound), which melt at 20° into a liquid of specific gravity 1·925 (Engel). Stannic chloride combines with ammonia (SnCl_{4},4NH_{3}), hydrocyanic acid, phosphoretted hydrogen, phosphorus pentachloride (SnCl_{4},PCl_{5}), nitrous anhydride and its chloranhydride (SnCl_{4},N_{2}O_{3} and SnCl_{4},2NOCl), and with metallic chlorides (for example, K_{2}SnCl_{6}, (NH_{4})_{2}SnCl_{6}, &c.) In general, a highly-developed faculty for combination is observed in it. Tin does not combine directly with iodine, but if its filings be heated in a closed tube with a solution of iodine in carbon bisulphide, it forms stannic iodide, SnI_{4}, in the form of red octahedra which fuse at 142° and volatilise at 295°. The fluorine compounds of tin have a special interest in the history of chemistry, because they give a series of double salts which are isomorphous with the salts of hydrofluosilicic acid, SiR_{2}F_{6}, and this fact served to confirm the formula SiO_{2} for silica, as the formula SnO_{2} was indubitable. Although _stannic fluoride_, SnF_{4}, is almost unknown in the free state, its corresponding double salts are very easily formed by the action of hydrofluoric acid on alkaline solutions of stannic oxide; thus, for example, a crystalline salt of the composition SnK_{2}F_{6},H_{2}O is obtained by dissolving stannic oxide in potassium hydroxide and then adding hydrofluoric acid to the solution. The barium salt, SnBaF_{6},3H_{2}O, is sparingly soluble like its corresponding silicofluoride. The more soluble salt of strontium, SnSrF_{6},2H_{2}O, crystallises very well, and is therefore more important for the purposes of research; it is isomorphous with the corresponding salt of silicon (and titanium); the magnesium salt contains 6H_{2}O.