Rock-salt is rarely found in an absolutely pure anhydrous state, in which it is colourless and perfectly transparent. In most rock-salt mines such specimens are regarded as curiosities, but in the deposits of Nevada and of Wieliezka, in Hungary (where the salt, containing 100 per cent. NaCl, is the purest in the world), large masses of quite transparent salt are encountered. The white opaque mass which the ordinary person is accustomed to think of as rock-salt, is the purified product of commerce. The colour of sea-water is affected by its percentage of salt, the colour changing from blue to green as the quantity of salt decreases; but sea-salt is generally white, although not transparent owing to the presence of minute particles of water, air, etc., in its intercrystalline spaces. But rock-salt is never more than whitish inclining to grey, and, as a general rule, it is coloured by earth or mineral impurities. The Salt Range in the Panjab yields a substance that varies from pink to red, according to the different quantities of iron present as impurities. That found at Marston, in Cheshire, varies from yellow to red and brownish-red in colour. Small blocks of transparent salt of a deep sapphire blue are occasionally found in the Wieliezka mines. The colour disappears on heating, and when the salt is ground to powder. It is attributed by some chemists to the presence of subchloride of sodium, by others to the presence of thin cavities having parallel surfaces with gas inclusions. Common salt, which is classed as “sweet” to distinguish it from the bitter-tasting salts of magnesium, has a peculiar saline taste which gains in pungency with refinement, and in its pure state is odourless. In solution, the smallest quantity perceptible to the taste is about 15 grains to the litre, roughly, 68 grains to the gallon. Common salt is highly soluble in cold water, and rather more so in hot water, but while it dissolves slightly in alcohol, neither ether nor oil has any effect upon it. One hundred parts of distilled water at 60° F. (15·5° C.) will dissolve 35·9 parts of chemically pure NaCl. A saturated solution of common salt, therefore, contains 26·42 per cent. NaCl. The increase of solubility of NaCl in proportion to the rise in temperature, calculated by Gay Lussac and Poggiale, is particularly marked between 100 deg. and 110 deg., when boiling point is passed, the increase amounting to ·74 parts of 10 deg., as compared with an increase of one 1·09 parts between freezing and boiling points. In a double solution of NaCl and some other more soluble salt, as sodium or magnesium sulphate or magnesium chloride, the solubility of sodium chloride is very greatly reduced.