The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS curious behaviour is available, for washing-soda, which normally contains ten molecules of water of crystallisation to each molecule of sodium carbonate, loses some of them on mere exposure to the air. This process is revealed to the observer by the fact that the crystals of washing-soda, originally clear and transparent, become gradually opaque, as if a white powder had been deposited on them. This is due simply to the partial removal of water from the surface layer of the crystals, which therefore exhibit signs of disintegration. In blue vitriol an instance has already been cited of the way in which the colour of a given salt varies according to the number of molecules of water of crystallisation which it contains. An even more striking example of this phenomenon is furnished by a substance known to chemists as magnesium platinocyanide. This salt can be obtained with seven, six, or two molecules of water, as well as in the anhydrous state, and these various products are respectively scarlet, lemon-yellow, colourless, and orange-yellow. The extraordinary influence which water thus has in altering the colour of a salt explains the action of the so-called “ sympathetic n or “ invisible 11 inks. One of these is a solution of cobalt chloride, a salt which crystallises with six molecules of water in the form of dark red crystals ; the anhydrous salt, on the other hand, is deep blue in colour. The water solution of cobalt chloride is merely pink, and if this is used to write on paper instead of ordinary ink, the impression left is so slight as to be scarcely noticeable, even when it has dried. The application of heat to the paper, however, makes the writing immediately visible, for the cobalt chloride is thereby converted into the blue anhydrous salt. Curiously 320