The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS water, and then immersing the flask in hot water. This treatment renders the contents of the flask fluid, and they remain in this condition even when cooled to the ordinary temperature. The solution is then supersatu- rated, and will deposit crystals only when it is irritated in some manner. This may be done by vigorous shaking or stirring, but most certainly by dropping in a crystal of sodium thiosulphate itself. This operation is described as “ inoculation ” or “ sowing,*’ and it is certainly a sowing which produces an immediate harvest. The presence of an already formed crystal acts as a stimulus to the molecules which have sluggishly lingered in the dissolved condition, and they hasten to arrange themselves in the regular manner which is characteristic of the crystalline state. One result of this is that the contents of the flask, formerly fluid, appear to have become nearly solid, and another obvious fact is a con- siderable rise in temperature. This evolution of heat which accompanies the crystal- lisation of a supersaturated solution is not to be wondered at; it is simply the repayment of a loan. For most salts absorb heat when they pass from the solid to the dissolved condition—a fact which any one can realise by putting a quantity of saltpetre in water and observing that the vessel containing the water becomes sensibly colder. This heat which the salt abstracts from the water and the containing vessel when it passes into solution, is duly returned by it when it comes out of solution; hence the remarkable evolution of heat when a supersaturated solution is suddenly stimulated into crystallisation. Another substance which resembles sodium thiosulphate in readily forming supersaturated solutions is acetate of 316