The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS when he is on the track of a new compound. He may have actually got it before him in a solution or in the form of an impure oil which will not crystallise. In such a case, seeing none of the already formed solid is available, the only thing to do is to try mechanical methods of inducing crystallisation. If the reader has ever gone into a research laboratory for organic chemistry, he may have seen some one eagerly stirring and scratching at an oily- looking substance on a watch-glass. This is all with the object of persuading the substance to crystallise, and it is wonderful how frequently this method is effective. An excellent illustration of the way in which scratching promotes crystallisation is furnished by the behaviour of potassium bitartrate. This substance is found in grape juice, and is more familiar, especially to housewives, under the name of “ cream of tartar ”; it is only sparingly soluble in water. If a saturated solution is made at 90° or 100° Fahrenheit, and is then cooled, it will be super- saturated at the ordinary temperature. If the solution, as soon as it has cooled, is poured on a glass plate, and the plate is scratched with a glass rod in such a way that the latter writes invisible letters on the plate, the writing soon becomes visible, because specially rapid crystallisation is induced along the lines where the glass rod and plate were in contact The letters are traced out by the deposited crystals. When a salt crystallises out from its solution in water it frequently happens that it carries water along with it. In the act of crystallisation each molecule of the salt hooks on to itself one or more molecules of water. This is not a mere mechanical adherence, for the crystals may be removed from the solution, and pressed between blotting-paper until they are absolutely dry, without 318