The Romance of Modern Chemistry

ACIDS AND ALKALIS are brought together by throwing down and breaking the glass vessel in which they are contained. The reader is doubtless aware that much of our building material consists of limestone, the chief con- stituent of which is carbonate of lime. Bath stone and dolomite, for example, are affected by acids in exactly the same way as ordinary carbonates, and inasmuch as the air in our large towns contains some acid constituents, derived mostly from the sulphur in coal, calcareous or chalky stones like these are liable to disintegration. The Houses of Parliament and York Minster furnish examples of the way in which a calcareous building stone decays under the influence, amongst other factors, of the acid constituents of the atmosphere. Bearing in mind another general characteristic of acids, we have a very simple clue to a conjuring trick which seems marvellous to the uninitiated. It is found that certain vegetable products assume one definite colour in the presence of acids, and another colour in the presence of alkalis, which, as we shall see presently, are the exact opposites of acids in many respects. A solution of litmus, for example, is turned red by acids, and blue by alkalis, while a solution of phenol phthalein is colour- less in the presence of acids, and intensely red in the presence of alkalis. These substances are called in- dicators, and are of the greatest use in chemical work, because they enable the chemist so to neutralise any solution that it is neither acid nor alkaline. In the conjuring trick a series of glasses are rinsed out alter- nately with acid and alkali, and then water containing some phenol phthalein is poured from the first glass into the second, from the second into the third, and so on. What the spectator sees and marvels at is a 87