NEW LIGHT ON ROBERT BOYLE AND THE DISCOVERY OF COLOUR INDICATORS

Abstract

ROBERTBoyle's Experimental History of Colours (1664) is rightly considered a landmark in the history of analytical chemistry. In that work, Boyle described a number of simple but elegant experiments which ultimately gave chemists a powerful tool for determining the composition of compounds. He found, for example, that the common acids turned the blue syrup of violets red, and that alkalis turned it green. Subsequent experiments with other vegetable juices, including privet berries (Ligustrunt v.ulgare), buckthorn berries, (Rhamnus cathartica), cornflowers (Centauris cyanis), and turns ole (Crozophora tinctoria), revealed similar colour changes when treated with acids and alkalis.1 Although some of these reactions had been noticed earlier, Boyle was the first to recognize the significance of these substances as chemical reagents, for he alone observed that all acids turned such blue vegetable juices red, and that all alkalis turned them green. Furthermore, he noticed that some substances caused no colour change; these Boyle classified as neutral, neither acid nor alkaline, and thus effectively disposed of the theory that all substances were one or the other.2 Recent scholarship has shown that an extensive literature on the analysis of aqueous solutions existed prior to Boyle's work, notably in the writings of physicians on the medicinal virtues of spa waters. Professor Allen Debus, for example, pointed out that Edward Jorden (1569-1632), an English physician trained at Padua, observed that bases turned scarlet cloth blue, while acids turned it red again; the colour change of tincture of violets and of rose leaves in the presence of oil of vitriol was also noticed in the early seventeenth century.3 Curiously, these writings are not mentioned by Boyle, and as Debus suggests, they do not seem to have made any noticeable contribution to Boyle's work on colour indicators.4 Another possible inspiration for Boyle's researches, however, has not thus far been seriously considered, and that was the empirical information accumulated by dyers and painters through generations of experimenting with colour changes. That Boyle should have observed these techniques in the workshops of dyers and colour-makers is entirely consistent with his professed adherence to Baconian philosophy. Indeed, if the scientific revolution of the seventeenth century was a matter in part of thinking in new ways about older data, Boyle's methodology in this instance is a remarkable example of it. It is not generally known that most of the colour changes that Boyle observed were noticed, and were induced methodically, long before anyone thought of using them as chemical indicators. Medieval dyers and painters knew empirically that plants could be made to yield a wide range of colours, depending on the season of the year in which they were collected and the mordants that were used with them. Unripe buckthorn berries, for example, yielded a fair but fugitive yellow that was used by painters to enrich greens, by leather dressers, and by bookbinders to colour the edges of books.5 If the berries were