Syrup of violets

I haven t devoted much space to discussions of Boyle s experimental work, because his theoretical ideas influenced chemistry much more, so perhaps it won t hurt to mention a few of his experiments here. He investigated the nature of acids and alkalis and discovered an indicator that allowed one to tell whether a solution was acidic or alkaline. This indicator was a liquid known as syrup of violets, which was prepared by boiling violet petals in water and adding sugar. Boyle found that acids turned the syrup red and alkalis turned it green. 

As Mr Blandy s health was deteriorating, a doctor was summoned. He was of the opinion that the patient was being poisoned and that Mary s position would be serious if he were to die. The contents of the soup pan and the white powder were examined by another doctor. Dr Addington, who carried out a series of tests and compared the results with those of a known sample of white arsenic. These tests were very thorough and included adding the powder to water, sprinkling it on red-hot iron, and adding various other substances such as syrup of violets and spirit of vitriol to a solution of the sediment. The doctor observed an exact similitude between the results with the sediment and the known sample of arsenic oxide. 

In Lemery s time the acid-alkah hypothesis was widely accepted and only changed after Robert Boyle argued that some substances are neither acid nor alkali, but are neutral. In order to show the presence of acids or alkalis he developed a colour indication test, which means that the presence of effervescence upon combining an acid and an alkali was no longer sufficient to prove the nature of a particular substance. Boyle showed that the combination of a particular substance with syrup of violets turns red if it is an acid and blue or green if it is an alkali. By the time Boerhaave wrote his l kmenta Chemiae, Boyle s ideas were widely accepted and Boerhaave strongly encouraged his students to read Boyle on the subject. 

Referring to acids and alkalis in order to understand chemical reactions was one thing, but thinking of acids and alkalis as basic to matter, or as principles of matter, was clearly another. Hermann Boerhaave accepted the former application and knew of Robert Boyle s technique of identifying acids, alkalis, and neutral substances by observing changes in color when a substance was dipped onto a little syrup of violets spread on white paper. However, in a work called A Short Recapitulation of Acid and Alkali that became part of his Elements of Chemistry, he noted how thinking of... 

Syrup of violets Violet scent mixed with syrup. 

It is also plain from his ample citations that Plot saw himself as part of a great Oxford chemical tradition, mentioning as he does Hooke, Mayow, Willis, and the Honorable Robert Boyle Esq. the Glory of his Nation and Pride of his Family , who had, in his own study of Oxfordshire waters, used Syrup of Violets as an early acid-alkali indicator. Then in November 1689, upon marrying, Robert Plot retired from the Chemistry Chair and left Oxford. He was succeeded to the post by Sir Edward Hannes, a medical man, who like so many seventeenth-century Oxford scientists (and most notably Robert Hooke) had followed the road from Westminster School to Christ Church. ... 

The name alcaloide is used by Dumas as an alternative to the alcalis veg taux of Berzelius. C. L. Derosne on diluting an evaporated extract of opium obtained a crystalline precipitate, which he recrystallised from alcohol. He found that the extract of opium is precipitated by alkali and the crystalline precipitate, even after washing, turned syrup of violets green, which he supposed was due to residual alkali, and since the first preparation did not he regarded it as a salt-like substance. Derosne concluded that opium contains a peculiar acid salt . He showed that the preparation had the same narcotic properties as a larger quantity of opium. 

Figure 14.2 Descroizilles s alkalimeter of 1806. Acid was poured from this tube into an alkaline solution until neutralisation occurred, as indicated by the colour change of syrup of violets. The flow of acid was controlled by restricting the rate at which air entered through the hole (marked b) by means of a finger. This piece of apparatus was the forerunner of the burette...

Carboxylic acids occur widely in nature and were among the first compounds isolated—both because of their acidity and their stability. It was common knowledge as early as the middle of the seventeenth century that a blue vegetable material called syrup of violets turned red with acids—an early working definition of something called an acid (vide infra). 

A solution of tartaric acid added to one of syrup of violets turns the blue colour of the latter to red a solution of caustic soda turns it green and a solution of alum turns it purple. 

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