E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
St. John Long.
An illiterate quack, who professed to have discovered a liniment which had the power of distinguishing between disease and health. The body was rubbed with it, and if irritation appeared it announced secret disease, which the quack undertook to cure. He was twice tried for manslaughter: once in 1830, when he was fined for his treatment of Miss Cashan, who died; and next in 1831, for the death of Mrs. Lloyd. Being acquitted, he was driven in triumph from the Old Bailey in a noblemans carriage, amid the congratulations of the aristocracy.
St. John is pronounced Sinjin, as in that verse of Popes