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The popularity and influence of Mrs. Marcet's "Conversations on Chemistry" first published anonymously in 1806 (and subsequently in innumerable English, European, and American editions) can hardly be overestimated. It contributed much to the field of popular education, and not only for women to whom it was primarily directed. Michael Faraday, when a bookseller's errand boy, first became familiar with chemistry through reading the book.1 Furthermore, medical apprentices sometimes used it to begin their study of chemistry.2

Texts and Documents WILLIAM CULLEN: HIS CALIBRE AS A TEACHER, AND AN UNPUBLISHED INTRODUCTION TO HIS A TREATISE OF THE MATERIA MEDICA, LONDON, 1773. WHILE THE main purpose of this paper is to make readily available Cullen's introduction to his first (1761) Edinburgh materia medica course, some characteristics of Cullen's teaching, which was possibly the most significant in eighteenth-century British medical education, will also be considered. Cullen's 1761 materia medica course-omitting the introduction-was published in the early 1770s, initially without Cullen's sanction, as Lectures on the Materia Medica.1 That Cullen's greatly enlarged and monumental Treatise of the Materia Medica (Edinburgh, 1789) was an entirely rewritten version of the Lectures is perhaps the best testimony to his feelings of the inadequacy of the pirated version. Unfortunately, however, the rewritten 1789 Treatise had little of the spontaneity of the Lectures, which, in turn, lacked some of the sparkle characteristic of many manuscripts of students' notes. The main value of the unpublished introduction (see pp. 84-87) is that it sets out in detail Cullen's plan and aims for his course. In contrast, the printed Lectures merely list the four headings under which Cullen considered each drug, with a comment that knowledge of the subject is of two kinds (the study of the natural history and of the crude, dried drug) and brief critical remarks on theories for discovering the therapeutic properties of drugs (e.g., the doctrine of signatures). All these points are amplified and made clear in the unpublished introduction. For instance, one learns of Cullen's attitudes to Linnaeus' classification and how far he intended to use it. Cullen emphasized, too, the difficulties of establishing a generally accepted scheme of classification and drew attention to writings 'best calculated for assisting the student, tho' each follow a different method'. Apart from Linnaeus' botanical scheme he referred to Lewis's alphabetical arrangement,2 Geoffroy's grouping into mineral, vegetable, and animal,3 Cartheuser's classification based on sensory characteristics,4 and Chomel's pharmacological classification.5 Additionally, the introduction includes strong and trenchant criticisms of the views and opinions of various authors on the therapeutic properties of medicines, as well as Cullen's own thoughts on the value of chemistry. But the unpublished introduction does more than give a clear understanding of 1 Much information on the furore over the publication can be found in J. Thomson, An Account

Sm,-The letter by Vickers in the August issue of this Journal (1961) has stimulated us to report the isolation of five anthracene glycosides from senna since our report of the separation of an active primary glycoside (Fairbairn, Friedmann and Ryan, 1958).This has been isolated from Alexandrian senna pod and appears to be similar to that reported by Vickers. We have shown it to be a rhein-monoglucoside by chemical analysis. Ultra-violet absorption measurements indicate that the compound is identical with synthetic rhein-8-glucoside, prepared from authentic rhein anthrone-8-glucoside by aerial oxidation in sodium borate solution at pH 9.2, and differs from a sample of synthetic rhein-1-glucoside, kindly supplied by Dr. A. C. Bellaart. Our experience is that the glucoside isolated by us is not readily decomposed by sodium bicarbonate solution with the formation of rhein, which does not accord with the inference that can be drawn from Vickers' letter.Rhein-8-diglucoside. A second glycoside isolated has been shown to be rhein-8-diglucoside. This glycoside, is much more soluble in water than the 1or 8-monoglucosides. It can be converted into the 8-monoglucoside by treatment with 0 . 1~ hydrochloric acid at room temperature.Rhein anthrone-8-glucoside, which is virtually half a sennoside molecule, was also found by us to be present in senna pod. This has been identified by hydrolysis to rhein anthrone, which was characterised spectroscopically, and by the formation of a green compound (Amax 640 mp) with p-nitrosodimethylaniline in pyridine (Tsukida and Suzuki, 1954). The conversion of rhein anthrone-8-