That dream is over. Tuberculin skin reactions have now been made by the million and we know that a positive reaction means nothing but that, at some time, the patient has been infected with tuberculosis. The skin test does not tell us whether he has recovered long ago and built up a good defense or whether he is still sick with tuberculosis and will die of it. It reacts equally well in the healthy, vigorous subject who at one time has had a mild tuberculosis and recovered, in the patient with early phthisis and in the advanced case. In Kraus and Levaditi's _Handbuch der Technik und Methodik der Immunitätslehre_, 1911, page 205, von Pirquet himself, the grandfather of all the skin tests, says, "A positive skin reaction indicates with certainty that the organism has been infected with tuberculosis. Of the localization, extent and prognosis of tubercular infection, a positive skin reaction gives no conclusion." Yet hundreds of physicians to-day are making diagnoses of tuberculosis by the skin test; for if there is one thing more difficult than to get a new idea into a doctor's head, it is getting it out again when the idea proves fallacious. So, I view these skin reactions for food and pollens with some suspicion of their real value in diagnosis and prognosis and as guides to treatment. Still, Talbot says, "Experience has shown that when a positive skin test is obtained for a food and that food is then excluded from the diet, the general condition of the patient almost invariably improves and in many instances a cure results." May his words prove true. CHAPTER XIV _ROSIN-WEED AGAIN HISTORICAL AND PHARMACOLOGICAL_ When we wish to learn anything about American medical literature, we turn to the big Index Catalogue of the Library of the Surgeon-General. The botanical name of rosin-weed is _silphium_. In the Index Catalogue, the word _rosin-weed_ does not appear, but, in the first series, under _silphium_, there are ten references, and thereby hangs a tale. =Ancient Silphium.= In ancient Greek and Roman medicine there was a famous gum called _silphion_ (Latin _silphium_) which, like all popular medicines, was the better for being brought from a far country and for being a little mysterious; for it was brought across the Mediterranean from Cyrene, where it had been originally presented to the inhabitants of that favored place by the gods. Learned botanists have discussed at length what plant produced this gum and have concluded that, like its neighbor in Egypt, the papyrus plant, it has disappeared from the earth. Even in Dioscorides' time the plant was getting scarce and there came a day when in all Cyrene there remained only a single silphium plant, which was piously presented to that worthy representative of the gods, the emperor Nero.