This section is from the book "Botanic Drugs Their Materia Medica, Pharmacology and Therapeutics", by Thomas S. Blair. Also available from Amazon: Botanic Drugs, Their Materia Medica, Pharmacology and Therapeutics.
Exogonium purga. Universally official. Jalap and scammony yield rather similar resins which are complex mixtures. The really active portion of these resins may be an eleterin-like body which is highly irritating. The resin of jalap is nearly tasteless.
Jalap is marked by a definite tendency to promote intestinal secretion, and this gives it a place of value when the feces are hard and dry and when watery evacuations may aid in the treatment of dropsy, as in Bright's disease. The compound powder (jalap and potassium bitartrate, dose, 30 grains) is commonly used for this purpose. The resin, in 2-grain doses, and also in various pill formulae, is an efficient cathartic, used in the beginning of fevers and cerebral congestion. Remember the irritating properties of jalap. The fl. (not official) is given in doses of 5 to 20 minims.