This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
In simple recent cases of ringworm affecting the body or the scalp, a few applications of iodine tincture or liniment may suffice to cure, but they seldom succeed in an aggravated case. The preparation introduced by Mr. Coster ("Coster's paste") is, however, more powerful. It is a solution of 1 part of pure iodine in 4 of "colorless oil of tar," and requires to be mixed carefully, for heat is developed during the combination; the resulting thick, dark-colored liquid should be thoroughly painted over the affected part and allowed to form a crust, which may remain for seven to ten days. One or two such applications will often cure, but to say that they do not cause pain is a mistake. The pain has seemed to me about equal to that caused by the iodine liniment, which is sometimes severe.
Iodine and the iodides have a similar action; the former is more stimulant to the general system, but more irritant to the gastric mucous membrane. It is probably better adapted for slowly modifying the general constitutional state, as, for instance, in struma; while the alkaline iodides, being more quickly passed out of the system, act better where some foreign material needs elimination, e.g., in syphilis, lead-poisoning, or rheumatism. Practically, however, the much less irritant effects of the alkaline compounds indicate, independently of other considerations, their employment in the majority of cases for which iodine in any form is needed.