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.
The parasitic growths upon which these unsightly maladies depend are destroyed by lotions containing 1 or 2 gr. of corrosive sublimate in the ounce, which should be applied once or twice daily after cleansing: ointments containing the same, or the ammonio-chloride, are also useful. Their curative effect, like that of all similar remedies, is dependent somewhat on the state of the general health in ringworm of the scalp, and in favus, but in ordinary ringworm of the body (tinea circinata), and in pityriasis versicolor, a few applications will suffice for cure.
Dr. Alder Smith has recommended the oleates of mercury as having more penetrating power, and records their proving curative in chronic and obstinate cases not amenable to lotions, blisters, etc.: for children under eight he uses a strength of 5 per cent., and, for others who can bear it, 10 per cent., mixed with acetic ether, 1 part to 7; after cutting the hair close, thorough washing, and drying, this is rubbed into the whole scalp regularly night and morning, a cap or turban being worn to keep any of the preparation off the face: it is important that the head should not be washed more than once a fortnight. Mercurial remedies should not be used too concentrated, or over too large a surface, for fear of producing severe constitutional effects; and it is well to remember that blistering increases the absorptive power of the skin (New York Medical Journal, July, 1858). Under the heading of "absorption" we have mentioned cases in which death followed inunction of the scalp for ringworm, and would refer again to one in which a single painting with a vesicating solution of sublimate (gr. x. ad3i.), caused salivation and death from mercurial poisoning (British Medical Journal, 1871). I have myself seen a case in which death resulted from the local use of a strong sublimate ointment, and more than one case in which serious symptoms resulted.