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.
Opinions have differed as to the possibility of preventing venereal infection by applications of nitrate to the sore soon after its appearance; authorities in favor of such practice are to be found among earlier writers, but modern opinion is decidedly against it. Hunter, Ricord, and Acton (writing in 1846) agree in stating that if the commencing chancre, the vesicle or pustule be thoroughly cauterized within three to five days of its origin the cure is rapid, and systemic infection very rare; but they agree also that if the sore be indurated no effect is produced, so that some of the cases they relied upon were probably "soft and non-infecting chancre:" on the other hand, Diday, Langston Parker, and others, have thus destroyed chancres within a few hours of their appearance, and yet an indurated sore and secondary symptoms have followed. We must conclude that cauterization of a true Hunterian chancre at any stage will not prevent its development or the occurrence of secondary symptoms.
Early cauterization of soft chancres will, however, sometimes cause rapid healing, and is a good treatment for sloughing or rapid spreading; but it is very painful, and the sore will usually heal under simple treatment. In syphilitic ulcers of the leg I have seen solid gelatinous fungating growths, which are well treated locally by pushing in a point of caustic and breaking them down freely with it, as already described under lupus. For syphilitic cracks, fissures, and ulcers on tongue and cheeks, the solid nitrate applied daily is very useful.