To-day we begin the study of the chemical elements termed halo- gens; Iodine, Bromine, Fluorine and Chlorine are the elements in this group. As a group the halogens may be remembered by this great characteristic symptom, they all act upon the larynx and bronchial tubes, and in fact upon mucous membranes generally. They are de- cidedly irritating to the mucous membranes, producing violent inflam- mation, rawness and excoriation, as anybody can testify who has once inhaled the fumes of Chlorine, Iodine or Bromine. They all produce spasms of the glottis and this is most marked in Chlorine, although they all have it. They all tend to produce pseudo-membranous forma- tions on the mucous membranes. All excepting Chlorine tend to pro- duce croupous membranes; Chlorine tends more to diphtheritic than pure croupous membrane. All of the halogens act upon the glandu- lar system, producing enlargement, induration and even abscess in 502 A CLINICAL MATERIA MEDICA.