SKIN AND MUCOUS MEMBRANES 153 



2. Caustic alkalies : Caustic potash, caustic soda, caustic lime, 

 Vienna paste. These dissolve the organic albumin by taking up 

 water and forming soluble alkaline albuminates. Their eschar 

 or slough is mostly moist, deep and diffuse, extending into the 

 surrounding tissue, a contrast to the dry, circumscribed, superficial 

 eschar of the mineral acids. 



3. Metallic caustics: Zinc chloride, corrosive sublimate, potas- 

 sium chromate, chromic acid, iron chloride, iron sulphate, copper 

 sulphate, zinc sulphate, silver nitrate, antimony trichloride, 

 arsenic, lead acetate and nitrate, biniodide of mercury, oxide of 

 mercury, copper acetate, potassium permanganate. These metal- 

 lic compounds act by forming an insoluble metallic albuminate, at 

 the same time setting free the acid components (sulphuric acid, 

 nitric acid, chlorine, iodine). Some of them, in addition, act also 

 by oxidation; these include the oxygen-containing caustics, chromic 

 acid and permanganate of potash, which give off oxygen, and also 

 arsenic, which first takes up oxygen and then gives it off. The 

 halogens, chlorine, bromine, iodine, absorb the hydrogen of the 

 tissues, in consequence of which oxygen in the nascent state (in 

 addition to hydrochloric acid) is set free. 



The degree of the caustic action differs with the chemical 

 nature of the individual drugs and also with the concentrations 

 employed. A distinction is made between a superficial scab forma- 

 tion and a deeper cauterization. The inorganic acids, silver 

 nitrate, copper sulphate, zinc sulphate, iron sulphate and lead 

 nitrate act superficially, while formaldehyde, the caustic alkalies, 

 sulphuric acid and chromic acid have a deep action. The caustic 

 effect upon the skin is much weaker than upon the mucous mem- 

 branes because of the protective layer of homy epidermal cells. 

 The dry caustics, especially the metallic salts, act upon the skin 

 generally only when they come in contact with fluids. The color 

 of the eschar differs very much. Nitric acid produces a yellow 

 eschar; hydrochloric acid, a gray; sulphuric acid, brown or dark 

 gray; chromic acid and formaldehyde, yellow, becoming brown 

 and finally black; acetic acid, white; silver nitrate, at first white, 

 then gray and finally black. 



