Certain pathologic conditions, as fatty degeneration of the liver, cloudy swelling of the kidney, nephritis, malignant jaundice, acute yellow atrophy of the liver, dysentery, and others may be caused by such poisons as phosphorus, arsenic, mercuric chloride, potassium chlorate, chloroform, etc. When such changes are found at autopsy the pathologist must always carefully differentiate between disease and poisoning. He must decide as to the actual occurrence of poisoning, the source and nature of the poison, how and when administered, amount of poison, number of poisons, primary and secondary effects, attendant circumstances, accidental, suicidal or criminal administration, motive, etc. Some poisons produce no characteristic gross or microscopic changes in the organs or tissues. In such cases no pathologic conditions sufficient to cause death may be found, and when there is doubt a chemical examination should be made. Other poisons produce more or less characteristic changes, either by their local action, by selective action upon certain organs, by excretion, or by acting upon the blood. The effects will vary according to the amount of the poison, its concentration, length of action, condition of gastro-intestinal tract, rapidity of excretion, etc. The most important and common poisons producing recognizable autopsy conditions are as follows:— =Acids.= In _carbolic-acid_ poisoning there may be dry, brown, leathery spots on the face about the lips; grayish-white eschars on mucosa of lips, mouth, tongue, pharynx and œsophagus; œdema of the glottis and pharyngeal submucosa; white or gray longitudinal eschars in stomach and duodenum; leathery appearance of stomach wall; cloudy swelling of kidneys, odor of phenol in urine, which is dark in color; general passive hyperæmia. In _sulphuric-acid_ poisoning there may be brown, leathery and dry eschars on lips and skin, grayish-white to black eschars in mucosa of mouth, stomach and œsophagus; black, dry and brittle clots in the blood-vessels; perforation of stomach; sloughing of mucosa; parenchymatous nephritis. _Hydrochloric acid_ has little or no action on the skin; on mucous membranes the action is similar to that of sulphuric acid except that the drying of the eschars and blood-clots is less marked. In _nitric-acid_ poisoning the eschars are yellowish; hæmatin is not separated and dissolved, so that the brown black eschars seen in sulphuric- and hydrochloric-acid poisoning are not formed. _Oxalic acid_ causes a white or grayish escharotic condition of the mouth, œsophagus and stomach; crystals of calcium oxalate may be found in the blood-clots and in the kidney-tubules. _Glacial acetic acid_ may produce a grayish-white escharotic condition of the mucosa of the upper respiratory tract, and pneumonia, when inhaled.