1. _Acids._ 2. _Caustic alkalies._ 3. _Caustic salts_, especially those of the heavy metals. 4. Locally irritating organic substances which neither can be classified as corrosive acids nor alkalies, nor as corrosive salts; such are:--_cantharidine_, _phrynine_, and others in the animal kingdom, _croton oil_ and _savin_ in the vegetable kingdom. Locally irritating colours, such as the _aniline dyes_. 5. Gases and vapours which cause local irritation when breathed, such as _ammonia_, _chlorine_, _iodine_, _bromine_, and _sulphur dioxide_. B. Those which have but little effect locally, but change anatomically other parts of the body; such as _lead_, _phosphorus_, and others. II. BLOOD POISONS. 1. Blood poisons interfering with the circulation in a purely physical manner, such as _peroxide of hydrogen_, _ricine_, _abrine_. 2. Poisons which have the property of dissolving the red blood corpuscle, such as the _saponins_. 3. Poisons which, with or without primary solution of the red blood corpuscles, produce in the blood methæmoglobin; such as _potassic chlorate_, _hydrazine_, _nitrobenzene_, _aniline_, _picric acid_, _carbon disulphide_. 4. Poisons having a peculiar action on the colouring matter of the blood, or on its decomposition products, such as _hydric sulphide_, _hydric cyanide_, and the _cyanides_ and _carbon monoxide_. III. POISONS WHICH KILL WITHOUT THE PRODUCTION OF COARSE ANATOMICAL CHANGE. 1. Poisons affecting the cerebro-spinal system; such as _chloroform_, _ether_, _nitrous oxide_, _alcohol_, _chloral_, _cocaine_, _atropine_, _morphine_, _nicotine_, _coniine_, _aconitine_, _strychnine_, _curarine_, and others. 2. Heart Poisons; such as, _digitalis_, _helleborin_, _muscarine_. IV. POISONOUS PRODUCTS OF TISSUE CHANGE. 1. Poisonous albumin. 2. Poisons developed in food. 3. Auto-poisoning, _e.g._ uræmia, glycosuria, oxaluria. 4. The more important products of tissue change; such as, _fatty acids_, _oxyacids_, _amido-fatty acids_, _amines_, _diamines_, and _ptomaines_. * * * * * § 19. I have preferred an arrangement which, as far as possible, follows the order in which a chemical expert would search for an unknown poison--hence an arrangement partly chemical and partly symptomatic. First the chief gases which figure in the mortality statistics are treated, and then follow in order other poisons. A chemist, given a liquid to examine, would naturally test first its reaction, and, if strongly alkaline or strongly acid, would at once direct his attention to the mineral acids or to the alkalies. In other cases, he would proceed to separate volatile matters from those that were fixed, lest substances such as prussic acid, chloroform, alcohol, and phosphorus be dissipated or destroyed by his subsequent operations.