5. Entrance of a nitro group strengthens the poisonous character. If a carboxyl or a sulpho group is present in the molecule, or if, in passing through the animal body, negative groups combine with the poison molecule, or carboxyl groups are formed in the said molecule; in such cases the poisonous character of the nitro group may not be apparent. 6. Substances with double carbon linkings are more poisonous than the corresponding saturated substances. Thus neurine with the double linking of the carbon of CH₂ is more poisonous than choline; vinylamine than ethylamine. CH==CH₂ / (CH₃)₃N \ OH Neurine. CH₂--CH₂OH / (CH₃)₃N \ OH Choline. CH₂ ║ CH.NH₂ Vinylamine. CH₃ | CH₂.NH₂ Ethylamine. § 27. M. Ch. Michet[40] has investigated the comparative toxicity of the metals by experiments on fish, using species of _Serranus_, _Crenolabrus_, and _Julius_. The chloride of the metal was dissolved in water and diluted until just that strength was attained in which the fish would live 48 hours; this, when expressed in grammes per litre, he called “_the limit of toxicity_.” [40] “_De la Toxicité comparée des différents Métaux._” _Note de M. Ch. Michet. Compt. Rend._, t. xciii., 1881, p. 649. The following is the main result of the inquiry, by which it will be seen that there was found no relation between “the limit of toxicity” and the atomic weight. TABLE SHOWING THE RESULTS OF EXPERIMENTS ON FISH. No. of Limit of Experiments. Metal. Toxicity. 20. Mercury, ·00029 7. Copper, ·0033 20. Zinc, ·0084 10. Iron, ·014 7. Cadmium, ·017 6. Ammonium, ·064 7. Potassium, ·10 10. Nickel, ·126 9. Cobalt, ·126 11. Lithium, ·3 20. Manganese ·30 6. Barium, ·78 4. Magnesium, 1·5 20. Strontium, 2·2 5. Calcium, 2·4 6. Sodium, 24·17 V.--Life-Tests; or the Identification of Poison by Experiments on Animals. § 28. A philosophical investigation of poisons demands a complete methodical examination into their action on every life form, from the lowest to the highest. Our knowledge is more definite with regard to the action of poisons on man, dogs, cats, rabbits, and frogs than on any other species. It may be convenient here to make a few general remarks as to the action of poisons on infusoria, the cephalopoda, and insects.