Observation has shown that both of these requirements are satisfied; phenylenediamine is more poisonous than aniline; toluylenediamine more poisonous than toluidine. Again, if an atom of hydrogen in the amido (NH₂) group in aniline be replaced by an alkyl, _e.g._ methyl or ethyl, the resulting substance does not produce muscular spasm; but if the same alkyl is substituted for an atom of hydrogen in the benzene nucleus the convulsive action remains unaffected. If an acidyl, as for example the radical of acetic acid, enter into the amido group, then the toxic action is notably weakened; thus, acetanilide is weaker than aniline, and acetylphenylhydrazine is weaker than phenylhydrazine. If the hydrogen of the imido group be replaced by an alkyl or an acid radical, and therefore tertiary bound nitrogen restored, the poisonous action is also weakened. In xanthin there are three imido groups; the hydrogen of two of these groups is replaced by methyl in theobromin; and in caffein the three hydrogens of the three imido groups are replaced by three methyls, thus:-- NH--CH | ║ CO C--NH | | \ | | CO | | / NH--C==N Xanthin. N.CH₃--CH | ║ CO C--N.CH₃ | | \ | | CO | | / NH-----C==N Theobromin. N.CH₃--CH | ║ CO C--N.CH₃ | | \ | | CO | | / N.CH₃--C==N Caffein. and experiment has shown that theobromin is weaker than xanthin, and caffein still weaker than theobromin. Loew[39] makes the following generalisations:-- [39] _Ein natürliches System der Gift-Wirkungen_, München, 1893. 1. Entrance of the carboxyl or sulpho groups weakens toxic action. 2. Entrance of a chlorine atom exalts the toxic character of the catalytic poisons (Loew’s catalytic poisons are alcohols, ether, chloroform, chloral, carbon tetrachloride, methylal, carbon disulphide and volatile hydrocarbons). 3. Entrance of hydroxyl groups in the catalytic poisons of the fatty series weakens toxic character; on the other hand, it exalts the toxicity of the substituting poisons. (Examples of Loew’s class of “substituting” poisons are hydroxylamine, phenylhydrazine, hydric cyanide, hydric sulphide, aldehyde, and the phenols.) 4. A substance increases in poisonous character through every influence which increases its power of reaction with aldehyde or amido groups. If, for example, an amido or imido group in the poison molecule be made more “labile,” or if thrice linked nitrogen is converted into nitrogen connected by two bands, whether through addition of water or transposition (_umlagerung_) or if a second amido group enters, the poisonous quality is increased. Presence of a negative group may modify the action.