I speak of these substances first because they are of all insecticides, the most important. There is 110 insect that can take either of these poisons into its system and live, hence their great value for the destruction of leaf-eating insects where the application does not endanger the lives of other beings. The poisonous principal in both of these substances is arsenic of which Paris green contains about fifty-seven per cent, and London purple about forty-seven per cent. This difference should be taken into account when these two poisons are being prepared. It should also be borne in mind that a given bulk of Paris green is fully double the weight of the same bulk of London purple.