A study was made of the action on the tick (Ixodes ricinus L.) of some toxic substances used in dips. Methods are described of attaching ticks to limited areas on the sheep and of suspending ticks for individual tests.

Chemical potentials rather than concentrations should be used in comparing the toxicity of different solutions. Phenol is less soluble in P31 paraffin oil than in water, and a solution of phenol in P31 paralyses the tick more rapidly than the same concentration in water. Solutions of phenol in paraffin and water which are in equilibrium show the same effect on the tick. The potentials are the same, but the concentrations are different.

Each toxic substance tested acts more rapidly at a given concentration in a medium in which it is less soluble. Acid solutions of arsenious acid act more rapidly than alkaline. Solid arsenious oxide as crystals or as a deposit on wool acts more slowly than sodium arsenite under the same conditions, owing probably to its slow rate of solution. Sodium arsenite and arsenious oxide readily penetrate the tips of the feet, and sodium arsenite readily penetrates the dorsal surface of the tick.

Previous work on arsenical dips and the contact toxicity of wool dipped in sodium arsenite indicate that arsenic is a contact and not a stomach poison to ticks.

Phenols are toxic at low concentrations in water, but their greater solubility in wool fat and rapid evaporation from thin films explain their ineffectiveness as dips.

DDT (2, 2 bis-p-chlor phenyl 1, 1, 1, trichlorethane) penetrates the feet of the tick more readily than the rest of the body surface. DDT acts more rapidly at 1% in paraffin oil (solubility 1-3%) than in sesame oil (solubility 10%). Saturated solutions have the same effect. Similarly, vaseline solutions are more effective than solutions in lanoline.

Rotenone penetrates the feet of ticks more rapidly than the back. Larval ticks are more rapidly intoxicated by 01% solutions in olive oil (solubility 0.2 %) than 0.1% solutions in castor oil (solubility 1-2%). Saturated solutions have the same effect.

The importance of a knowledge of the site of entry of insecticides into the organism is stressed.

The less soluble the substance in the medium, the less is needed to give a high chemical potential and the more rapid will be the penetration of the organism. With low solubility, however, a small loss by diffusion causes a large fall in potential. Solubility is analogous to capacity. In practice, media will be needed in which the toxic substance is neither so soluble that a wasteful amount is needed to produce a high potential nor so insoluble that losses by diffusion cause large falls in potential.

The carrier activity of different oil bases for insecticides and the synergistic action of insecticides applied together may in part be due to changes in solubility and hence chemical potential.