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1. The degree of toxicity of the hydrogen ion and the ions of eighteen metals to Polycelis nigra (Müller) has been assessed by determining in each case the concentration the animal survives for approximately 48 hr. at 15-18°C. 2. On a mg./l. basis their order of increasing toxicity is: Sr Na Ca Mg Mn'' Pb'' K Al Co'' Cr''' Ni'' As''' Zn Cd'' H Au''' Cu'' Hg'' Ag 3. On a molar concentration basis the order is as follows: Na Sr Ca Mg Mn'' K Al Pb'' Cr''' Co'' Ni'' H As''' Zn Cd'' Cu'' Au''' Ag Hg'' 4. The position of iron is uncertain; the toxicity of ferric chloride solutions appears to be due to their acidity. Irregular results were obtained with solutions of barium salts, which appear to effect excessive stimulation of the musculature of the animal and induce convulsive movements which eventually result in the rupture of the body and extrusion of the tissues. 5. On a mg./l. or molar concentration basis there is a decided relationship between the solution pressures of the metals and the degree of toxity of their salts, the general result being very similar to that obtained in an earlier investigation with Gasterosteus aculeatus as test animal. This relationship suggests that the degree of toxicity of ions is largely determined by their affinity for their electrical charges, this affinity determining the readiness with which they tend to abandon the ionic state to enter into chemical combination with protoplasmic compounds.

A Further Study of the Relation Between Toxicity and Solution Pressure, with Polycelis Nigra as Test Animal