1. Stimulation in the rock barnacle Balanus balanoides by hydrochloric, sulfuric, and nitric acids, and by the first seven members of the normal aliphatic acid series has been studied. The hydrogen ion concentrations of the solutions tested varied from 3.2 x 10–8 to 5.889 x 10–6. The criterion of response was percentage closure in groups of individuals, recorded at 1 minute intervals until maximum closure occurred.

2. The intensity of stimulation by these acids is proportional to the effects of two forces, one related to the change in the (H+), and the other to the field of force around the anion of the acid added to the environment.

3. A preliminary interpretation of the results led to the development of the following expression which fits approximately the data obtained at the end of 4 minutes:

Per cent closure = 100 – 100e–0.1z+(0.003125)2–0.1z+(0.003125)2n(z–0.4)

where z is the (H+) x 107 and n is the number of carbon atoms (if present) in the anion of the acid. This equation assumes that the anions of the mineral acids enter into the reaction stoichiometrically, and emphasizes the difference in the fields of force around the anion of the fatty acids, a difference which is correlated with the length of the carbon chain.

4. A further analysis of the data revealed the presence of three or more receptor groups which appeared to be differentially affected by forces originating from the anions of the acids.

5. The order of stimulating efficiency for the mineral acids was found to be: HCl>H2SO4>HNO3.

6. The order of stimulating efficiency for the fatty acids was found to be: heptylic>caproic>valeric>butyric = acetic>propionic = formic.

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