FOULING OF SHIPS' BOTTOMS 



235 



Table 7- 



-Resistance (in minutes) of larval barnacles (nauplii) to several concentrations of various 

 compounds (from report of A. W. Bray) 



Thus, it is seen that Bray has shown that certain compounds have a very toxic 

 effect on the earliest larval stages of barnacles, provided the concentration is sufficient 

 in the medium surrounding the organism to have its maximum effect. It must be 

 understood at this time that the barnacles attach by means of long antennae, and 

 that in the case of merciu-ial compounds a concentration of more than one part per 

 hundred thousand must be maintained in order to have any effect at all. With the 

 entire ocean as a solvent, and less than 14 per cent of an extremely thin film to act 

 upon, it seems questionable if such poisons can build up a concentration sufficient 

 to be lethal for any considerable period of time. Of course, it is remotely possible 

 that chemical action with sea water might have some effect, as suggested by Gardner 

 (1922, p. 55). He states: 



The toxicity of free substances such as mercury and copper compounds to young organisms 

 does not necessarily give a true indication of tlieir toxicity wlien mixed with other ingredients of a 

 paint, and the influence of the component parts of the sea water upon the toxic substance through 

 longer periods may render it more or less toxic by dilution or by chemical interaction * * *. It 

 is well known that when two substances are mixed together in varying proportions the resulting 

 mixture is frequently more toxic than the same quantity of either component if used separately. 

 The "why" of this action is not known; it is merely an empirical result. 



However, this type of speculation has no evidence whatsoever for its support 

 and perhaps is indicative of the methods sometimes employed in the preparation of 

 antifouling paints. 



Many paints have been tested by actual application on the bottoms of ships, 

 both by the United States Navy and by the American Society for Testing Materials, 

 through cooperation with the United States Shipping Board. In such tests the vessel 

 to be painted usually was marked off into four divisions, and the forward port quarter 

 and aft stern quarter were painted with the test paint while the other two quarters 

 were painted with the regulation "Navy standard;" or vice versa, as the case might 

 be. In such tests a true comparison of the relative efficiency of the two paints could 

 be determined. 



