MARINE ANIMALS ON SUBMERGED METALS. 



133 



In both instances the relative small amount of growth when Sn 

 and Pb were combined with Zn is probably due to the Zn ions or 

 compounds from the neighboring Zn half-plate rather than to the 

 Sn and Pb themselves. 



The last metal in the set is Cu, which is below the other five in 

 the electromotive series. In combination with Al, Zn, Fe, or Sn, 

 the half-plate of Cu in seawater remained bright and uncorroded 

 as was to be expected. When combined with Pb, however, it ac- 

 quired a green coating like that which it developed when, as. an 

 isolated single metal, it was exposed to seawater. The solution 

 pressure of Pb is so near that of Cu that probably the Pb is unable 

 to protect the Cu in the way that the other metals do, and hence 

 when it is combined with Pb, Cu corrodes (Table VII.). 



TABLE VII. 



COUPLES BETWEEN Cu AND THE FlVE OTHER METALS, ALL OF WHICH STAND 

 ABOVE Cu IN THE ELECTROMOTIVE SERIES. 



The animals are indicated as in Table I. 



Although Cu, when immersed by itself in seawater, is absolutely 

 free of animal growth, it will support a maximum general growth 

 when rendered inactive by Al. This general growth is also present 

 when the Cu is united with Zn, with Fe, and with Sn, but the 

 quantity falls off as one passes down the series from Al to Sn 

 (Plate V.). Cu has long been known to be most poisonous to the 

 lower organisms and the Cu ions liberated from this metal, when 

 it is immersed alone in seawater, are without doubt the occasion of 

 this quality. It is interesting to observe that in the Cu couples 

 used in these tests the animal growth was at a maximum only with 

 Al and diminished in sequence with Zn, Fe, Sn till it reached noth- 

 ing with Pb. The orderly diminution in the organisms of different 

 Cu couples just pointed out probably indicates that the Cu was 

 completely inactive only when paired with Al, and that when it was 

 paired with the other metals lower in the electromotive series a 



