134 



G. H. PARKER. 



very slight but increasing activity appeared, which, though ex- 

 tremely small, was sufficient to check growth. The Cu series also 

 illustrates a historical discovery made by Sir Humphrey Davy, who 

 in the early days of electrochemistry showed that copper-covered 

 ships'-bottoms could be kept from corroding by coupling the Cu 

 with Fe, but that under such circumstances organisms grew upon 

 the Cu in great abundance and thus defeated the object for which 

 the Cu had been applied. It is not impossible, however, that in 

 seaside laboratories and aquaria, where it may be convenient to 

 conduct seawater through copper pipes, the deleterious effect of 

 the Cu can be overcome, at least for short distances, by combining 

 it with some metal high in the electromotive series, such, for in- 

 stance, as Al. 



The observations contained in this paper lead to the conclusion 

 that marine animals will grow upon any heavy metal, provided that 

 metal does not liberate ions or soluble compounds. The ions and 

 soluble compounds of the heavy metals are usually extremely 

 poisonous and where they are liberated freely from a metallic sur- 

 face that surface is protected against organic growth. Such seems 

 to be the case with Zn and Cu. With Al, Fe, Sn, and Pb the 

 products of marine corrosion are essentially insoluble and hence 

 organisms grow upon these metals in the sea. By coupling Cu 

 with members higher in the electromotive series, this metal can be 

 rendered chemically inactive in seawater and, under such circum- 

 stances, animals will grow freely upon it. Zn in this respect is 

 much less easily controlled, for it lies high in the electromotive 

 series and consequently it is not open to the electrochemical pro- 

 tection that Cu is. Its compounds, moreover, are relatively freely 

 soluble and thus become very effective in checking the growth of 

 animals. 



PLATE I. 



Photographs of plates, 15 cm. square, of six different metals which, after a 

 submergence of about six weeks in seawater, were more or less covered by 

 organic growth: i, aluminum; 2, zinc; 3, iron; 4, tin; 5, lead; 6, copper. For 

 these and the succeeding photographs the author is indebted to Mr. George 

 Nelson. 



