234 NATIONAL ACADEMY OF SCIENCES 



a coating of zinc, of greater thickness on the surface containing the letters and 

 figures. 



" From well-established principles of galvanism, and from the direct experi- 

 ments of the committee, it is certain that while the zinc coating covers every part 

 of the surface of the iron, no other galvanic action will take place than that 

 exhibited in the ordinary corroding of a single metal, but that as soon as the 

 smallest portion of the iron is exposed to the liquid precipitation from the atmos- 

 phere, a galvanic current will be established passing through the liquid from the 

 zinc to the iron, that the former will be more rapidly corroded than it was 

 previous to the exposure of the iron, and that this action will go on until all the 

 zinc is dissolved. The iron, during the process, will be protected from the action 

 of the atmosphere at the expense of the zinc. After all the zinc has been dis- 

 solved, the iron, being unprotected, will then be corroded in the usual manner. 



" From this statement it is evident that the coating of zinc will tend to prolong 

 the existence of the iron in its metallic state, though it will not afiford a perpetual 

 protection such as may be given by a coating of enamel like that used in covering 

 the basins of iron sinks, kettles, etc. 



" The experiment made by the committee consisted in attaching to one end of 

 the wire of a galvanometer a plate of zinc and to the other end a plate of iron. 

 These two plunged in a vessel of water slightly acidulated by sulphuric acid, gave 

 rise to a powerful current of galvanism from the zinc to the iron. While the 

 zinc was rapidly corroded the iron remained unaiifected. By substituting for the 

 zinc a plate of copper, a still more powerful current was produced in the opposite 

 direction. The iron in this case was violently acted upon, while the copper 

 retained its brightness. 



" The committee may state, as a general rule, that when two different metals 

 are placed in metallic contact, the one most readily acted on by an acid will be 

 dissolved, while the other will be protected, and that the action on the metal dis- 

 solved will be increased in intensity by the association. Thus, iron in association 

 with zinc is protected, while the same metal in connection with copper is more 

 rapidly corroded than it is without such connection. 



'' Respectfully submitted, 



" Joseph Henry, 

 " J. H. C. Coffin, 

 " Joseph Saxton. 

 " General D. H. Rucker, U. S. A., 



" Acting Quartermaster General." 



It will be observed that this report refers exclusively to the 

 metals composing the headstones and that the opinion expressed 

 was that iron blocks would not endure perpetually. While the 

 report was pending, a discussion of another character regarding 



