421 
electrical and chemical changes . 
negative to rolled copper ; copper ( to use the technical 
language of manufacturers) both overpoled and underpoled , 
containing in one case probably a little charcoal and in the 
other a little oxide, negative to pure copper. A specimen 
of brittle copper, put into my hands by Mr. Vivian, but in 
which no impurity could be detected, was negative with 
respect to soft copper. 
In general, very minute quantities of the oxidable metals 
render the alloy positive, unless it becomes harder, in which 
case it is generally negative. As I have mentioned before, 
amalgams of the oxidable metals are usually positive, not 
only to mercury, but even to the pure metals. 
There are probably few chemical operations which electri- 
cal changes do not influence, and either increase or modify. 
In the rusting of iron, for instance, the oxide formed by the 
contact of moisture becomes the negative surface, and exalts 
the oxidability of the mass of metallic iron, and the rust con- 
sequently extends in a circle. 
The precipitations of metals have been already traced to 
causes of this kind, and many metallic solutions must belong 
to the same order of phenomena. 
I have pointed out in former papers some of the cases of 
electro-chemical protection, which I have no doubt, when the 
principles are well understood, will be generally adopted ; 
and others are constantly occurring. I shall mention one, — 
the preservation of the iron boilers of steam engines by intro- 
ducing a piece of zinc or tin. This in the case of steam boats, 
particularly when salt water is used, may be of the greatest 
advantage, and prevent the danger of explosion, which gene- 
rally arises from the wear of one part of the boiler. 
mdcccxxvi. 3 1 
