the prevention of metals by electro-chemical means. 335 
with a small piece of zinc wire of about half an inch in 
length. The zinc and a portion of the copper were intro- 
duced into one glass, and the coils of copper wire were intro- 
duced into other glasses, so as to form a series of six or seven 
glasses, which were filled with sea water, and made part of 
the same voltaic arrangement, by being connected with pieces 
of tow moistened in sea water. 
It was found in these experiments, that when the pieces of 
tow connecting the glasses were half an inch in thickness, 
the preserving effect of the zinc in the first glass was no 
where diminished, but extended apparently equally through 
the whole series. 
When the pieces of tow were about the fifth of an inch in 
thickness, a diminution of the preserving effects of the zinc 
was perceived in the fourth glass, in which there was a slight 
solution of copper ; in the fifth glass this result was still more 
distinct, and so on till in the seventh glass there was a con- 
siderable corrosion of the copper. 
When the tow was only the tenth of an inch in thickness, 
the preserving effect of the zinc extended only to the third 
glass ; and in each glass more remote, the effect of corrosion 
was more distinct, till in the seventh glass it was nearly the 
same as if there had been no protecting metal. All the che- 
mical changes dependent upon negative electricity were suc- 
cessively and elegantly exhibited in this experim.ent. In the 
first glass containing the zinc, there was a considerable and 
hasty deposition of earthy and alkaline matter, and crystals 
of carbonate of soda adhered to the copper at the surface 
where it was clean and bright ; but in the lower part it was 
coated with revived metallic zinc. In the second glass the 
