252 Sir Hiimpnry Davy OH the [April, 



as those just described, and with precisely the same conse- 

 quences. 



In my first experiments upon the effects of increasing the 

 length or diminishing the mass of the imperfect or fluid conduct- 

 inff surface in interfering with the preserving effects of metals, 

 I used long narrow tubes ; but I found them very inconvenient ; 

 and I had recourse to the more simple method of employing 

 cotton or tow for this purpose. 



Several feet of copper wire in a spiral form were connected 

 with a small piece of zinc wire of about half an inch in length. 

 The zinc and a portion of the copper were introduced into one 

 glass, and the coils of copper wire were introduced 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 dimi- 

 nished, but extended apparently equally through the whole 

 series. 



When the pieces of tow were about the filth 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 class this result was still more distinct, 

 and so on till in the seventh glass there was a considerable cor- 

 rosion 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 chemical changes 

 dependent upon negative electricity were successively and 

 elegantly exhibited in this experiment. In the first glass con- 

 taining 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 waa coated with revived metallic 

 zinc. In the second glass the wire was covered over with fine 

 crystals of carbonate of lime; and the same phenomenon of the 

 separation of carbonate of soda occurred, but in a less degree. 

 In the third glass the wire was clean, but without depositions ; 

 and the presence of alkaline matter could only be distinguished 

 by chemical tests. In the fourth glass the copper was bright, 

 evidently in consequence of a slight but general corrosion, but 

 with a scarcely sensible deposit; in the fifth, the deposit was 

 very visible ; and in the seventh the wire was covered with 

 green rust. 



