344 Mr. M. Donovan on the supposed Identity of the Agent 



With this view I made the following experiments. A numher 

 of strips of thin milled zinc were prepared : each was half an inch 

 in breadth and 10 inches long; one end of each terminated in a 

 copper wire of about 6 inches in length ; towards the other 

 end, at an inch from the extremity, the strip was bent at a right 

 angle; that inch of zinc was amalgamated at both sides, and 

 well covered with a strong alcoholic solution of sealing-wax, ex- 

 cept a quarter of an inch at the extreme end. By this method 

 I intended, when these plates were dipped in dilute acid, to ex- 

 pose that portion of the metal only which was uncovered (viz. 

 \ inch x \ inch) to its action. I had already found that mere 

 immersion of the zinc strip to a regulated depth would not con- 

 fine the action of the acid to the immersed surface ; the efferves- 

 cence produced always created an elevated ridge of bubbles round 

 the zinc, and so the chemical action was extended above the level 

 of the liquid more or less. I also prepared a number of zinc 

 strips the same in all respects as the foregoing, except that they 

 were not amalgamated. 



A number of copper strips of the same size, shape and con- 

 struction, were also prepared ; they were well cleaned with sand- 

 paper, and waxed as the rest. 



A shallow porcelain tray was fitted into a stand, and from its 

 sides diametrically opposite rose two standards, with a horizontal 

 sliding cross-bar which moved up and down the standards ; there 

 were two notches in the cross-bar of such size as would confine 

 the metallic slips when immersed in the tray. 



The terminal wires of a strip of zinc and of a strip of copper 

 being tightened by the binding-screws of an excellent galvano- 

 meter, the other ends of the plates which had been bent to a 

 right angle now stood vertically over the porcelain tray, and were 

 confined in the notches of the cross-bar; the chief length of each 

 plate, therefore, lay horizontally from the galvanometer to the 

 cross-bar. Two ounces measure of the exciting acid was poured 

 into the porcelain tray previously levelled. Everything being 

 ready, the time Mas noted, and the cross-bar carrying with it 

 both the plates was moved downwards until their ends touched 

 the bottom of the tray, and the unwaxed terminal portion of 

 bright metal was immersed in the acid. The galvanometer 

 needle started off; in a minute or less, steadily pointed to a cer- 

 tain degree, and after varying a little, suddenly fell, when the 

 exposed portion of the zinc was dissolved away. The time was 

 again noted, as also every period when the needle moved a de- 

 gree or two. 



I was thus precise, because the results seemed to be important : 

 forty experiments were successively made with pairs of zinc and 

 copper, one pair for each acid or strength of acid. It would be 

 useless to give the details of all ; the results of four experiments 



