262 The Rev. Prof. Callan on a New Single 



wanted, by exciting zinc and cast iron with dilute sulphuric 

 acid. 



During the evening of the day on which I tried the cast iron cell 

 excited with dilute sulphuric acid, it struck me that, on having 

 increased the strength of the acid solution by pouring a little 

 acid into the cell, a strong impulse was given to the needle of 

 the galvanometer, and that perhaps, by increasing very much 

 the strength of the solution, I might get a very powerful galvanic 

 current. I then resolved to try concentrated sulphuric acid and 

 dilute acid of the various degrees of strength between concen- 

 trated acicFand one part of acid mixed with eight of water. 



The sulphuric acid which I used was very strong and pure, 

 being made, not from pyrites, but from sulphur. In comparing 

 the galvanic powers of the various batteries which I tried, I 

 always used our large galvanometer, and sent the voltaic current 

 only through the outermost coil, the diameter of which is about 

 25 inches. The coil is made of copper wire fths of an inch 

 thick. The outside coil is nearly 7 feet long. The galvano- 

 meter may be used as a sine or tangent galvanometer. 



I began by exciting the cast-iron cell and zinc plate with con- 

 centrated sulphuric acid : the galvanic current produced was 

 exceedingly feeble. I then mixed a small quantity of water with 

 the acid : the deflection of the needle was greatly increased. I con- 

 tinued to pour water gradually into the cast-iron cell, and found 

 that the deflection of the needle increased until the acid was diluted 

 to a certain degree, and that when it was diluted still further, 

 the deflection was diminished, and became constantly less as the 

 quantity of water was increased. After five or six hours spent 

 in frequently charging the cell with dilute sulphuric acid of 

 various degrees of strength, I found that the acid which I used, 

 diluted with twice its bulk of water, produced the most powerful 

 galvanic current ; and that when the acid was diluted with more 

 or less than twice its bulk of water, the voltaic power of the cell 

 was diminished. I afterwards found that acid which was still 

 stronger than the very strong acid which I first used, and which 

 was also made from sulphur, and not from pyrites, required to be 

 diluted with three times its bulk of water in order to produce 

 the maximum of galvanic power, or to produce a current equal 

 in power to that which was produced by one part of the first acid 

 diluted with twice its bulk of water. Either of the two diluted 

 acids which I have described act very little on amalgamated zinc. 

 I am inclined to think that the common sulphuric acid, which 

 is generally made from pyrites, and is not so pure as that which is 

 made from sulphur, nor so strong as that which I first used, should 

 not be diluted with more than about once and a half its own bulk 

 of water. 



