270 The Rev. Prof. Callan on a New Single 



batteries ; it is less expensive in construction, since it does not 

 require porous cells ; it is far more cecononiical in use ; it is free 

 from the trouble, annoyance, and all the very serious disadvan- 

 tages which arise from the use of porous cells and nitric acid ; 

 it is extremely constant in its action, and may by a simple con- 

 trivance be made to act with almost perfect constancy till the 

 zinc plates are worn out. The metals will remain clean for any 

 length of time the battery may be in action : no hydrogen ad- 

 heres to the cast iron. The only cause, then, of the diminution 

 of galvanic power is the exhaustion of the exciting fluid. By 

 siphon tubes, one of which reaches near the bottom of each cell, 

 passes through a hole in the side of the cell a little above the 

 upper edge of the zinc plate, and projects about an inch from 

 the outside of the cell, and dropping tubes connected with the 

 vessel which contains the exciting fluid, one of the tubes being 

 set over each cell, a supply of fresh exciting fluid may be given 

 to each cell, and the exhausted fluid may be made to pass out 

 through the siphon tube. The dropping tubes, should project 

 from the bottom of the vessel which holds the fluid, and should 

 be at such distances from each other that one will be over each 

 cell of the battery. The vessel should be air-tight, and a stop- 

 cock fitted to the top of it, so that the communication between 

 the inside of the vessel and the external air may be cut off by 

 closing the stopcock, and may be renewed by opening the stop- 

 cock. When the communication between the inside of the vessel 

 and the external air is cut off, the fluid ceases to flow into the 

 cells from the dropping tubes : as soon as the communication is 

 renewed, the fluid begins again to flow into the cells from the 

 dropping tubes, and the exhausted fluid, as well as the sulphate 

 of iron and zinc, at the bottom begins to flow out through the 

 siphon tubes. By suitable clockwork, the stopcock may be 

 opened aud closed as may be required, in order to give the neces- 

 sary supply, and no more, of the exciting fluid to each cell. 



I have not yet tried a large series in which the zinc and cast 

 iron were very near to each other. I am preparing for the Col- 

 lege a battery of about 300 pairs of 4-inch cast-iron and zinc 

 plates separated from each other about j-g-th of an inch, and 20 

 pairs of plates, each about a foot square. The former are intended 

 for effects which require considerable intensity, such as heat and 

 light ; and the latter for the decomposition of water and the 

 lime light. 



Having found that the galvanic power of cast iron and zinc 

 excited by one part of sulphuric acid and two of water, was 

 increased by dissolving sulphate of copper in the acid solution, 

 it occurred to me to try the effect of cast iron instead of the 

 copper used in Darnell's battery. I put into a saturated solu- 



