PHYSICS OF NINETEENTH CENT. ELECTRICITY. 559 



which appears on the copper or other metal of the couple not acted 

 upon by the acid. In 1836 Professor Daniell contrived an ingenious 

 method of avoiding this inconvenience. In his battery the zinc is in con 

 tact with one liquid, and the copper with an- 

 other, the two liquids being separated by a 

 porous partition. Fig. 269 shows a cell in 

 section. A A is a cylindrical copper vessel, 

 within which is a cylinder of porous earthen- 

 ware c c ; within which, again, is a rod of zinc, 

 externally covered with a little mercury, which 

 dissolves only the pure zinc, and presents it 

 to the action of the dilute sulphuric acid con- 

 tained in the porous cell. Outside of the 

 porous cell is a saturated solution of sulphate 

 of copper. In the action of the cell no gas 

 appears, but metallic copper is deposited on 

 the inside of the copper vessel. The theory 

 was that the hydrogen, instead of appearing 

 on the copper plate as a gas, replaces the dis- 

 solved copper in the solution. Several other 

 arrangements of "constant batteries" have 

 been devised since, and some of these are 



very energetic, or have other special advantages. It will suffice to 

 mention Bunsen's battery, the arrangement of which is exhibited in 



FIG. 269. 



FIG. 270. 



Fig. 270, where the complete cell and its parts are severally shown. 

 A is a glass or porcelain vessel containing dilute sulphuric acid. B 

 is a thick plate of zinc bent round and amalgamated, c is a vessel 



