(I . 



SCIENCE AND PRACTICE. 221 



Many other improvements have been made in the forms 

 of zinc- copper elements, the chief of which are those of 

 Sturgeon and Daniell. Sturgeon recommended the use of 

 amalgamated zinc as being more electro-positive than common 

 zinc, and only dissolved when current passes ; and Daniell 

 succeeded in constructing a battery of more constancy than 

 those hitherto employed, 



4. Daniell' s Constant Battery. The principle of the electro- 

 motive combination arrived at by Professor Daniell, as giving 

 the best results, consists in immersing each of the electrodes 

 or metallic plates in a different solution, by which the polari- 

 sation, rendering the working of the previous piles of so 

 short duration, was in a great measure prevented ; the element 

 retained its electro-motive force for a longer period, and 

 thence obtained the name, not absolutely correct, of a constant 

 battery. 



To prevent their mechanical mixture, the solutions in 

 which the plates are placed are divided from each other by 

 a porous diaphragm, whose pores are not so close, however, 

 as to prevent the necessary transfer between the atoms. 

 In the earlier forms of his battery, Daniell employed a 

 diaphragm of ox-bladder ; but this has been long since 

 replaced by a cylinder, or vessel of unglazed porcelain. 



A very ordinary form of Daniell' s element at present in 

 use is the cylindrical. The copper-plate is bent round to 

 fit inside a cylindrical porous pot, filled with a saturated 

 solution of sulphate of copper. A cylinder of amalgamated 

 zinc is placed between the porous pot and outer glass vessel 

 in a space filled with dilute sulphuric acid. Crystals of 

 sulphate of copper are kept in the inner chamber, to insure 

 the necessary degree of concentration, and for this purpose 

 the copper cylinder is sometimes partly closed at the bottom 

 to form a cup to contain the crystals. The dilute sulphuric 

 acid in which the zinc is immersed contains usually about 

 five per cent, of commercial sulphuric acid. When these 

 elements are used in numbers, in the form of a battery, 

 the copper cylinder of one element is generally connected 

 permanently with the zinc cylinder of its neighbour, by 



