Chap. II.] 



D AN i ELL 1 s ELEMENT. 



Daiiiell's element (Fig. 9) was the first constant 

 element, and was devised in 1836. It consists of an 

 outer vessel of glass or glazed earthenware, in which 

 is placed a cylinder of copper c, perforated with 

 holes, and open at both ends. A saturated solution 

 of sulphate of copper is placed in the outer vessel. 

 Within the copper cylinder is placed a vessel of 

 porous earthenware P, which contains dilute sul- 

 phuric acid, having im- 

 mersed in it a cylinder of 

 zinc z. The porous vessel 

 thus marks off an outer 

 and inner compartment, 

 while, being porous, it 

 permits communication be- 

 tween the two. When the 

 element is in action, the 

 acid attacks the zinc of 

 the inner compartment, 

 produces sulphate of zinc, 

 and liberates hydrogen. 

 The hydrogen passing into 

 the outer compartment re- 

 duces the copper sulphate, 

 and deposits metallic copper on the copper cylinder, 

 which is thereby kept always bright. By this decom- 

 position, the hydrogen forms sulphuric acid, which re- 

 places that used in the inner compartment. The sul- 

 phate of copper is thus the only thing used up. To 

 replace it, the copper cylinder is supplied with a small 

 shelf G, on which crystals of the copper salt rest. 

 The fluid reaches up to the shelf; the crystals are dis- 

 solved as required, and thus the strength of the solu- 

 tion is maintained. Let it be noted how polarisation, 

 the cause of the inconstancy of elements, is got 

 lid of in this case : (1) The copper is kept clean 

 and fresh by deposition from the sulphate of copper 



Fig. 9. Darnell's Element. 



