VOLTAIC INSTRUMENTS. 



237 



cipally to three circumstances : to the amalgamation of the 

 zinc, which prevents the waste of that metal by solution when 

 the circuit is not completed ; to the non-occurrence of the 

 precipitation of zinc upon the copper surface ; and to the com- 

 plete absorption of the hydrogen at the copper surface, the 

 adhesion of globules of gas to the metallic plates greatly di- 

 minishing, and introducing much irregularity into the action 

 of a circle. 



Bird's battery and decomposing cell. To M. Becquerel we 

 are particularly indebted for the investigation of the decom- 

 posing powers of feeble currents, sustained for a long time, the 

 results of which are of extreme interest, both from the nature of 

 the substances that can be thus decomposed, and from the form 

 in which the elements of the body decomposed are presented, 

 the slow formation of these bodies permitting their deposition in 

 regular crystals.* Dr. Golding Bird has also added to the 

 number of bodies decomposed by such means, and contrived a 

 simple form of the battery, which with Becquerel's decomposing 

 cell, renders such decompositions certain and easy, and forms 

 indeed the voltaic instrument, perhaps above all others, the 

 most directly useful to the chemist.f The decomposing cell 

 FIG. 26'. consists of a glass cylinder a, 



(Fig. 26) within another glass 

 cylinder b. The inner cylin- 

 der a is 4 inches long and \\ 

 inch in diameter, and is closed 

 at the lower end by a plug of 

 plaster of Paris 0.7 inch in 

 thickness : this cylinder is fixed 

 by "means of wedges of cork 

 within the other, which is a 

 plain jar, about 8 inches deep 



by 2 inches in diameter. A piece of sheet copper c, 4 inches 

 long and 3 inches wide, having a copper conducting wire sol- 

 dered to it, is loosely coiled up and placed in the inner cylin- 

 der with the plaster bottom : a piece of sheet zinc z, of equal 

 size, is also loosely coiled, and placed in the outer cylinder; 

 this zinc likewise being furnished with a conducting wire. The 



* Trait^ Experimental de TElectricit^ et du Magn&isme, par M. Becquerel. 

 t Phil. Trans. 1837, p. 37. 



