NATURAL PHILOSOPHY. 161 



porous cup, wmch is immersed in the liquid of the trough, two centimetres 

 above the level of the mercury. The battery, once in action, requires 

 no other care than that of drawing off with a glass syphon, the liquid satu- 

 rated with iodide of mercury, which is to be restored to its primitive elements. 

 The couple thus arranged and exhibited recently before the French Acad- 

 emy, possessed a feeble electro-motive force. It was but little stronger than 

 a couple with sulphate of copper, and only one third that of a couple with 

 nitric acid. Its force was such that, for a trough of about five decametres 

 square, and with a thickness for the bed of iodide of potassium of about three 

 centemetrcs, it was equivalent to ten metres and a half annealed copper 

 wire, one millimetre in diameter, this wire being at centigrade in tempe- 

 rature. 



The process adopted by Mr. Doat for economizing the residues, admitting 

 of some improvement, he made changes which have increased the power of 

 his batteries. 



The main point consists in substituting zinc amalgam for mercury ; he 

 obtains thence iodide of zinc, and the restoration of this compound to its 

 elements, which at first appeared difficult, he has rendered easy by using a 

 hydrated carbonate of copper. Whilst the soluble salts of oxide of copper 

 in reacting on the alkaline iodides precipitate only one half, the basic salts, 

 and especially the carbonate, exercise hardly a sensible action on the alka- 

 line iodides, but act with the greatest rapidity on the alkalino-earthy or 

 metallic iodides, and eliminate the whole of the iodine, the oxide passing to 

 the state of a suboxide, and the metal combining with the iodine becoming 

 oxidized. This action, which goes on rapidly at the ordinary temperature, 

 is instantaneous at 50 C. 



On the flat carbon pole, there is placed a broad filter of porous earth, con- 

 taining hydrated carbonate of copper. When the battery has been for a 

 while in action, the liquid, consisting of double iodide of zinc and potassium, 

 is drawn from the troughs and thrown upon the filter, where it is decom- 

 posed by the copper salt. The alkaline iodide remains pure and the iodide 

 of zinc is changed into an oxide of this metal, whilst the iodine set at liberty 

 is dissolved in the alkaline iodide, and passes with it through the filter and 

 fulls upon the carbon pole. Thus the processes for recovering the iodine 

 requires only the drawing off the liquid and putting it in a filter charged 

 with hydrated carbonate of copper. The products left on the filter are oxide 

 of zinc and carbonate of copper. They are mixed with charcoal and fused 

 at a red heat. The result is a brass always in demand in commerce. The 

 hydrated carbonate of copper is prepared by double decomposition by means 

 of sulphate of copper and carbonate of soda The latter is the only product 

 which is lost ; all the others, the iodine, iodide of potassium, mercury, zinc 

 and copper, are re-obtained and may serve again in the battery, or be useful 

 elsewhere. 



Mr. Doat does not perform the reduction of the zinc and copper except 

 when it can be done on a large scale ; for he then obtains a casting of brass, 

 of greater commercial value. 



A Battery, called a Battery with triple contact. One element of this bat- 



14* 



