SCIENCE AND PRACTICE. 229 



of the time, and the periods of closed circuit are not of long 

 duration, this inconstancy is of little importance, for which 

 reason Stohrer considers it well adapted to the general work 

 of telegraph lines, particularly as it continues in good order 

 for a very long time if treated with a little fresh alum water 

 about once a month. 



11. Marie Davy's Proto-sulphate of Mercury Battery. 

 Davy discovered the greater electro- motive force resulting 

 from the decomposition of a mercury salt at a carbon plate 

 than that of a copper salt at a copper plate, as in DanielPs 

 arrangement. His battery is extensively used now in the 

 French and in some of the English telegraph bureaux, where 

 it is preferred to Daniel!' s on account of its greater electro- 

 motive force, and the comparative ease with which it is kept 

 in order. 



The element is composed of a zinc-plate, immersed in pure 

 water, separated by a porous diaphragm from a carbon plate 

 immersed in a paste of proto-sulphate of mercury and water. 

 The internal action is similar to that of Daniell's ; the 

 mercury salt is decomposed by the current, metallic mer- 

 cury being precipitated upon the carbon. 



These elements are commonly arranged in France in boxes 

 of ten ; the zinc cylinders packed in sponge, which holds 

 the water and retards its evaporation ; and the porous 

 pots covered with cork, by which the carbon plates are 

 suspended in the salt. The battery requires very little 

 attention, and according to the inventor's statement, will 

 retain its original force for six months. The intensity which 

 a battery of these elements may have is limited in practice, 

 because if the strength of the current exceeds a certain 

 amount, the dissolution of the zinc goes on faster than the 

 decomposition of the salt, and therefore there is an insuf- 

 ficiency of oxygen liberated to combine with the hydrogen, 

 which, collecting upon the carbon, weakens the proper 

 polarisation of the system ; or, the mobility of the sulphate 

 of mercury paste being small, the salt becomes quickly 

 exhausted in the immediate vicinity of the carbon plates, 

 and is replaced by a stratum of water. This is a phenomenon 



