314 LOCALIZED ELECTRIZATION. 



The electro-dynamic instruments of moderate force, even when 

 they are graduated to a minimum, are still too powerful for certain 

 delicate applications, such as faradization of the chorda tympani. 

 I have desired the makers to add to these instruments a tube con- 

 taining water, which I have called a moderator (see F, fig. 52), 

 which is designed to reduce the most powerful apparatus to one 

 of extreme feebleness, which can then be graduated by means of 

 the metallic cylinders. 



D. — Battery. — The inconveniences connected with the use of the 

 battery, in the electro dynamic instruments, are such, that they have 

 induced many to prefer the electro-magnetic instruments. Practi- 

 tioners recoil from the more or less tedious and disagreeable mani- 

 pulation that is requii-ed ; and the least neglect or awkwardness 

 may occasion more or less serious injury to the instruments. For 

 this reason, makers have endeavoured to remove or to diminish 

 these inconveniences ; and, on this part of the subject, I have to 

 record some progress. 



(a). The numerous disadvantages of nitric acid, principally on 

 account of the disengagement of nitrous gas, which attacks the 

 metallic portions of the instruments, have caused the rejection of 

 Bunsen's battery by the majority of makers. 



MM. Legendre and Morin have continued its employment for 

 their instrument, because it is much more powerful than other 

 batteries (such as Daniell's, or the bisulphate of mercury of 

 Marie-Davy), and, while giving a notable quantity of electricity, it 

 may be prepared several hours in advance, will remain several 

 hours in action, and, lastly, may be dismantled at leisure. These 

 makers have, indeed, removed the inconveniences due to the dis- 

 engagement of nitrous gas, by adopting to their battery (fig. 85) 

 a cover of gutta percha, B, which closes the whole system almost 

 hermetically. Notwithstanding this improvement, MM. Legendre 

 and Morin should abandon the Bunsen's battery, or they will see 

 their little instrument lose, especially in hospitals, the favour 

 which it has hitherto enjoyed, and which it, in other respects, 

 merits. 



(J). The battery of Marie-Davy (carbon, zinc, and solution of 

 bisulphate of mercury) may advantageously replace that of Bun- 

 sen, as being free from the inconveniences which I have described. 

 It is for this reason that I have employed it for the flat battery of 

 my volta-faradic apparatus. 



The majority of makers have followed this example, and they 

 have constructed the elements in a flat form, similar to that which 

 I have used during many years in my instruments; the carbon 

 elements being below, covered with powdered bisulphate of mercury, 



