TREATMENT OF BATTERY, 667 



bottle, during which operation the evolution of fumes is worst, 

 should in any case be done in the open air. 



The bottle with the used nitric acid should be kept in a place 

 where nitrous fumes can do least harm, for the stopper is apt to be 

 lifted by the pressure of the nitrous acid gas which is given off by 

 the acid, and the gas escapes. A cork must never be used for 

 closing the bottle ; it is soon destroyed by the action of the acid, 

 and as long as it still closes the bottle so tightly that the gas 

 cannot lift it, there is great danger of the bottle bursting by the 

 pressure of the gas within. 



Such of the clamps, copper strips, or pieces of wire as show a 

 deposit of verdigris upon their surface, due to the action of the 

 acids or acid fames, must be well polished with fine emery-paper 

 after they have been washed and dried. 



In consequence of the high price of platinum, the platinum foils 

 used for batteries are very thin. They require a somewhat careful 

 treatment, for they are easily torn or apt to become crumpled. A 

 creased foil is made red hot in the ;flame to soften it, and placing it 

 upon the table it is smoothed by the finger-nail. The carbon 

 pieces of Bunsen's battery are not liable to this disadvantage, but 

 they require on the other hand some amount of trouble in washing 

 them thoroughly Out, and thereby always cause the loss of some 

 acid which has penetrated into the porous mass. 



The carbon plates are made of a very hard, dense variety of 

 carbon, which is deposited in gas-retorts during the process of 

 manufacture, and is designated as ' graphitoidal ' carbon. The 

 carbon is also to be had in the form of hollow or solid cylinders 

 made of an artificially prepared calcined mixture of coke and bitu- 

 minous coal, which is first finely powdered and then strongly com- 

 pressed. These are not so dense as those previously mentioned, 

 and the cylindrical form is also less convenient for the attachment 

 of the connecting parts. It is therefore not advisable to purchase 

 them, and still less any of the various combinations of elements 

 constructed with a view to avoiding the inconveniences of Grove's 

 and Bunsen's elements, or recommended as being cheaper than 

 these. All these elements either produce a much feebler current, 

 for example, the so-called chromic acid batteries, or they are still 

 more inconvenient than those described, as for instance, the zinc-iron 

 elements. 



Two Grove's or Bunsen's elements are absolutely required for the 

 experiments which will be described in the following articles. If 

 the student can afford more, four or six, the experiments may be 

 performed on a somewhat larger scale. , 



