ELECTRIC LIGHT. Ill 



live character from the wire, and will not, therefore, 

 take positive metal from the solution. This is obvia- 

 ted by covering the block with a fine powder of plum- 

 bago or black lead, which has high conducting power. 



267. This process is very extensively 



It what cases r / 



is the process practised. Where a large number of cuts 

 pra of the same kind are wanted, as for exam- 



ple, to print labels for dry goods, only one engraving 

 on wood is made, and numerous copies are taken by 

 the above process, which is much less costly. 



268. HEATING EFFECTS OF THE CUR- 



Descrtbe the 



heating e/ect RENT. If the electrodes are connected 

 of Recurrent? while the b attery j s j n actlO n, the wire be- 



comes heated more or less strongly, according to the 

 size of the plates. If the plates are very large, the 

 wire melts, even though it be of platinum, the most 

 infusible of metals. Gold may even be converted in- 

 to vapor by the same means. Carbon, supposed a few 

 years since to be entirely infusible may be also super- 

 ficially fused, and even volatalized between the electro- 

 des. It condenses again at a little distance, in the form 

 of microscopic crystals. Imperfect diamonds have been 

 thus artificially produced. With such a battery as 

 has been described the elevation of temperature would 

 be scarcely perceptible. 



269. THE ELECTRIC LIGHT. If the current 



How is the . 



electric light be allowed to pass between two points of 

 produced ? prepared charcoal, an exceedingly intense 

 light is produced, accompanied by great heat. Char- 

 coal is employed because it is a comparatively infu- 

 sible, and inferior in conducting power. A metallic 



