416 WELLS'S NATURAL PHILOSOPHY. 



change, and the copper will be preserved. Sir Humphrey Davy attempted to 

 apply this principle to the protection of the copper sheathing of ships, by 

 placing at intervals over the copper small strips of zinc. The experiment 

 was tried, and a piece of zinc as large as a pea was found adequate to pre- 

 serve forty or fifty square inches of copper ; and this wherever it was placed, 

 whether at the top, bottom, or middle of the sheet, or under whatever form 

 it was used. The value of the application was, however, neutralized by a 

 consequence which had not been foreseen. The protected copper bottom 

 rapidly acquired a coating of sea-weeds and shell-fish, whose friction on the 

 water became a serious resistance to the motion of the vessel, and it was dis* 

 covered that the bitter, poisonous taste of the copper surface, when corroded, 

 acted in preventing the adhesion of living objects. The principle, however, 

 has been applied wilh success to protect the iron pans used in evaporating 

 sea- water. 



CHAPTER XVII. 



TnERMO-ELECTRICITT. 



What is Ther- ^86. If two dissimilar metallic bars be sol- 

 mo-eiectricityr ^^^^^ together, and heated at the point of 

 junction, an electric current will circulate through them, 

 and may be carried oiF by connection with any good con- 

 ductor. Electricity thus generated or developed is called 

 Thermo-electricity. 



Thus, if two bars, one of German silver and the other of brass, as repre* 

 sented in Fig. 344 (the dark one being the brass), be heated at their junction, 

 FiQ. 344. ^^ electric current will flow in the direction of the 



arrows from the German silver lo the brass. 



Different degrees of temperature, also, in the same 

 metal, will occasion an electric current to flow from 

 the colder to the warmer portions. 



The properties of thermo-electricity 

 are the same as those of ordinary electricity. 



The metals best adapted for showing its effects are 

 German silver, bismuth, brass, iron, and antimony. 



„ ^. Thermo-electric batteries of considerable power maybe con- 



How are tner- ^ '' 



mo-electric bat- structed by combining together alternate plates of German silver 



Btrm!ted» *^°'^" ^^^ brass, or bismuth and antimony, thick cards of pasteboard 



being so placed between the plates, that a contact of tho 



metals is prevented, except at the ends. Such a battery, represented by Fig. 



