GALVANISM 



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Thou knowest not ! 'tis doubt, 'tis darkness all ! 

 E'en here on earth our thoughts benighted stray, 

 And all is mystery through this worldly ball — 

 Who then can reach or read yon milky way ? 

 Creation's heights and depths are all unknown, untrod ; 

 Who then shall say how vast, how great, creation's God? 



Questions. — 1. Why is it supposed that those meteoric appear- 

 ances called falling stars owe their origin to electricity ? 2. How may 

 the analogy between electricity and the water-spout be made visible ? 

 3. Describe the northern light. 4. What is Dr. Franklin's idea of it ? 

 [Note. A similar light calitd aurora australis has been long since ob- 

 served towards the south pole.] 



LESSON 70. 



Galvanism. 

 Mus'cle, the fleshy fibrous part of an animal body. 

 Galvanism is another mode of exciting electricity. In 

 electricity the effects are chiefly produced by mechanical 

 action ; but the effects of galvanism are produced by the 

 chemical action of bodies upon each other. This branch 

 of philosophy has been denominated galvanism, from Galva- 

 ni, an Italian professor, whose experiments led to its dis- 

 covery. In 1789, he was by accident led to the fact of 

 electricity having the property of exciting contractions in 

 the muscles of animals. After having observed that com- 

 mon electricity, even that of lightning, produced vivid 

 convulsions in the limbs of recently killed animals, he ascer- 

 tained that metallic substances, by mere contact, under par- 

 ticular circumstances, excited similar commotions. He 

 found it to be essential that the forces of metals employed 

 should be of different kinds. He applied one piece of metal 

 to tlje nerve of the part, and the other to the muscle, and 

 afterwards connected the metals, either by bringing them 

 together, or by connecting them by an arch of a metallic 

 substance ; every time this connexion was formed, the con- 

 vulsions took place. The greatest muscular contractions 

 were found to be produced by zinc, silver, and gold. A per- 

 son may be made sensible of this kind of electric action by 

 the following experiments. If he place a piece of one metal, 

 as a half crown above, and a piece of some other metal, as 

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