1 84 ELEMENTARY LESSONS ON [CHAP. in. 



green vitriol, while each piece apart produces no such 

 sensation." This galvanic taste, not then suspected 

 to have any connection with electricity, may be ex- 

 perienced by placing a silver coin on the tongue and a 

 steel pen under it, the edges of them being then brought 

 into metallic contact. The same taste is noticed if the 

 two wires from the poles of a voltaic cell are placed in 

 contact with the tongue. 



228. Ritter discovered that a feeble current trans- 

 mitted through the eyeball produces the sensation as of 

 a bright flash of light by its sudden stimulation of the 

 optic nerve. A stronger current transmitted by means 

 of moistened conductors attached to the battery terminals 

 gave a sensation of blue and green colours in flowing 

 between the forehead and the hand. Helmholtz, re- 

 peating this experiment, observed only a wild rush of 

 colour. Dr. Hunter saw flashes of light when a piece 

 of metal placed under the tongue was touched against 

 another which touched the moist tissues of the eye. 

 Volta and Ritter heard musical sounds when a current 

 was passed through the ears ; and Humboldt found a 

 sensation to be produced in the organs of smell when a 

 current was passed from the nostril to the soft palate. 

 Each of the specialised senses can be stimulated into 

 activity by the current. Man possesses no specialised 

 sense for the perception of electrical forces, as he does 

 for light and for sound ; but there is no reason for denying 

 the possibility that some of the lower creatures may be 

 endowed with a special electrical sense. 



The following experiment shows the effect of feeble 

 currents on cold-blooded creatures. If a copper (or silver) 

 coin be laid on a piece of sheet zinc, and a common 

 garden snail be set to crawl over the zinc, directly 

 it comes into contact with the copper it will suddenly 

 pull in its horns, and shrink in its body. If it is set to 

 crawl over two copper wires, which are then placed in 

 contact with a feeble voltaic cell, it immediately an- 



