192 life's beginning on the earth 



that the electric charge can travel along a lead wire. We 

 can charge the wire at one end, but this charge will not 

 travel on. The solution of the problem of the nerve im- 

 pulse was greatly handicapped by this failure, as well as the 

 futility of other hypotheses propounded until, in 1918, 

 Dr. R. S. Lillie, now a professor at the University of Chi- 

 cago, demonstrated the correct way of carrying out the 

 experiment. All we have to do, according to Lillie's sug- 

 gestion, is to substitute an iron wire for a lead wire in the 

 set-up and, instead of moistening it with sulfuric acid, im- 

 merse it in a narrow vertical tube filled with nitric acid 

 solution containing 55% of that acid. (A previous treat- 

 ment of the iron wire is also necessary : it is immersed for 

 a short time in concentrated acid.) 



If such a wire is charged at one end in the manner indi- 

 cated in figure 76, the charge will actually travel along the 

 wire. The electric negative charge of an iron wire mani- 

 fests itself through visible chemical changes: a darkening 

 of the formerly bright surface of the wire, and a develop- 

 ment of small bubbles of hydrogen gas. 



These visible changes are seen to sweep along the surface 

 of the wire. Here we have a propagation of an electrical 

 wave that is of the same type as that which travels along 

 the nerve. It is not an electrical current passing through 

 the wire. If it were, it would pass with a velocity far 

 greater than our vision could follow. We can see that it 

 moves at the rate of a few inches a second, which is the aver- 

 age velocity of the propagation of the nerve impulse. In 

 many other respects, this traveling charge resembles the 

 nerve impulse. The more we study it, the more similarities 

 we find, so that we are finally forced to the conclusion that 

 both are identical in type. 



Dr. Lillie has demonstrated a close similarity between 

 the traveling charge on the iron wire and the nerve impulse 

 with respect to the following details: 



