360 



SCIENCE 



[N. S. Vol. LIV. No. 1398 



change of density of 133 parts per million, 

 or 0.02Y unit of atomic weight. 



The evidence that a separation has actually- 

 been obtained rests in the quantitative agree- 

 ment between our results and those of Bron- 

 sted and von Hevesy, with respect to the rate 

 of separation (efficiency of process). If we 

 consider the efficiency of our more ideal ap- 

 paratus as 100 per cent., that of the other 

 investigators is 75 per cent, while that of our 

 less ideal apparatus used in the greater part 

 of the work in order to save the expense of 

 carbon dioxide as a cooling agent, was 93 

 per cent, when the vaporization was slow, and 

 as low as 80 per cent, for a rapid vaporiza- 

 tion. We have obtained evidence that there 

 is a slight separation of isotopes produced 

 when mercury is distilled slowly at a suffi- 

 ciently low pressure. 



The rate of separation of two isotopes 

 varies as the square of the difference of their 

 atomic (or molecular) weights, and the prod- 

 uct of their mol fractions, as the logarithm 

 of the cut, and inversely as the atomic (or 

 molecular) weight. 



A diffusion coefficient has been calculated 

 to represent the relative separation of isotopes 

 attained in terms of the atomic weight change, 

 when a definite cut is made. The values are 

 0.00843 for neon, 0.00868 for magnesium, 

 0.00450 for lithium, 0.00Y58 for nickel, while 

 the experimentally determined coefficient for 

 mercury is 0.00570. For chlorine the coeffi- 

 cient is 0.00950 for hydrogen chloride, 0.00690 

 for methyl chloride, 0.00494 for chlorine, 

 0.00413 for methylene chloride, 0.00295 for 

 chloroform, and 0.00229 for carbon tetrachlo- 

 ride. 



It is of interest to note that there are 9 

 isotopic forms of MgCl, (or more if there 

 is a chlorine of atomic weight equal to 39), 

 7 of CgClg, and if mercury consists of 6 

 isotopes, there are 63 isotopic forms of 

 Hg.,Cl„. In addition to this most of the 

 isotopic forms of Gfi\ consist of a number 

 of space isomers. 



William D. Haekins 

 TjNrvERsiTT OP Chicago, 

 August 30, 1921 



AN ARTIFICIAL NERVE 



Physiologists are keenly interested in all 

 attempts to discover an explanation or an 

 analogy for the passage of the nerve stimulus. 

 Most enlightening suggestions have recently 

 been presented by Lillie^ in his studies of 

 passivity phenom'ena in pure iron wires. It 

 seems that the transmission of the momen- 

 tary wave of activity which occurs in a pas- 

 sive iron wire on activation in 70% nitric 

 acid is closely analogous both chemically and 

 electrically to the passage of the nerve im- 

 pulse. 



The general similarity of the two phenom- 

 ena was apparently first noticed by Wilhelm 

 Ostwald and subsequently elaborated by his 

 student Heathcote.- In a paper published in 

 1907 under the caption " Transmission along 

 a nerve " (p. 909) Heathcote writes as fol- 

 lows: 



In 1900, then, Prof. Ostwald called our attention 

 to the possibility of nerve transmission being a 

 process akin to the transmission of activity. . . . 

 It is to be expected . . . that transmission of ac- 

 tivity would be slower immediately after the first 

 transmission owing to products of reaction around 

 the iron. This has been confirmed by direct ex- 

 periments in the case of iron in nitric acid. An 

 eifect of this kind in a nerve would explain the 

 nature of ' ' fatigue ' ' so far as it concerns nerves. 



After discussing the small amount of energy 

 consumption in both transmissions Heathcote 

 summarizes his conclusions as follows: 



There is nothing in the structure of nerve which 

 renders it impossible to regard transmission as 

 occurring in a way which is analogous to the 

 transmission of activity along passive iron. ... It 

 appears possible too that the network in proto- 

 plasm may be a layer capable of transmitting 

 changes in a similar way and which manifest them- 

 selves as an essential part of the mechanism of 

 irritability. 



It is not surprising that Heathcote's paper 

 should have escaped the attention of physiolo- 



iLillie, E. S., '18, Science, 43, 51; '20, J. 

 General Physiol, 3, 107. 



2 Heathcote, H. L., '07, J. Soc. Chem. Indus- 

 tries, 26, 899. 



