150 RALPH S. LILLIE. 



metal depends in general upon three chief factors; these are, 

 (i) the nature of the second metal, (2) its distance from the 

 point under consideration, and (3) the electrical conductivity 

 of the solution. The electromotive force of the local circuit 

 depends primarily upon the difference between the specific solu- 

 tion-tensions of the two metals; hence under otherwise equal 

 conditions zinc is less effective than manganese, and manganese 

 than magnesium, in inhibiting the formation of filaments from 

 an iron surface. The other two factors, (2) the distance between 

 the contact of the metals and the point in question, and (3) the 

 electrical conductivity of the solution, determine the resistance 

 of the portion of the local circuit which includes that point; 

 this determines the intensity of the current and hence the rate 

 of local electrolysis at the point. The effect is thus greatest 

 near the region of contact and diminishes as the distance from 

 this region increases. 1 



The actual phenomena may now be illustrated by the descrip- 

 tion of typical experiments. In order to show the accelerating 

 influence of a nobler metal, zinc is more satisfactory than iron, 

 for the reason that it can more readily be obtained in com- 

 paratively pure and homogeneous condition, in which state it 

 forms precipitate in ferricyanide solutions very slowly unless in 

 contact with the other metal. It is difficult to procure non- 

 reactive specimens of iron; pure iron wire of the kind used in 

 standardizing (e. g., Baker's labeled 99.7 per cent, pure) forms 

 filaments rapidly in a 2 per cent, solution of KaFeCye in dilute 

 egg-white; the rate of formation is accelerated by encircling 

 the iron with copper or platinum wire, but the degree of accelera- 

 tion is comparatively slight, and the effect is much less striking 

 than when zinc is used. 



Such experiments as the following are easy to perform and 

 invariably give striking results. A small strip, e. g., 2 cm. long 

 by i mm. wide, cut from a bar of pure zinc, is encircled at one 

 end with a ring of fine copper wire, and placed in a 2 per cent, 

 solution of K 3 FeCy 6 in dilute egg-white. Within a minute or 

 two the characteristic thin-walled transparent vesicles appear 



1 Cf. the second paper of my recent series on the physical chemistry of the con- 

 duction-process, Amer. Journ. Physiol., 1915, Vol. 37, p. 348. 



