332 



Prof. W. M. Thornton. 



[Jan. 12, 



more freely than before. Lortet's observations would be at once explained if 

 his emulsion had been sterilised by the addition of an ionising liquid, such as 

 perchloride of mercury, the conductivity of which is greater than that of the 

 germs. The fact is that dead or living bacteria orientate equally well. 



[April 24th. — In this paper the cause of the observed orientation is con- 

 sidered to be the influence of the electric field upon charges induced on the 

 surface of the bacteria. The surface density a of charge on the interface 

 between two media of resistivities pi, p 2 , and dielectric constants k h k 2 , with 

 a current density i normal to the surface, is given by the relation 

 47TO- = (kipx — k 2 p 2 )i. Thus, when k is about 80, as for water, p 100 for 

 saline solutions, and i 1 ampere per square centimetre, a is of order unity.* 

 This is comparable with the surface density on metallic conductors charged 

 to several thousand volts in air, and is much greater than that on a surface 

 between good conductors. 



The dielectric constants of saline solutions have still to be experimentally 

 determined ; the product kp can, however, be found from existing data in 

 terms of the refractive index. The dispersion terms in the Helmholtz- 

 Ketteler formula for the dispersion of light contain the number of electrons 

 N" in unit volume as a factor, and since electrical conductivity is also 

 proportional to jST, the sum of these terms can be written b/p in low frequency 

 fields. 



Thus when the formula is applied to solutions of varied conductivity 

 n 2 = k + b/p, or kp = n 2 p — b, n being the refractive index. From tabulated 

 dataf the conductivity of saline solutions is nearly proportional to the 

 percentage g of added salt, and we may write gp = a, a constant. 

 Schiitt has shownj that for these solutions the change of refractive index 

 is also proportional to the added salt, so that (n — n )/g = a constant, c. 

 Thus p = ae/(n—n ) and kp = acn 2 /(n—n Q )—b. 



The values of n 2 /(n—n ) for different strengths of solution are as 

 follows : — 



g. n*/(n-n ). 

 1 1010 

 5 204 

 10 103 

 30 36 

 The product kp therefore decreases as salt is added. In the expression 

 for a the current density is proportional to the strength of the solution, so 

 * See Jeans, ' Electricity and Magnetism,' p. 336. 

 t See Whetham, 'Theory of Solution,' p. 413. 

 X See Landolt and Bornstein, ' Tabellen,' p. 684. 



