268 Messrs Burton and Phillips, Susceptibility of Iron, etc. 



This leads us to enquire as to the force which a given mag- 

 netic field would exert on the colloidal solution if the particles 

 were small spheres of pure iron. 



If n = number of particles per unit volume, 



A = cross-section of the cell, 



M — magnetic moment of a small spherical particle, 



then the pull dP exerted by the magnetic field H on an 

 elemental horizontal layer of thickness dz would be 



dP = — dz M -v grams. 

 g dz & 



Now M=Iv, where / is the intensity of magnetisation in the 

 sphere and v is the volume of the sphere. Substituting this value 

 of M in the expression for dP and integrating we have 



A [ s * A 



P = nv . — I / dH . grams = nv — {H 1 — H 2 ) 1650, 

 9 J n, g 



for i~ may be taken at its limiting value about 1650. vn is 

 equal to the volume of iron per c.c. of the colloidal solution, 



44 x 10 -6 



i.e. 5 c.c.s. A as before is 2'63 sq. cms. Supplying these 



values in the expression for P and neglecting H 2 in comparison 

 with .//j (= 10000) we have P = "243 grams. Since the actual 

 pull exerted by this field on the cell, full of colloidal solution, was 

 *0870 — '0665 — *0205 grams, it is quite evident that the particles 

 do not consist wholly of pure iron. 



These results lead to the following alternative conclusions 

 regarding the state of iron in colloidal solution : (1) the particles 

 in suspension are made up uniformly of iron which is in a con- 

 dition in which it has magnetic properties different from those of 

 iron in any state of combination which has been studied ; or 

 (2) each particle consists of a core of pure iron surrounded by 

 a layer of some compound of iron, e.g. hydroxide. 



In conclusion we wish to acknowledge our extreme indebted- 

 ness to Prof. Poynting for so kindly placing at our disposal the 

 resources of the Physical Laboratory at the University of Bir- 

 mingham, and we desire to express our thanks to Mr G. F. C. 

 Searle for some helpful suggestions. 



