34 



unit volume in solid solutions gives the total mass of submicrons 

 and also amicrons* present. Since the amicrons are not included 

 in the number of particles counted, their mass should be subtrated 

 in order to obtain the true concentration of the submicrons. For 

 solid solutions there is no satisfactory method of correcting for this 

 error, but with hydrosols it is possible to separate the amicrons by 

 ultra-filtration, and it is found that the error they introduce does 

 not affect the order of the magnitude, and is seldom more than 50 per 

 cent. The more recent ultra-microscopes cannot be used for the 

 examination of soUd substances. 



II. — Continuous Phase Liquid. 

 (a) Microns. 



1. Filtration. — For the rapid grading of microns filtration through 

 special earthenware fUters is convenient, this was the method of 

 Linder and Picton^^ (1892). Bechold^ extended the method by 

 preparing a series of graduated filter papers which are standardised 

 for known solutions, and used under pressure in a special apparatus. 



2. Stokes' Law. — The viscous resistance of spheres of radius " r " 

 density " p " moving with uniform velocity " v " in a medium of 

 density p^ and viscosity t) is given by Stokes' Law F = Gvr'^rv. This 

 Law was first appUed to coUoidal solutions by Barus and Schneider 

 (1891) to measure the radius of particles precipitated by electrolytes. 

 Since the force acting is gravity " g " the equation becomes — 



r= /g JK- 



It was shown by Perrin^" that Brownian Movement does not 

 interfere with the mean velocity of the particle in coUoidal suspensions 

 and solutions so that, assuming they are spheres, we have a ready 

 means of determining accurately their size from diameters of 1 mm. 

 to and beyond the microscopic limit. 



(b) Sub-Microns. 



Methods may be classified as (1) ultra-fUtration ; (2) ultra- 

 microscopy; application of single reflecting condensers, using 

 (3) Stokes' Law ; and (4) density of distribution ; (5) double reflecting 

 condensers for direct measurement; (6) improved ultra- microscopy ; 

 application of (7) light absorption; (8) Rayleigh Formula; (9) 

 diffusion constant. 



1. Ultra- Filtration. — Bechold's method (referred to under Microns) 

 may be used but the prepared papers, owing to the negative charge 

 in the capillaries, cause adsorption and precipitation of the positive 

 coUoid. 



2. Ultra - Microscopy. — The Zsigmondy - Siedentopf apparatus 

 (described above) was adapted for the examination of liquids and 

 gases by introducing an observation cell. This cell — improved by 



* The term amicron is used for particles which cannot be detected in the 

 ultra-microscope. 



