38 



objective and condenser near enough to enable the point of focus of 

 the condenser to be observed through the objective set at right angles. 

 The hydrosol to be examined is introduced into the small space 

 between the front lens of condenser and objective, in fact a drop 

 will remain suspended in this position, so that the path of the beam 

 through the solution can be seen. For convenience a smaU ebonite 

 cup is used to hold the drop in position and this cup is so arranged 

 that fresh hydrosol or water can be rapidly brought into the field by 

 opening a spring clip. The instrument is thus an immersion ultra- 

 microscope with a dark field superior to any other. The illumination 

 is b}' sunlight or arc lamp and the dimensions of the beam are 

 accurately defined by a special micrometer sht and eye-piece so that 

 particles of gold of 3ju./x are countable and smaller particles are visible. 

 This apparatus is by far the best for the examination of hydrosols* 

 and no difficulty is experienced when using viscous or jelly-like 

 hydrosols. It has the disadvantage that, unlike the Cardioid or 

 Jentzsch thread-like structures and bacteria are observed as points of 

 light. Observations with this instrument have been reported by 

 Zsigmondy" ; Zsigmondy and Bachmann^i, and King^". 



7. Apj)Iication of Light Absorption. — The investigations of Garnett^* 

 and Mie" should be referred to but so far insufficient is known of the 

 fundamental relationship to enable the size of the particle to be 

 determined by this means (c./. however Voigt*^, Svedberg, Ivruyt^^. 



8. RayleigVs Formula. — See under SoUd Continuous Phase, (b) 1. 



9. Diffusion Constant. — The Sutherland*^ Einsteini^ equation will 

 be discussed later under gas as continuous phase. The equation has 

 been appHed to measurements obtained in highly disperse liquid 

 systems c.f. Dabrowsldi", Svedberg, but the method is too tedious 

 to be of praptical value. 



III. — Continuous Phase-Gas (Smokes). 



(a) Microns. 



On accoimt of the rapid Broiniian Movement direct microscopic 

 observation is impossible, unless the particles are collected on a 

 microscope slide. Stokes' Law may be applied to gases up to the 

 microscopic Hmit. 



(b) Sub-Microns. 



The methods are classified as (1) Rayleigh's Law {see above, (b) 1); 

 (2) Stokes' Law; (3) Diffusion constant; (4) Ultra-microscopy; 

 (5) Oscillation in alternating field. 



2. StoJces' Law. — The formula may be used for particles greater 

 than 10~* cms. diameter, granules below this size are comparable 

 with the mean free path of the molecules of the gas and Cunningham's* 

 correction factor must be apphed. For particles 10~* cms. the rate 



* In 1914, when the author was associated with Zsigmondy in the development 

 of this instrument, only aqueous solutions could be used, since the cement used 

 for fastening the front lens was not chemically resistant. The instrument is 

 now made by R. Winkol, Gottingen. 



