39 



of fall in air, clue to gravity is about 11 cms. per hoar, but 

 convection currents interfere with the observation in highly disperse 

 smokes. 



3. Diffusion Constant.— The value of the diffusion constant D in 



TIT 1 



Sutherland's^^ formula D = j^-^—,- (where R =8-316 x 10' 



N = 606 X 10^^) may be substituted in the Einstein equation 

 X = v'2Dt for the displacement x due to BrowTiian Movement. 

 This gives a simplified equation d = 4-7 x 10 -^^ t/x^ for calculating 

 the diameter d of a particle in air as continuous phase at room 

 temperature (20° C.) when the mean displacement x in time t is 

 measured. This displacement was recorded photographicallj' by 

 De BrogUe^, but such records are reliable only if a photographic plate 

 moving at a known rate be used. Owing to the impossibility of 

 excluding convection effects this method is not recommended for 

 measurmg the size of granules in highly disperse gaseous systems. 



4. Ultra- Microscopy. — Smokes are conveniently observed in the 

 Jentzsch or " slit " ultra-microscope, but not in any of the others. 

 Precaution should be taken to prevent disturbance due to convection 

 and heating by using a heat filter (ferrous-ammonium sulphate 

 solution 1 : 5) in the path of the beam. 



5. Oscillation in an Alternating Electric Field. — If the particles are 

 charged, they will move in an electric field, according to Stokes' Law, 

 wdth a uniform mean velocity. 



V= ^^ 



3 77 7J d 



where X is the field in volts per centimetre and e the electronic charge 

 (c./. Hevesy^^). By measuring the velocity and the field the diameter 

 of any individual particle is determined. This method has recently 

 been appUed by WeUs and Grerke^^ to the examination of smokes. 

 By means of a rotating commutator the particles were caused to 

 oscillate, and their motion was observed, either photographically or by 

 means of a micrometer eye-piece, in a modified Zsigmondy ultra- 

 microscope constructed for this purpose. Since the motion due to 

 convection was perpendicular to the oscillation a zig-zag line was 

 obtained. It was thus possible to measure the oscillation ampUtude 

 which, multipUed by the frequency of the field reversal gave the 

 velocity. By varying the field and determining the corresponding 

 velocities good agreement was obtained for the diameter of a tobacco 

 smoke particle (2-73 X 10"* cms.). The method can be used for the 

 measurement of the size of individual particles in a non-uniform 

 disperse phase. This represents a distinct step forward, von 

 Weimarn proposes to construct an ultra- microscope with quartz or 

 fluorite lens and to use ultra-violet Ught, which by reason of its short 

 wave length wiU increase the intensity of scattered Ught. With such 

 an instrument, using the photographic plate method of WeUs and 

 Gerke", it should be possible to make stiU further progress into the 

 realms bordering on the molecular. 



