LIGHT (OPTICAL) MICROSCOPY 



transmitted light is picked up by a photo- sidered to be the loose aggregates, the "ulti- 



multiplicr tube. A similar result can be mate" particles of which the aggregates are 



achieved by illuminating the sample with, a composed, or even individual mineral grains 



very narrow fixed beam of light and scan- in the ultimate particles, 



ning the sample by automatically moving The term particle size is also usually am- 



the stage. The beam, after passing through biguous unless defined for each type of ap- 



the sample, passes through the microscope plication. Particle size is usually described 



and into a photomultiplier tube. In each in terms of rather artificially defined "diame- 



case the signal from the photomultiplier ters" which generally fall in one of two 



tube is analyzed electronically and presented classes. Diameters of one class, called sta- 



as a size distribution. tistical diameters, are defined in terms of the 



Equipment has been developed for ob- geometry of the individual particles and are 



taining photomicrographs of aerosol parti- determined for a large number of particles, 



cles without having to remove them from Definitions of some statistical diameters are: 



the air or other gas. Actual images of the (1) Martin's diameter: the distance be- 



particles are obtained in one technique, while tween opposite sides of the particle, meas- 



in others diffraction patterns are obtained ured crosswise of the particle, and on a line 



whose intensities are functions of the particle bisecting the projected area. 



sizes. Such methods are especially useful for (2) The diameter of the circle whose area 



obtaining the size distributions of particles is the same as the projected area of the 



such as fog droplets which might be changed particle. 



during a collection process. (3) The shorter of the two dimensions ex- 

 Particles which are smaller than the limit hibited. 

 of resolution of a microscope can be observed (4) The average of the two dimensions ex- 

 as unresolved dots of light using the ultra- hibited. 



microscope. A mean particle size can be de- (5) The average of the three dimensions 



termined by suspending a known weight of of the particle. 



powder in a known volume of some liquid (6) The distance between two tangents to 



and determining the number of particles per the particle, measured crosswise of the field 



unit volume. A hemacytometer- type cell is (for example, of a microscope), and per- 



often convenient for holding the suspension pendicular to the tangents, 



during the counting process. The bottom of Diameters determined by sieving are also 



the cell is ruled off into squares and "snap statistical diameters. 



counts" are made of whether there are none. Diameters of the other class are defined in 



1, 2, 3, etc., particles per square at any mo- terms of the physical properties of the parti- 



ment. cles. Examples are diameters determined by 



Theory and Statistics of Analysis.* sedimentation or elutriation. 

 The word particle, in its most general sense. Numerous definitions of the mean particle 

 refers to any object having definite physical size of a powder may be used, the choice de- 

 boundaries, without any limit with respect pending largely on the use to which the data 

 to size. Particles varying from about 0.01 to are to be applied. It is convenient to define 

 1000 microns are considered in this discus- such means in terms of data which have 

 sion. The term is often ambiguous unless been classified into groups (classes) which 

 carefully defined for a specific case. For ex- are defined by means of particle size limits 

 ample, the particles in a soil may be con- called class boundaries. The midpoint of 

 *From "Encyclopedia of Chemistry Supple- each interval is called the class mark (rf,), 

 ment," Reinhold, New York. 1958, p. 210. the number of particles in each interval is 



466 



