July IS, 1919 Disperse Colloids in Bituminous Road Materials 1 73 



of concentric halos or diffraction rims around each particle that de- 

 tracted greatly from the definition of the images. In order to overcome 

 this so far as possible, it was found necessary to employ cells below 

 o.io mm. in depth, having a capacity less than o.io cmm. (Table I, 

 No. 3, 5, and 6). Counts were made of all particles in suspension as 

 well as those that might have settled out on the cell floor or become 

 attached to the cover glass during the process of counting. The area 

 examined was taken from within the central millimeter circle of the 

 cell and represented exactly one-fourth of i square millimeter (equiva- 

 lent to four fields of the cross-line eyepiece micrometer at a magnifi- 

 cation of 320 diameters), while the volume of liqtiid was obtained from 

 this area and the depth of cell employed (0.043-0.089 mm.). The 

 number of particles counted in each of the 25 square subdi\asions of 

 the micrometer through the entire depth of liquid was recorded, and 

 from the average of four such determinations the value for i cmm. of 

 solution and i gm. of bitumen was computed. 



The results of the analyses were recorded on a special form which, 

 in addition to the data indicated above, contained information regarding 

 the physical properties of the bitumen, together with the relative size 

 and distribution of the colloidal particles. In general, it may be stated 

 that these particles varied in size from submicrons having a minimum 

 diameter of about 15/^^1 (0.000015 rnm) to particles within the visibility 

 of the ordinary microscope (above 0.25 ^ = 0.00025 mm.).^ These 

 dimensions may be determined by direct microscopic measurement or 

 they may be calculated by dividing the total volume of particles con- 

 tained in a definite quantity of solution by the number of particles 

 found where the volume represents the weight of the particles divided 

 by their specific gravity. The quotient thus obtained will equal the 

 volume of one particle (x). Assuming the particles to be spheres of 

 diameter a, 



Then - t: a^ = x 



3 



3/ X 



RESULTS OF THE ULTRA-MICROSCOPIC EXAMINATION OF 

 BITUMINOUS SOLUTIONS 



In order to standardize the method of ultra-microscopic analysis 

 outlined above, a number of determinations were made of various 

 colloidal materials contained in different types of bitumens. The 

 results of these determinations are shown in Table II. 



' ZsiGMONDY, Richard, op. cit., p. S8b-97. 



