ULTRA-MICROSCOPIC EXAMINATION OF DISPERSE COL- 

 LOIDS PRESENT IN BITUMINOUS ROAD MATERIALS 



By E. C. E. Lord 

 Petrographer, Bureau of Public Roads, United States Department of Agrictilture 



INTRODUCTION 



On a cursory examination of bituminous solutions by means of the 

 ultra-microscope, varying amounts of finely divided solid material held 

 permanently in suspension will invariably be found. In certain hard 

 native asphalts highly dispersed mineral matter is present in large quan- 

 tities, and the high adhesive properties of such asphalts have been at- 

 tributed largely to the selective absorption exerted by these colloids on 

 certain portions of the bitumen.^ Further investigations have led Rich- 

 ardson to conclude that some bitumens were absorbed in larger quantities, 

 and consequently had a greater colloid -carrying capacity than others, 

 and that this variation was apparently in accordance with their viscosity 

 and the general character of the particular bitumen.^ 



As a result of these investigations, it seemed desirable to develop a 

 reliable method of ultra-microscopic analysis whereby the number of dis- 

 perse colloidal particles could be determined accurately in any type of 

 bitumen, thereby furnishing a ready means for comparing their colloidal 

 capacities and at the same time establishing a possible method for esti- 

 mating the relative value of this property from a road-making standpoint. 

 The fact, however, should be emphasized that the present investigations 

 were undertaken essentially to develop a method for counting colloidal 

 particles in bituminous solutions, and that in drawing comparisons of the 

 relative supporting values from the results obtained, the original con- 

 sistency of the materials employed should receive due consideration. 



METHODS OF ULTRA-MICROSCOPIC EXAMINATION 



When examined under the ordinary microscope, the great bulk of the 

 colloidal material common to bituminous solutions is invisible. Early 

 investigations by Siedentopf and Zsigmondy ^ have shown that the re- 

 solving power of the microscope is very greatly increased when particles 

 are viewed in a powerful light against a dark background. This illumi- 

 nation was obtained originally by allowing a beam of light to enter the 

 cell through a narrow slit at right angles to the axis of the instrument, 



' Richardson, Clifford, the theory of the perfect sheet asphalt surface. In Jour. Indus, and 

 Engin. Chem., v. 7, no. 6, p. 463-465. 1915. 



* . importance of the REL.'^TIOX of solid surfaces and liquid films in SO.ME TYPES OP EN- 



Glneering CONSTRUCTION. In Sci. Amer. Sup., v. 83, no. 2152, p. 198-199. 1917. Printed also in sepa- 

 rate {onn. 

 ' Zsigmondy, Richard, erkenntnis der kolloide. 1S6 p. Jena, 1905. 



Journal of Agricultural Research, Vol. XVII, No. 4, 



Washington, D. C. July 15, 1919 



sa Key No. D-16 



167 



