July IS. 1919 Disperse Colloids in Bituminous Road Materials 1 69 



PREPARATION OF THE ULTRA-MICROSCOPE CELL 



In order to carry out a quantitative analysis of bituminous solutions 

 it was found necessary to employ a cell of minimum capacity that might 

 be readily cleaned and hermetically sealed to prevent the escape of the 

 volatile solvent. Efforts were made to utilize a container constructed 

 on the principle of the Zeiss haemocytometer, as employed by Burton 

 and Perrin in their examination of colloidal water solutions (hydrosols)/ 

 but it was found that all types of cement used in constructing this cell 

 were attacked by the benzol solution and, furthermore, that the rulings 

 on the bottom of the cell when filled with the solution were almost 

 invisible under the microscope.^ 



In order to overcome the above-mentioned difficulties, it was found 

 necessary to excavate a suitable cavity in the object glass itself, thereby 

 doing away entirely with the superimposed glass plates of the haemo- 

 cytometer slide. The object glass selected was as free as possible from 

 air bubbles and other inclusions and had perfectly smooth plane sur- 

 faces and a thickness varying from 1.25 mm. to 1.75 mm. to assure 

 a proper focus within the cell of rays from the dark held illuminator, 

 he excavation was carried out by means of a stationary upright drill 

 provided with a pointed vulcanized liber cylinder having a flat grinding 

 surface about 2 mm. in diameter. The drill was run by an electric 

 motor at i,Soo revolutions per minute, using coarse emery mixed with 

 a little heavy lubricating oil as an abrasive. In operating the drill 

 great care was taken to apply a moderate uniform pressure, and the 

 glass plate was protected from sudden strain by a folded towel or 

 felt cushion placed beneath it. After grinding for one or two minutes 

 the drill was removed and the cavity examined. In general, the central 

 portion was found to be essentially flat and surrounded by deeper cir- 

 cular grooves, produced by the larger fragments of emery becoming 

 lodged in the drill during the process of grinding. 



From this stage in the operation the grinding was carried on by 

 means of an electrically driven, flexible shaft drill constructed on the 

 principle of the dental drill and using volcanic ash or ground pumice 

 with water as an abrasive. This drill, operating at a speed of 1,540 

 revolutions per minute, was provided with a grinding point of vulcanized 

 rubber or belata gum which also proved very effective in polishing the 

 cell. The polishing was begun with diatomaceous earth and water and 



1 Burton, E. F. op. cit., p. 118-120. 



Perrin, jean, mouvement brownien et realite molecul-mre. In Ann. Chim. et Phys., s. 8, t. 

 18, p. 40-42. 1909. 



2 The indices of refraction for ordinary light flint glass and benzol at 21.5° C. are 1.5710 (D) and 1.5304 

 (H), respectively. Smithsonian physicai, tables, ed. 6, p. 184, 192. Washington, D.C, 1914. (Smithsn. 

 Misc. Collect., v. 63, no. 6.) Hence light passing through glass and meeting etched lines on a cell bottom 

 mounted in benzol are but slightly diffracted and consequently appear indistinct under ultra-microscopic 

 illumination while plainly visible when viewed in water or air. 



