﻿404 Mr. H. D. Murray on Influence of Size of Colloid 

 Concentration of Mastic. 



Thirty c.c. of the three solutions were evaporated slowly 

 to dryness in a steam oven, and, as a mean of several deter- 

 minations, gave the following weights of mastic in 10 c.c. of 

 solution : — 



Weight found. 



Solution B -00463 gms. 



Fr. VI -00171 „ 



Fr. II -00339 „ 



Number of Particles. 



A true ultramicroscope was not used to count the particles, 

 but a cardioid condenser, fitted to an ordinary microscope. 

 The chief difficulty in work of this nature is to ascertain 

 accurately the volume of the liquid within the field of view. 

 A cell was made according to the recommendations of 

 Siedentoff'*, the only alteration made being the substitution 

 of heavy glass for fused quartz. Fluorescence due to the 

 glass was not sufficient to render difficult the counting of the 

 comparatively large mastic particles. The cell consists of a 

 glass plate, 5 cm. in diameter and 1*0 mm. in thickness, 

 provided with a circular groove. The portion enclosed by 

 the groove, 1 cm. in diameter, was polished exactly 2 /jl 

 deeper than the surface of the plate. This was used with a 

 cover slip about *25 mm. in thickness. The cell was soaked 

 in concentrated sulphuric and chromic acids, washed with 

 water, and then passed through two solutions of re-distilled 

 alcohol. It was finally flamed. The source of illumination 

 was a Pointolight lamp, fitted with a condenser. All the 

 solutions examined were diluted with water which had been 

 carefully distilled and allowed to stand for a month undis- 

 turbed. It contained on an average 1 particle in 20 

 counts in a volume of 14'1 x 10" 5 cu. mm. and could, there- 

 fore, be considered optically pure to the degree of accuracy 

 to which work was carried. All the solutions were contained 

 in vessels of resistance glass, closed with corks covered with 

 tinfoil. The method of procedure was to transfer, by 

 means of a clean platinum loop, a very small drop of the 

 solution to be examined to the central portion of the cell. 

 The cover slip was laid on and pressed down until the 

 Newton interference rings appeared at the edges. The 

 dilutions were such that, when viewed with a convenient 

 stop in the eyepiece, about three or four particles appeared 



* Siedentoff, Verhd. Deut. Phys. Ges. xii. p. 6 (1910). 



