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



It is obvious that the rate of coagulation decreases con- 

 siderably with decreasing concentration of the mastic. The 

 point of complete coagulation was taken to be " clear with 

 large flocks." These flocks do not necessarily settle to the 

 bottom of the vessel ; some adhere to the side. The interior 

 of the liquid, however, appears quite clear. It is noticeable 

 that the flocks adhering to the sides are more numerous 

 upon, if not confined to, the side of the vessel away from 

 the source of daylight illumination. 



It is apparent that, in order to arrive at the point of 

 complete coagulation in the case of the more dilute solution, 

 it is necessary to leave it undisturbed for a good many days, 

 a course of action to which there are several objections, 

 apart from that of: mere convenience, in carrying out a series 

 of numerous determinations. During this time external 

 influences, such as chemical action at the surface of the 

 particle, have more time to show themselves. 



These objections can be obviated by centrifuging the 

 solution after a definite time at a constant speed. The 

 rate of coagulation is made up of two factors, the rate of 

 aggregation of the particles and that of settling of the aggre- 

 gates so formed. By centrifuging, the influence of the 

 latter is reduced to a minimum, and we arrive at a truer 

 measure of the former. An examination of this method 

 shows that it is possible to obtain complete coagulation after 

 a reasonable length of time. 



The solutions were treated in the way to be described, and 

 the following tables (and figs. 1 and 2) show the minimal 

 concentrations of A1 2 (S0 4 ) 3 and NaCl at various intervals 

 after the moment of addition for complete coagulation. 

 Similar results were obtained with BaCl 2 . 



It will be seen that the minimal concentration of electro- 

 l} r te decreases rapidly with time until from 12 to 22 hours 

 after mixing, thereafter, it remains fairly constant. In 

 order, however, to ensure reaching the true end-point, the 

 solutions in the subsequent experiments were allowed to stand 

 48 hours after mixing and before centrifuging. 



Several workers have pointed out that the rate and 

 method of addition of the electrolyte affect the end-point. 

 Weiser and Middleton * devised an apparatus, by the use of 

 which they obtained concordant results, and a modification 

 of it made by the writer was found to give equally good 

 results. In order to ensure perfect cleanliness, it was made 

 of glass throughout. The modified apparatus consisted of 



* Weiser and Middleton, Jour. Phys. Chem. xxiv. p. 30 (1920). 



