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



different size, and is stable when precipitated, redispersed, 

 and dialysed. Gum mastic was found best to meet the 

 requirements, and was used in the subsequent experiments. 

 To show that the solutions employed were comparatively 

 stable, the concentration of NaCl required to precipitate one 

 of the fractions at the beginning and end of the experiments 

 was measured and found to be : — 



Feb. 15th 433 millemols. 



Mar. 29th 439 



Fractionation. 



One gram of finely-powdered picked gum mastic was 

 dissolved in about 20 c.c. of alcohol, and poured slowly with 

 vigorous stirring into one litre of distilled water. By this 

 method seven litres of mastic solution were prepared. Oden 

 recommends that in all cases NaCl should be used for the 

 precipitation. With mastic this necessitates a very large 

 concentration of salt, which appears to be strongly adsorbed, 

 and comes slowly through the dialyser. It was thought 

 better to employ HC1, which precipitates in smaller concen- 

 tration. It was found convenient to separate the mastic into 

 seven fractions with these concentrations of HC1 : — 



Concentration of HC1 Condition of 

 Fraction, in millemols. Precipitate. 



I 0-1-1 trace 



II ri-l"4 good 



III 1-4-1-7 



IV 17-2-0 



V 20-2-3 



VI 2-3-2-6 small 



VII. 26-2-9 trace 



The procedure was as follows: — 200 c.c. of the mastic 



N 

 solution were mixed with a quantity of ^0^01 *° §^ Ye * ne 



required concentration, and then poured into the centrifuge 

 vessels and allowed to stand for 60 minutes from the moment 

 of mixing. It was then centrifuged at 3000 r.p.m. for 

 30 minutes. At the end of this time the supernatant liquid 

 was poured off and the precipitate carefully shaken up with 

 about 100 c.c. of distilled water. Fractions II. and VI. were 

 retained until about 1500 c.c. of each had accumulated ; the 

 other fractions were rejected. At the same time 1500 c.c. 



