348 



Messrs. I. Jorgensen and F. Kidd. 



be introduced so as to be uniformly distributed among the original components 

 of the heterogeneous system. 



It is important to bear in mind that actions in heterogeneous systems are 

 not comparable with those taking place in true solutions. 



Preparation. — The chlorophyll sol was prepared by dissolving - 40 grm. of 

 pure chlorophyll (" a " + " b ") in 3 c.c. of absolute alcohol, and adding the 

 solution to 300 c.c. of distilled water. This was the sol used in most of our 

 experiments. 



The way in which the alcohol solution is mixed with the water influences 

 the dispersion, but by always making up the sol under exactly the same 

 conditions, uniformity can be obtained. 



Properties. — The chlorophyll sol is pure green in colour, somewhat 

 opalescent, without fluorescence, and of a much greater depth of colour than a 

 true solution of the same strength. 



Chlorophyll sol offers opportunities for very exact investigations into the 

 properties of suspensoids owing to the fact, first pointed out by Willstatter, 

 that certain of the solvents of chlorophyll which are immiscible with water, 

 such as ether, petrol-ether, benzene, do not remove chlorophyll from colloidal 

 solution when shaken up with it, but immediately remove it if it is 

 precipitated. Consequently the extent of precipitation of the colloid can 

 always be estimated by colorimetric examination of the ethereal extract. 



We found that in 24 hours chlorophyll was completely precipitated from 

 the colloidal solution by sodium chloride when the latter was present in 

 concentration N/25, but in N/30 sodium chloride no precipitation took place. 

 Similarly with magnesium chloride a concentration of N/100 causes complete 

 precipitation of the chlorophyll, while there is no precipitation when the 

 concentration of the magnesium chloride is reduced to N/125. Addition of 

 small quantities of alkalies stabilises the sol, while precipitation is facilitated 

 by additions of small quantity of acid. Strong acids are particularly efficient 

 m effecting precipitation. Thus the sol described above was precipitated in 

 N/ 500 sulphuric acid. If, however, a weak acid of a much higher concen- 

 tration, for example, N/5 boric acid, is used, no precipitation takes place. 



If such a colloidal solution without any added substance is exposed to«air 

 there is no precipitation, but it will gradually change in colour to yellow- 

 green and finally bleach, the rate at which this takes place being greater in 

 the light than in the dark. 



Owing to the method of preparation of the chlorophyll sol described above 

 it will be observed that the colloidal solution contains a small quantity of 

 solvent {e.g., absolute alcohol) in which the chlorophyll is first dissolved. The 

 influence of the presence of this solvent in any system used must obviously 



