THE NEW YORK JOURNAL OF PHARMACY 



often appears whitish to pale yellow. 

 Gold "sols" may be violet, red, blue, 

 green, orange. The intensity of the 

 color is measured by the amount of 

 white which must be added to get a cer- 

 tain tint. 



(6) Electric Properties: The dis- 

 perse-phase is charged either positively 

 or negatively. 



Positive charge have : Metal hydrox- 

 ides, silicic acid and certain dyes like 

 methylene blue. 



Negative charge have : Metals and 

 their sulfides, dyes like indigo, eosin, 

 fuchsin. also starch, mastic and acacia. 



Filter paper in water becomes nega- 

 tively charged — a substance, therefore, 

 which does not ascend the filter paper by 

 capillarity, is positive in charge, viz., it 

 coagulates on the paper. 



The kind of charge is usually stated 

 for water, for it ma\' be dififerent in 

 other solvents ; for example, it is the op- 

 posite charge from that in oil of turpen- 

 tine. Graphite in oil is positive, in water 

 it is negative. Addition of certain sub- 

 stances may reverse the electric charge ; 

 albumin becomes negative by addition 

 of an alkali : it becomes postive on addi- 

 tion of an acid. In passing the electric 

 current through a colloidal solution, the 

 particles traxx'l either to the anode or 

 cathode, according to their charge. This 

 is Electro-osmosis or Kataphoresis. Col- 

 loidal solutions conduct electricity con- 

 siderably less than dissociated solutions. 

 The addition of a so-called "protective 

 colloid," such as albumose, acacia, etc., 

 imparts to the dispersoid the charge of 

 the protective colloid. So, collargol, ly- 

 sargin and other colloidal preparations 

 carry the charge of the protective colloid 

 used. 



(7) Jellification : Under this head we 

 distinguish (a) setting, and (b) swell- 

 ing. 



The setting or jellifying of colloidal 

 solutions is due to changes in the in- 

 ternal dispersion and in hydration. Set- 

 ting takes place upon warming and con- 

 sequent concentration, or upon the addi- 

 tion of electrolytes. The amount of the 

 latter must exceed certain minimal values 

 and amounts, which differ greatly with 

 the nature of colloid as well as electro- 

 lyte. Hydrophilous colloids increase 

 their volume on being brought in contact 

 with water — they swell. This swelling 

 may go on to such an extent that the 

 particles of the substance are torn apart 

 and a colloidal solution results, or it may 

 reach a certain limit and then stop. The 

 setting and swelling of collloids is a very 

 important phenomenon. Every substance 

 or organ has a certain definite swelling- 

 value, which may counter-balance or 

 even overcome osmotic pressure. The 

 force of swelling is often very great. 

 Ostwald states that swelling peas lifted 

 the cover of an iron pot which had been 

 weighted down with 83.5 Kg. The 

 swelling-pressure of starch has been 

 found to be equal to a pressure of 2523 

 atmospheres. The old Egyptians drove 

 wooden sticks into stones and poured 

 water in them. The swelling of the wood 

 broke the stones, which were used for 

 building purposes. The curling of gela- 

 tine-films is due to a swelling, also the 

 straightening or curling of hair in dry 

 or moist air respectively. Interesting 

 work has been done on the structure of 

 jellies. When we dissolve a small 

 amount of gelatine in a mixture of alco- 

 hol and water and allow to cool slowly, 

 we note under the mocroscipe, that small 

 semi-liquid drops separate and, as their 



