vin THE GENERAL PHYSICAL PROPERTIES OF ALBUMINS 257 



3. As a solid ; for example, as dry silicic acid or glass. 

 Graham first noticed that colloidal substances are held in solution 

 by a singularly feeble force, being "precipitated by the addition to 

 their solution of any substance from the other class " (i.e. crystalloids), 

 and that they are also altered by heat. He called the solid con- 

 stituent of gels, which contracts on heating, the clot, and the exuding 

 liquid the serum. Colloids, which, by a reversal of the causes pro- 

 ducing their precipitation, may be rendered soluble again, Hardy calls 

 'reversible.' If a colloid cannot be brought back to its original 

 soluble state it is irreversible. 



The author distinguishes between insoluble, semi -soluble, and 

 soluble colloids. A soluble colloid is one in which all the component 

 particles carry definite electro-positive or electro-negative charges, as 

 will be shown later, while an insoluble ' colloid ' is iso-electric, i.e. carries 

 no electrical charges, and as long as a colloid remains in this insoluble 

 state it exhibits none of the characteristics usually attributed to colloids 

 and enumerated below. According to the nature of the particular 

 colloid we are working with, the conversion of the insoluble into the 

 soluble state is either comparatively easy or very difficult, and the 

 more a colloid is rendered truly iso-electric, the more difficult is it, 

 other things being equal, to reconvert it into the soluble form. This 

 reconversion in the case of albumins is often quite impossible, 

 because when the iso-electric point is approached, the different groups 

 of amino-acids in the albumin -molecule rearrange themselves intra- 

 molecular ly to compensate for the removal of the electrically charged 

 ions by means of which they were kept in solution. In addition to 

 this change, amino-acids may also be converted from real acids and 

 bases into pseudo-acids and into pseudo-bases (see pp. 218, 219). 



For a historical account of investigations into the nature of colloids 

 up to the year 1902, see the author's Physiological Histology, Clarendon 

 Press, 1902, pp. 28-70. 



Summing up our present knowledge, colloids, when 'in solution/ 

 have the following characteristics : 



1. They polarise transmitted light. 



2. Possessing a low osmotic pressure, they raise the boiling-point 

 or affect the freezing-point of water only very slightly. 



3. They are not coagulated irreversibly by a rise of temperature, 

 provided electrolytes are absent and provided their chemical constitu- 

 tion does not become permanently altered. 



4. They move either with or against an electrical stream which is 

 being passed through them, and they are therefore either electro- 

 positive or electro-negative, but they offer a great resistance to the 



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