164 



PHYSIOLOGY 



The factors involved in the formation of adsorption or absorption 

 combinations are therefore : 



(1) Extent of surface. In a colloidal solution this must be 

 enormous in proportion to the mass of substance in solution. Thus 

 a 10 c.c. sphere with a surface of 22 sq. cm., if reduced to a fine powder 

 consisting of spherules of -00000025 cm. in diameter, will have a 

 surface of 20,000,000 sq. cm., i.e. nearly half an acre. At the whole 

 of this surface adsorption may take place, involving the concentration 

 of dissolved electrolytes, ions, or gases. 



(2) Chemical nature of particle. 



(3) Electric charge on the surface. The sign of this may be deter- 

 mined by the chemical nature of the colloid and its relation to the 

 electrolytes in the surrounding medium. 



Another factor which may determine the character of the charge on the 

 particles has been pointed out by Coehn. This observer finds that, when various 

 non-conducting bodies are immersed in fluids of different dielectric constants, 

 they assume a positive or negative charge according as their own dielectric 

 constants are higher or lower than the fluid with which they are in contact. 

 For instance, glass (5 to 6) is negative in water (80) or alcohol (26), whereas in 

 turpentine (2-2) it is positive. In water, as Quincke has found, nearly all non- 

 conducting bodies take on a negative charge. Among these are cotton-wool 

 and silk. Particles of these in water, exposed to an electric field, move towards 

 the anode. The same is true, as Bayliss has shown, of paper. 



The conditions which determine the formation of these adsorption 

 compounds can be studied in their simplest form on the adsorption 

 of dyestuffs by substances such as paper. If we take a series of 

 solutions of a dye, such as Congo-red, in progressively diminishing 

 concentration, and place in each solution the same amount of filter- 

 paper, we find that a part of the dye is taken up by the paper, and the 

 proportion taken up is larger the more dilute the solution. This 

 relation has been spoken of by Bayliss as the law of adsorption. 

 This is illustrated by the following Table of results of such an 

 experiment : 



If put into the form of a curve, where the ordinates represent the 



