148 



PHYSIOLOGY 



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-con- 

 ducting 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 com- 

 pounds 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 per- 

 centage of dye left in solution, and the abscissae the original concentration 

 of the solution, the curve only approaches the axis (i.e. zero concentration) 

 asymptotically. In other words, however dilute the original solution may 

 be, there will always be a certain amount of the dye left unabsorbed by 

 the paper. Similar relations are found to exist between proteins and electro- 

 lytes. By continuously washing a protein or gelatin with distilled water, 

 the removal of electrolytes becomes slower and slower, but it is practically 

 impossible within finite time to get rid in this way of the last traces of ash. 



Although the chemical behaviour of colloids is largely determined by 

 surface phenomena, it presents at the same time analogies with more strictly 

 chemical reactions, since it is conditioned by the chemical structure of the 

 colloid molecule as well as by the charge carried by the latter. A 

 good example of these adsorption combinations is presented by globulin, 

 the behaviour of which has been studied by Hardy. This may be obtained 

 from diluted blood-serum by precipitation with acetic acid. Four states 



