COLLOIDAL THEORY OF ANTIBODIES 321 



and must therefore carry a positive charge. Further researches 

 show that absolutely pure colloids, free from all traces of electro- 

 lytes, carry no charge at all, and are not conveyed by an electrical 

 current. The charge depends upon the nature of the ions present : 

 traces of acid and of acid salts give it a positive charge, whilst 

 alkalis and alkaline salts do the reverse (Pauli). 



The process of coagulation of proteids, therefore, must depend 

 upon the neutralization of this electrical charge, and this can be 

 accomplished either by electrolytes or by colloids. Non-electro- 

 lytes (i.e., substances which do not split up into electrified ions in 

 solution) do not bring about coagulation in this way. Many of 

 them, such as sugar and urea, are inert ; others, such as alcohol, 

 act in an entirely different manner. The precipitation of an albu- 

 minous solution by means of a strong acid takes place thus : The 

 negatively-charged particles attract to themselves the positively- 

 charged hydrogen ions ; their charge is now neutralized, and the 

 force of attraction due to their surface tension is no longer counter- 

 balanced by an electrical repulsion ; the particles are drawn 

 together, form larger and larger aggregates, and finally cohere into 

 masses so large as to come under the influence of gravity, when 

 precipitation takes place. The coagulation of albumin by alcohol 

 is due to the fact that proteids are not soluble in that fluid, so that 

 when it is added to a watery solution of proteid, the water is with- 

 drawn and the particles brought so close together that surface 

 tension comes into play, and makes them coalesce into aggregates. 

 The truth of this explanation appears from Pauli's observation 

 that when no electrolytes are present alcohol acts very readily as 

 a precipitant there is no electrical repulsion between the particles. 

 But if a little acid or alkali be added, and the molecules be thus 

 given a mutually repellent electrical charge, the precipitation is 

 inhibited or entirely prevented. Colloids can also be precipitated 

 by colloids as well as by electrolytes, but in this case they must 

 be of opposite sign. Thus, in testing for albumin in urine by 

 means of acetic acid and ferrocyanide of potash, the addition of 

 the acid insures that the particles shall acquire a positive charge 

 (if they have it not already), which is then neutralized by the 

 colloidal ferrocyanic acid of negative sign. Numerous other 

 examples might be quoted. 



This taking up of substances by colloidal particles is an example 

 of the phenomenon known as adsorption. This process is difficult 

 of definition, but in general terms we may regard it as a combina- 



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