LIFE AND ENERGY 35 



velocity of the former ions is the cause, but the matter is not quite 

 cleared up. 



Precipitation by Electrolytes. The neutralisation of the 

 electric charge on colloidal particles will have the effect of throw- 

 ing them down from suspension, since the removal of the charge 

 acts both by increasing the surface tension and by abolishing the 

 mutual repulsion of the particles. The addition of an electrolyte 

 is an effective way of -doing this. Suppose that we add to a 

 colloidal solution of arsenious sulphide, whose particles have a 

 negative charge, some sodium chloride in solution. There are now 

 present sodium ions, with a positive charge, and chlorine ions, 

 negatively charged. The sodium ions neutralise the negative 

 charge of the particles by being deposited on their surfaces, the 

 colloid is precipitated, carrying with it the ions required to 

 neutralise the charges on the particles. Since it needs several 

 univalent ions to neutralise the charge on each particle, it is clear 

 that many ions have to be met with by each particle before 

 sufficient opposite charge has been obtained. Bivalent or pluri- 

 valent ions afford two or more electrons at each encounter, so that 

 they are much more effective, as would be expected by the law of 

 chances (E., p. 178). When we have an electro-positive colloid, it 

 is the anions of the added electrolyte that are the active ones. We 

 see that a decrease of free energy occurs by such abolition of 

 charge, whereas if ions of the same sign as the surface were de- 

 posited on it, a gain of free energy would result. 



Emulsoid colloids are, as a rule, much less sensitive than sus- 

 pensoids to the action of electrolytes. But it is only a matter of 

 degree (P., p. 92). If we call to mind that the two phases of which 

 the former consist differ only in the amount of water contained, it 

 will be understood that the forces at the interface of contact, whose 

 magnitude depends on the difference in nature of the two phases, 

 must be less than when the two phases are altogether different in 

 chemical composition. 



This is an appropriate place to remind the student that the 

 various physical properties to which, for the time, our attention is 

 being directed, depend on the chemical nature of the substances 

 concerned. While we discuss the properties which belong to 

 certain constituents of the cell on account of their being in the 

 colloidal state, we must not forget that they also react chemically 

 with other constituents and with substances coming from the 

 outside. Substances in the colloidal state, however, do not so 

 readily enter into chemical reaction with other substances, since it 

 is only the surface of the matter of which they are composed that 

 comes into relation with other reagents. On the other hand, the 

 physical properties of the surface can be brought into play very 



