SUSPENSOIDS 293 



when immediate precipitation results. Further, the assump- 

 tion accounts for the fact that suspensoid colloids cannot be 

 kept for any length of time unless carefully freed from electro- 

 lytes by dialysis. 



The behaviour of suspensoids under the influence of a 

 powerful electric current bears out the above views. In most 

 cases the particles appear to be negatively charged since they 

 travel towards the anode ; exceptions to this are the hydrosols 

 of the metallic hydroxides, such as iron, aluminium, etc., also 

 silicic acid and basic dyes, such as methylene-blue and methyl- 

 violet, all of which are positive. It should, however, be noted 

 that these charges are reversed if the same substances are dis- 

 solved in turpentine. 



A fundamental difference between suspensoid colloidal solu- 

 tions and true suspensions is that the latter when left at rest 

 will ultimately deposit their suspended matter in the form of 

 sediment as a result of gravitational attraction ; by stirring up 

 the sediment and again allowing it to settle the process can be 

 repeated indefinitely, and it is, in fact, a reversible one, even 

 though the sediment may have been heated or otherwise 

 treated with a view to bringing about coagulation, A colloidal 

 solution, on the other hand, is not in the same way affected by 

 the force of gravity, and if effectually coagulated the change 

 brought about is irreversible, and the precipitated substance 

 will not go back into solution. 



PROPERTIES OF EMULSOIDS. 



Examination under the ultramicroscope reveals no distinct 

 particles, but whether this is due to the particles being too 

 small or not having a refractive index sufficiently distinct from 

 that of the liquid in which they are suspended, is not as yet 

 known. 



As already stated, emulsoids are supposed to be composed 

 of liquid particles consisting of a more concentrated solution 

 suspended in a liquid medium composed of a much diluter 

 solution. This assumption readily accounts for the viscosity 

 of such solutions, which, being in reality suspensions of one 

 liquid in another, would naturally be expected to have some 

 of the properties of emulsions ; that emulsions have a high 



