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the surface exposed to each c.c. of the dispersed phase. There 

 would be a continuous series of systems ranging from a non- 

 dispersed two-phase system on the one hand to a homogeneous 

 mixture of an ionised salt in water, i.e. a true solution. Colloids 

 may thus be regarded as intermediate in this series, e.g. Gold coin 

 in water, gold dust suspended in water, very fine gold dust sus- 

 pended in water, range of colloidal gold in water ((Zsigmondy), 

 solution of gold salt (undissociated) and, finally, completely 

 dissociated gold salt in aqueous solution. 



Physiological colloids differ from this metallic series in one 

 respect at least. They dissolve in water and they also imbibe 

 water. A solution of albumin, for instance, cannot be regarded 

 as a solid dispersed throughout a liquid, but is a strong solution 

 of albumin dispersed throughout a weaker solution. 



This state is not peculiar to natural organic colloids, but, as 

 has been amply demonstrated by Von Weimarn, can be obtained 

 from such materials as NaCl, A1(OH) 3 and silver salts. He has 

 enunciated a postulate called the law of corresponding states, 

 which is as follows : " The degree of dispersion and the general 

 physical appearance of precipitates are always the same irre- 

 spective of the chemical nature of the precipitates provided that 

 the precipitation takes place under corresponding conditions." 

 Working with substances as widely apart in their chemical nature 

 as the various salts of aluminium, barium, silver, sodium and 

 many others, Von Weimarn has prepared precipitates with almost 

 any desired degree of dispersion ranging in each instance, all the 

 way from coarse and obviously crystalline precipitates, to gela- 

 tinous precipitates and thick transparent jellies. 



Colloidal matter may be further divided into two groups. 

 White of egg is an emulsoid colloid. In its ordinary state, as 

 obtained from the egg, it can be dissolved in water to form a clear 

 solution. Boiling the solution causes coagulation of the egg 

 white. It comes out of solution in the form of a white solid, 

 insoluble in water. Those colloids which form solutions like egg 

 white are called sols. According to the medium in which they 

 were dispersed they were termed by Graham, hydrosols, alcosols, 

 glycerosols, etc.. These colloids which assume a semi-solid form 

 like coagulated egg white are called gels. In a gel the more 

 liquid phase is dispersed through the less liquid phase. 



An examination of the optical properties of these various disperse 

 systems makes it clear that there is a regular gradation in the 

 size of the particles dispersed, which passes from the easily visible 



