LAW or COR RES PONDING STATES 75 



tive 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. 



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. Because of 

 their affinity for water such colloids are termed hydrophilic, i.e. 

 water-loving, in contradistinction to the hydrophobic suspensoids, 

 which are readily separated from their dispersion medium. 



Colloidal matter may be further divided into two groups. 

 White of egg is a hydrophilic 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 semi- 

 soHd, 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. 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 (see p. 98). 



Preparation of Colloidal Dispersions. 



Some substances easily assume the colloidal state, but fairly 

 strenuous methods have to be adopted to induce others to do so. 

 The naturally occurring colloids, such as proteins of all kinds and 

 polysaccharides, are caused to crystallise with difficulty, while 

 substances which crystallise easily become colloids under compul- 

 sion. The methods used in the preparation of colloids fall, in 

 general, into two classes, chemical and electrical methods. In the 

 former class is included all methods which entail reduction, double- 

 decomposition, hydration, substitution of solvent, peptisation, etc. 

 In Part II. are given directions for the preparation of typical 

 colloids by these methods. Electrical dispersion methods con- 

 sist in the passage of an oscillating discharge between iron or 

 aluminium electrodes immersed in water (or other dispersion 

 medium) in which are suspended coarse fragments of the metal to 

 be dispersed. 



