130 ORIGIN AND NATURE OF LIFE 



the initial step in crystal formation. The 

 distinguishing feature of the colloid is that 

 the molecular unions shall be of a feeble 

 unstable kind with very little evolution of 

 energy. This condition has been termed the 

 meta-stable condition, and is seen at its 

 acme in living colloids. The true inorganic 

 colloids showing typically those characteristic 

 properties which distinguish the colloidal 

 class from the crystalloidal class of bodies 

 are also formed without much evolution of 

 energy. Between the two classes lie all 

 kinds of intermediate grades, and it is only 

 by contrasting a typical colloid with a typical 

 crystalloid that those characteristic features 

 have become obvious which will presently be 

 described. 



Crystallization of a substance from solution 

 in well-formed crystals of definite form is 

 usually regarded as one of the best criteria 

 of definite chemical constitution and purity. 

 As a rule, to which there are some notable 

 exceptions, colloids do not form saturated 

 solutions like crystalloids, and then crystallize 

 out giving an increasing crop of crystals 

 as the solvent evaporates off. After crystals 

 once begin to form in a solution of a crystal- 

 loid, the mother liquor over the crystals 

 remains of a constant concentration, and this 



