72 COLLOIDS IN BIOLOGY AND MEDICINE 



The crystallization of alkaline salts of the higher fatty and arylic 

 acids is well known. 



The crystals of colloids are distinguishable from those of crystal- 

 loids in many respects. That their solution is preceded by a swelling 

 is not surprising in view of the hydrophile colloidal character of the 

 substances under consideration. On the other hand, it is remarkable 

 that other constituents may always be demonstrated as inclusions. 

 The crystallized globulins from vegetable seeds always contain com- 

 mon salt. K. A. H. MORNER* showed that only the sulphates of 

 egg and serum albumin were crystallizable. As E. AsDERHALDEN* 2 

 has shown, oxyhemoglobin crystals do not contain their proper pro- 

 portion of albumin, and although numerous researches on crystallized 

 egg albumin have been undertaken, in different instances the amount 

 of contained carbohydrate varied. 



In spite of these facts, we are of the opinion that colloids can 

 actually crystallize, and that their crystal form is not controlled by 

 the crystalloid impurities. We know that crystalloids frequently 

 include mother liquor, that they may form mixed crystals and 

 that it is often impossible to remove impurities by ordinary re- 

 crystallization. In view of the persistent salt content of crystallized 

 albumins it is probable that only their salt-like compounds possess a 

 definite crystalline shape. Especially favorable to this view is the fact 

 reported by DABROWSKI, that crystallized egg albumin, when placed 

 in a 3.6 per cent solution of ammonium sulphate, exhibits a more 

 rapid diffusion than salt free egg albumin, and has about one-sixth 

 of the atomic volume of the latter. The crystallized egg albumin, 

 therefore, is formed of smaller particles. 



The Life Curve of Colloids. 



Though in the absence of chemical changes, crystalloids retain their 

 physical properties, in the case of colloids after a lapse of time changes 

 occur which are commonly called aging. For instance, silicic acid 

 which has been freshly prepared from water-glass solution and HC1 

 is at first dialyzable but loses this property after a few days. Most 

 of the "aging phenomena" of sols are characterized by the fact that 

 the particles of a highly dispersed solution gather together to form 

 larger particles, that their sensitiveness to flocculation is increased 

 or that they spontaneously coagulate. In the case of gels, their 

 elasticity suffers changes and they become optically inhomogeneous 

 or turbid. 



Bearing in mind that colloids are metastable systems, it is obvious 

 that in the course of time they must change, since they tend to be- 

 come stable systems. In the examination of a colloid, the properties 



