30 PHYSIOLOGICAL CHEMISTRY 



loidal state, so that the properties of colloids are both inter- 

 esting and important. The group is divided into two classes, 

 hydrophile or emulsoid colloids, and suspensoid colloids. Hy- 

 drophile colloids more closely approach the crystalloids in their 

 properties, whereas suspensoid colloids are more nearly like 

 suspensions. As a matter of fact there is no sharp dividing 

 line either between the groups, or between emulsoids and true 

 solutions, or suspensoids and suspensions, since all gradations 

 exist. Peptones, although belonging to the class of derived pro- 

 teins, will pass through a parchment membrane fairly well, and 

 are thus between the emulsoids and crystalloids. Certain metal 

 hydroxides form gels, but are precipitated easily by electro- 

 lytes, and are thus between the emulsoids and suspensoids. 

 Kaolin shaken up with water is midway between the suspensoids 

 and true suspensions. 



Emulsoid colloids are characterized by the fact that they 

 form gels if sufficiently concentrated, and are not easily pre- 

 cipitated by the addition of salts. If in solution, the substance 

 is called a hydrosol, if in a gel form, a hydrogel. Examples of 

 emulsoids are albumin, gelatine and other proteins, starch, etc. 

 The emulsoid colloids have some attraction for, or relation with 

 the water surrounding them. This property is of the greatest 

 importance, for the colloids of the living protoplasm aid in 

 holding the water which is essential to the life and functioning 

 of living cells. 



The suspensoid colloids do not form gels, and are easily pre- 

 cipitated by the addition of even a small amount of a salt. Ex- 

 amples of this class are colloidal metals, sulphides, etc. They 

 seem to have little relation to the water surrounding them. 



The particles in colloidal solutions are so small that they will 

 pass through an ordinary filter with ease. Filters impregnated 

 with collodion have been prepared, however, by means of which 

 colloid particles can be held back. In this, and other ways, the 

 size of the particles has been estimated. 



Tyndall's Phenomenon. Colloidal solutions show an inter- 

 esting behavior known as Tyndall's phenomenon. If a beam 



