500 INFECTION AND RESISTANCE 



crystalline or amorphous form. This class of bodies he named col- 

 loids, since glue (Greek KoAAa meaning glue) presented a typical case. 

 Colloid substances may appear in highly dispersed states, such as. 

 dilute glue, arsenic sulphid suspensions, oil or rosin emulsions, milk 

 (casein in highly dispersed condition), and the like, in which case 

 they are spoken of as sols. They may also occur in the undispersed 

 or only slightly dispersed state, as the amorphous precipitated sul- 

 phids of the heavy metals, precipitated casein, or dry glue. In this 

 state they are spoken of as gels. 



When a colloid substance has once been converted from the sol 

 or dispersed state into the gel or undispersed state, its properties 

 may differ greatly in different cases. Thus if a dispersed soap (soap- 

 solution, or more correctly soap sol) be coagulated by the addition 

 of common salt, the coagulum or soap gel may be removed from the 

 salt solution, and if again placed in pure water it will redisperse 

 and again assume the sol condition. Such a colloid is spoken of as a 

 reversible colloid. If, however, an arsenious sulphid suspension be 

 put through precisely the same course of treatment it will, in the 

 last stage of the treatment, refuse to redisperse, and is therefore 

 spoken of as an irreversible colloid. Some authorities prefer to 

 speak of these two classes of colloids as emulsion and suspension col- 

 loids, respectively, 2 since in general those colloids which are reversi- 

 ble tend to separate out in soft masses, and in general to gelatinize 

 rather than to flocculate, while the irreversible colloids rather tend 

 to truly flocculate and form very compact and frequently more or 

 less granular coagula. Since, however, we seem more likely at the 

 present time to suffer more from an excess of classification than from 

 a lack of it, the attempt will be made to get along in this discussion 

 with the earlier nomenclature. It may, indeed, be added that 

 it is highly probable that the distinction between reversible and irre- 

 versible colloids is only one of degree. For example, many of the 

 metallic sulphids which are typically irreversible may be made 

 to some extent reversible by means of thorough washing and re- 

 treatment with hydrogen sulphid which had been originally used 

 in their preparation. It is probable that certain colloids are 

 apparently irreversible only because we do not truly reverse 

 the conditions. 



Heretofore in this discussion the term "colloid' 7 substance has 

 been used as if to imply that certain chemical individuals were 

 characteristically colloid, while others were not. It was much in this 

 sense that Graham used the term. Investigations since his time have 

 shown this to be a misconception, and it is now apparent that any 

 and all substances may be either colloid or crystalloid, the form they 

 assume depending upon treatment. Thus albumin may be crystal- 

 lized and common salt may be obtained in the state of a colloid solu- 



2 V. Weimarn. Ztschr. Chem. Ind. KolL, 1908, 3, 26. 



