GELATIN 33 



by water and with a dilute acid (hydrochloric), with rela- 

 tively little exposure to heat, so that as few as possible of 

 the fluid disintegration products of the stock are formed and 

 the jellying power of the resultant solution is not destroyed. 



The term gelatin is derived from the Latin verb gelare, 

 to congeal, and calls to mind the principal attribute of this 

 substance, that of its stiffening or jellying property. 



Gelatin belongs to that interesting class of substances 

 called colloids. It is a typical example of the class, and 

 exhibits the characteristic properties of the class. Colloids, 

 in marked contrast to crystalloids, do not crystallize, do 

 not readily diffuse and are impermeable to each other. 

 The ultimate particles of colloids are much smaller than 

 what we would ordinarily term a physical subdivision, but 

 rather larger than chemical molecules; the diameter of the 

 smallest particles in a colloidal solution, e.g., red colloidal 

 gold, which have been counted by means of the ultra-micro- 

 scope, is 6 millimicrons or 6 thousandths of a micron. A 

 micron is one thousandth of a millimeter. (Bacteria are 

 much larger, the smallest visible by means of the ordinary 

 microscope being from 0.3 to 1.0 micron in diameter.) 

 Consequently their reactions stand midway between the 

 physical and the chemical changes of matter, as may be 

 seen by considering the properties of gelatin. 



Gelatin will absorb a considerable quantity of warm 

 water (it is almost insoluble in cold water) and swells up, 

 yielding a jelly which, upon application of heat, melts to 

 a viscous, sticky solution that gelatinizes again upon cooling. 

 The name of hydrogel is applied to colloids showing this 

 property. Ordinary gelatin media for microbiological work 

 contain 12% to 15% gelatin. When dried at medium tem- 

 peratures, gelatin can again be redissolved and redried in- 

 definitely. From this property it is called a reversible 

 colloid to distinguish it from other colloids which, when 

 their physical state is once changed, are insoluble, e.g., 

 casein and silicic acid. 



