Properties and Activities of Living Protoplasm 77 



solid, gel state). Certain materials which are considered to be nonliv- 

 ing, such as gelatin, etc., form colloidal suspensions in water and exhibit 

 the fluid, sol state when warm and the semisolid gel state when cooler. 

 These states are reversible, as they may be in living protoplasm. 



When a strong beam of light is passed through a colloid, the small 

 colloidal particles suspended in the liquid reflect the light, and the path 

 of the light appears as a visible cone known as TyndalVs cone, named 

 for John Tyndall, the British physicist (1820-1893) who discovered it. 

 In this manner the same effect is observed when light passes through 

 fog or smoke. However, if the same strong beam of light is passed 

 through a true solution of a substance, no such cone is visible. 



If the proper colloidal particles suspended in a liquid are viewed 

 through a microscope, the motion of the light reflected from the colloidal 

 particles reveals that the latter are moving. This unordered movement 

 (in all directions, back and forth) is called Brownian movement because 

 it was first observed in 1827 by the Scotch botanist Robert Brown. 



Molecules in the interior of a colloidal particle are attracted equally 

 in all directions by other surrounding molecules, while those on the sur- 

 face of a colloidal particle are subject to unequal forces of attraction 

 (similar to the unequal attraction of molecules on the surface of a 

 liquid). Because colloidal particles are so small and numerous, they 

 possess a great total surface area, so that there are great numbers of 

 molecules on the surface of each particle. As a result, these surface 

 molecules are able to attract and hold other molecules, atoms, or ions 

 through a process called adsorption (L. ad, to; sorhere, to draw in). 

 This property plays an important role in many phenomena in the non- 

 living and living worlds. 



When a colloid is placed between two electrodes of a cell with a rela- 

 tively high voltage, the colloidal particles migrate either toward the posi- 

 tive or negative electrodes, depending^ on the specific colloid. Colloidal 

 metals (and metal sulfides) tend to migrate toward the positive electrode; 

 hence they must bear a negative electrical charge. Most colloidal hy- 

 droxides of metals (containing the hydroxyl OH) move toward the nega- 

 tive electrode; hence they bear a positive charge. The electrical charge 

 borne by colloidal particles is due to the somewhat selective adsorption 

 of positive or negative ions from the surrounding medium, the specific 

 type of ion adsorbed depending on the particular colloid. Ions (Gr. 

 ion, going) are atoms, or groups of atoms, with either positive or nega- 

 tive electrical charges. Colloidal particles bearing electrical charges may 

 explain some of the electrical phenomena of nonliving as well as living 

 substances. 



