142 PROTOPLASM 



in preparation, silica gel has now attained wide commercial use 

 in America as an adsorbent, for drying blast-furnace air, decolor- 

 izing solutions, deodorizing refrigeration air, and purifying air 

 for breathing by adsorbing poisonous and obnoxious gases. The 

 efficiency of silica gel as an adsorbent is due to its great number 



of, and exceedingly minute, pores (5 m/x), 



which offer a tremendous surface for adsorption. 



In other countries, "active" charcoal is still the 



preferred adsorbent. 



Dififusion. — The kinetic energy of molecules 

 -,. ^^ ^ causes them to diffuse in all directions, so that a 



Fig. 90. — Por- c ■ ■ ■ , 



ous structure of a gas Set free m a room is m time equally distributed 

 sihca gel. throughout the room, and a crystal of salt put 



into a glass of water becomes uniformly distributed through- 

 out the water. This movement of gases and dissolved substances 

 from a region of high to one of low concentration is termed 

 diffusion. Ions migrate at different speeds, hydrogen being the 

 fastest. The rate of migration or diffusion of a molecule, atom, 

 or ion is in part determined by its size; therefore hydrogen, being 

 the smallest, should and does travel the fastest. Calcium and 

 chlorine travel at about the same rate; they are separated in the 

 periodic table by but one element, which indicates that they are 

 very nearly the same size. Lithium, on the other hand, is 

 separated by but one element from hydrogen, yet it moves much 

 more slowly. The greater speed of hydrogen over that of any 

 other element is probably due to the unique condition of this 

 atom when ionized, for then it is stripped of its lone negative 

 electron, and there remains but a single proton or positive charge, 

 unhampered by outer electrons. The much slower diffusion rate 

 of the nearby lithium atom is probably also due to the fact that 

 it is highly hydrated. Size, mass, temperature, and viscosity 

 of the medium are determining factors in the rate of diffusion of 

 atoms, but hydration may dominate over these. Thus, the 

 order of diffusion rate of three elements is Li < Na < K, 

 while the order of size is the reverse; furthermore, chlorine, 

 bromine, and iodine have the same diffusion rate in spite of 

 considerable difference in size. 



Size and mass probably play the chief part in determining the 

 rate of movement of the mammoth-sized molecules of proteins, 

 although here, again, hydration is a prominent factor. Egg 



