EARLY HISTORY OF LIFE 47 



be expected from our knowledge of the in concentration of bombardment that 



movement of objects that come within our causes the particle to move in the random 



experience. Physicists tell us that the move- fashion that is observed. This type of ac- 



ment of particles is dependent on the ab- tivity is essential in keeping the particulate 



sorption of heat; the higher the tempera- matter dispersed. 



ture, the faster the movement and the lower Other factors complicate the behavior of 



the temperature, the slower the movement, particles in solution. We learned earlier that 



Movement is governed by the size of the atoms and molecules may become ionized, 



particle; when the diameter of the particles that is, they may carry an electrical charge, 



is halved, the rate of movement is doubled, positive when electrons are short, and nega- 



Therefore, the huge lumbering molecules tive when electrons are in excess. We also 



in a colloidal system move slowly compared know that particles of the same charge 



to the tiny molecules of a salt solution. repel one another while those of unlike 



Particles that are smaller than 0.001 mi- charges attract. This fact has a profound 



cron in diameter are called crystalloids; effect on the behavior of particles in solu- 



sugar or table salt dissolved in water forms tion. 



a crystalloid solution. Such systems appear In a solution the dispersion medium is 



clear and transparent to the naked eye be- called the solvent, whereas the dispersed 



cause light passes directly through without particles are the solute. Because of the elec- 



being changed in any way by the tiny trical charges on the various particles of 



molecules of salt and sugar. Furthermore, the solute, some are attracted to one an- 



crystalloids pass readily through some mem- other, while others are kept apart. Fur- 



branes such as frog skin, and their individ- thermore, some of the molecules of the sol- 



ual particles move much faster than those vent are attracted to those of the solute, thus 



in the colloidal state. increasing their bulk. The strange thing 



Protoplasm contains numerous crystal- about the charge on a particle is that it may 

 loidal and colloidal particles in the form of be stronger at one side or end than at the 

 atoms, molecules, and molecular aggre- other; in other words, it can exhibit electri- 

 gates. The particles remain evenly dis- cal polarity just as a magnet does. Because 

 persed and do not respond to the pull of such substances as water, salts, and pro- 

 gravity because of their continuous move- teins exhibit polarity they are called polar 

 ment. Each particle is being bombarded by compounds. Fats and starches do not pos- 

 others of its own kind as well as by those sess these properties, so are called non-polar 

 of a different sort. This can be verified by compounds. This property of particles has 

 observing even larger particles, such as cer- considerable bearing on their behavior in 

 tain pollen grains, under the highest powers protoplasm. 



of a light microscope. They will be seen to Wlien the dispersed particles are molec- 



jostle about in a random manner, seeming ular aggregates of solid material they are 



to get nowhere. The apparent aimless mo- spoken of as suspensions; if the aggregates 



tion has been given the name Brownian are fluid they are referred to as emulsions. 



Movement. In an aqueous solution much of There may be a wide range in size of these 



the activity is due to the bombardment of particles from those that are so small as to 



water molecules which, of course, are much constitute a colloidal solution to those that 



smaller. It requires millions of hits of water are visible under the light microscope. A 



molecules to move the huge visible particles familiar emulsion is milk, in which droplets 



and these must be concentrated more on of fat are dispersed in a watery fluid of 



one side than the other if movement is to sugar, salts, and a soluble protein, 



occur in any one direction. It is the change It is essential to distinguish between the 



