22 ELEMENTS OF BIOLOGY 



not merely the boundary between the environment and the proto- 

 plasm. Typical colloids are regarded as being composed of particles 

 not greater in diameter than o.i of a thousandth of a millimeter, or 

 O.I micron, and not less than cooi of a micron, or i millimicron. 

 Now if a solid having a diameter of i centimeter is divided into 

 particles with diameters of o.i micron, that is, the largest of col- 

 loidal particles, the combined surfaces of the particles is 126 square 

 meters, equal to the area of one side of a fence a little more than a 

 yard high and nearly 130 yards long. If the substance is divided into 

 particles with diameters of i.o millimicron, the smallest of colloidal 

 particles, the entire surface exposed by the particles is 12,000 square 

 meters, or equivalent to the surface on one side of a fence a bit 

 more than a yard high and over seven miles long. Since the col- 

 loidal substances in protoplasm are between the extreme diameters 

 given here, the total surface in a mass of protoplasm with a diameter 

 of I centimeter is somewhere between 126 square meters and 12,000 

 square meters, in any case a tremendous area. It is therefore not 

 difficult to understand that the actual surface in protoplasm that is 

 constantly responding to the laws of surfaces is relatively very great 

 in proportion to the volume of protoplasm. 



Surface Tension. We are all familiar with the fact that the 

 surface of a container of water is a film that behaves as if it were 

 stretched. The surface of the water is in reality the boundary be- 

 tween water, H2O, and air, which is a mixture of nitrogen, oxygen, 

 carbon dioxide, and traces of other gases. If a fine droplet of oil is 

 suspended in water, the surface of the droplet is the boundary 

 between the oil and the water; it is more properly called an inter- 

 face. Such boundaries are in a state of tension and therefore repre- 

 sent an equilibrium between some sort of forces. 



According to commonly accepted theories, each material particle 

 in the cosmos exerts an attractive force on all other material bodies. 

 This is regarded as true among molecules of a substance as well as 

 between celestial bodies of enormous size. Thus, in a container of 



