LIFE: ITS BEGINNINGS AND NATURE 



24 



other activities that go on within the animal 



body. 



Particles of matter, especially small par- 

 ticles where the effect is more apparent, at- 

 tract one another of the same kind and also 

 have an attraction for other particles of a 

 different sort. The first is given the name 

 cohesion, and the latter adhesion. Particles 

 of matter such as water tend to cling to- 

 gether to the extent that they actually form 

 a film at their surfaces. This gives the effect 

 of a stretched membrane and is spoken of 

 as surface tension. Water has a rather high 

 surface tension, but mercury surpasses it 

 by a considerable margin. Anyone knows 

 that when mercury is dropped it breaks up 

 into hundreds of small perfect spheres. 

 Water gives a similar but lesser response 

 when dropped on a dry, dusty surface. Rain 

 drops are usually near-spheres. Liquids, 

 when free of external forces, will assume 

 the shape of a sphere because the cohesive 

 forces of the particles of which it is made 

 form a surface membrane. This fact makes 

 it possible for certain species of insects to 

 walk on the water (Fig. 2-2). Their bodies 

 are li^ht and the weight is distributed over 

 a large area, so that the cohesive force of 

 the water is sufficiendy strong to keep the 

 membrane intact. 



Fig. 2-2. Some insects, lii<e the water stridor, can walk 

 on water because their evenly spread weight is not 

 sufficiently great to break the surface tension. 



Adhesive forces are simultaneously at 

 work with cohesive forces. These have no 

 significant influence on large bodies, but 

 on small particles they become very im- 

 portant, just as important as gravity is 

 to the larger bodies. Certain gaseous mole- 



cules, for example, will adhere to carbon 

 particles so tenaciously that it requires high 

 temperatures to remove them; this is the 

 principle of the gas mask and certain puri- 

 fication processes. It is important in bio- 

 logical systems where enzymes, for ex- 

 ample, adhere or adsorb (the process is 

 called adsorption) to food particles, thus 

 enhancing digestion. This physical property 

 of matter manifests itself in many ways in 

 the bodies of animals. 



Composition of matter 



Almost every school child knows that 

 matter is made up of molecules, and some 

 know that the molecules are composed of 

 atoms. Any college student could hardly 

 have missed newspaper discussions of the 

 nature of atoms, how they are being in- 

 vestigated, and particularly what effect 

 this knowledge is having on our lives today. 

 There is a great deal of excellent experi- 

 mental proof for the existence of these small 

 particles, some of which we should consider 



briefly. 



Matter exists in space in one of the three 

 forms: solids, liquids, or gases. Water is a 

 convenient example of matter. Below freez- 

 ing it exists as ice, a solid; between freezing 

 and boiling, it exists as water, a liquid; 

 above boiling, it exists as a vapor or steam, 

 a gas. In all of these states the substance is 

 still water, its chemical composition un- 

 changed. Whether it is in one state or 

 another depends on the interrelationship of 

 the molecules in space. In the solid they 

 are close together and rather static in their 

 position, although free to vibrate; in the 

 liquid they are still close together, but not 

 so close as in the solid and they are free to 

 move within the liquid itself; in the gaseous 

 state they are far apart and free to move. 

 The particular state in which matter exists 

 depends on the speed of movement of the 

 individual molecules. In the sohd state they 

 vibrate, but stay in fixed positions within 

 the solid; in the liquid they move faster and 

 are free to move about within the liquid. 



