EARLY HISTORY OF LIFE 



23 



weigh 30 pounds on the moon, although his 

 mass would be the same. Weight simply 

 measures the pull of gravity. 



Animal bodies are constructed to com- 

 pensate for the pull of gravity. For ex- 

 ample, small animals, such as mice, have 

 relatively light skeletons in comparison to 

 their weights, whereas larger animals, such 

 as the elephant, have much heavier skele- 

 tons with respect to weight. This fact limits 

 the size of animals, for should they go on 

 increasing in bulk, they would reach a 

 point where the skeleton alone could not 

 bear its own weight. If life, as we know it, 

 occurs on other planets, it too would show 

 relation to weight. Animals on Jupiter, for 

 example, would have to be constructed on 

 an entirely different plan from those on the 

 earth, because the pull of gravity is so much 

 greater. They would probably be heavily 

 boned animals and greatly flattened. 



One cannot mention the motion of matter 

 without referring to another force that 

 operates on bodies, namely, inertia. When 

 a swift elevator starts up, one is con- 

 scious of a sudden increase in weight; like- 

 wise, when it comes to rest, one seems 

 suddenly and momentarily lighter than 

 usual. There may be a simultaneous pecul- 

 iar feeling in the mid-section as the in- 

 ternal organs respond to the effects of 

 inertia. This force is the resistance of a 

 body to change in its rate of motion. If 

 standing still, it resists movement; if mov- 

 ing at a certain rate of speed it resists any 

 change in this rate. That is why we need 

 low gear and good brakes on our cars — it 

 requires more power to get started or to 

 stop tlian to keep rolling. 



Most animals are little affected by inertia 

 except when suddenly stopped, such as a 

 bird flying into the side of a building. The 

 sudden cessation of forward motion can be 

 fatal to bird or man, as attested by car 

 accidents. Another modern machine that 

 brings the effects of inertia into promi- 

 nence is the airplane. Pilots often "black 

 out" because their forward motion is sud- 



denly changed, as in coming out of a power 

 dive. The blood, being fluid, tends to fol- 

 low the forward motion it has attained and 

 is thus pulled away from the head, causing 

 the "black out" or faint. The normal move- 



Fig. 2-1. Two cubes, one with dimensions l/lOth of the 

 other. In respect to volume, the smaller has 10 times 

 the surface area of the larger. 



ments of animals are little affected by in- 

 ertia, and the problem arises in man only 

 when he steps into one of his mechanical 

 contrivances which carry his body faster 

 than it was made to go. 



Surface phenomena 



The behavior of matter depends to a 

 large extent on its surface area. Large 

 bodies have smaller surface areas in respect 

 to volume than do small bodies. A mouse 

 has more surface area in respect to its vol- 

 ume than does an elephant. This can easily 

 be computed by simply measuring the 

 surface area of a cube, say 10 inches on a 

 side or 600 square inches, and then its 

 volume, which is 1,000 cubic inches (Fig. 

 2-1). This is a ratio of 0.6 square inch of 

 surface to every cubic inch of volume. If 

 a cube one inch on a side is cut out of 

 the larger cube, and the surface computed 

 in respect to the volume, it will be found 

 that each inch cube will have 6 square 

 inches of surface, a tenfold increase. Since 

 chemical reactions take place at surfaces, it 

 is obvious that the activity will be much 

 greater as matter is divided into smaller 

 and smaller particles. This is a very im- 

 portant physical property, as we shall see 

 when we discuss enzyme action and many 



