THE CHEMISTRY OF LIFE 



35 



all states of matter, but may vary in their speed of motion and in their 

 position with relation to other molecules around them. In a solid the 

 molecules vibrate back and forth, but remain in fixed positions. In a 

 liquid they move faster and are free to move about one another. This 

 gives the fluidity to a liquid. In a gas the molecules attain a very 

 high speed, a speed so fast that the molecules separate from one another 

 and become independent free-floating bodies. The movement of mole- 



|N Other minerals 4-,% 

 \ Nitrogen 3Vo 



Hydrogen f0% 



Carbon 18 % 



Oxygen 65% 



Fig. 3.1. Proportion of the different chemical elements in the human body by weight. 

 Note that oxygen, carbon, and hydrogen make up 93 per cent of the body weight. 



cules is dependent upon the energy of heat, and the higher the tempera- 

 ture the faster the movement. This explains why temperature changes 

 cause changes in the states of matter. There is always some molecular 

 movement, however, even in heavy solids. This movement would stop 

 if the temperature ever reached absolute zero (minus 273°C. or minus 

 460°F.). This temperature has never yet been obtained, although sci- 

 entists have come within a fraction of a degree of it. This gives us a 

 lower limit of temperature, but there seems to be no upper limit. We 

 have special ovens that produce temperatures of several thousand de- 



