32 THE MECHANISM OF LIFE 



syrup), or liquid (as in the case of water). When they do not 

 cohere at all, so that the material can take any shape and expand 

 to any extent, we call it a gas. Thus in ice the molecules cohere 

 strongly, but not nearly so much when the ice is melted, and 

 hardly at all when the water is converted into steam. 



Temperature and Heat. — Now all bodies of which we have any 

 • experience possess some heat. Heat is a " mode of molecular 

 motion " — a form of energy — and the intensity of heat (but not 

 its quantity) depends on the rapidity of motion of the molecules. 

 At a temperature of 273° C. below the freezing-point of water all 

 movements of the molecules themselves (but not of the parts of 

 'the molecules) cease, and this temperature is the absolute zero, 

 and is approximately that of cosmic space. When the tempera- 

 ture rises, the molecules move more rapidly until they become 

 loosened from, but still attract, each other ; then the body melts. 

 Rising in temperature still more, the molecules finally cease to 

 attract each other, and each of them moves freely so that they 

 tend to fly apart; then the body becomes a gas. The tempera- 

 ^tures at which melting and vaporisation occur are, of course, 

 different ones in different materials, and depend on the nature of 

 the molecules of which the body is composed. 



When bodies are at diflerent temperatures, heat tends to flow 

 of itself from the warmer to the colder body. If, when we touch 

 a body, heat flows from it to our skin, we say that the body is 

 warm, and if the converse happens, we say that the body is cold. 



Density. — The more molecules there are in the same bulk the 

 . denser, we say, the body is. Thus a square inch of ice contains 

 a certain number of molecules of HgO, and the substance has a 

 certain density ; but when the ice is melted and the temperature 

 of the water rises to, say, 65° F., the same number of molecules 

 now occupies a greater volume (for they are further apart), and 

 so the density becomes less. 



When the water is raised to, say, 220° F., it is converted into 

 steam, and it now occupies over 1,700 times the volume it had in 

 the liquid state. Therefore its density is very much less. This 

 means that the density of a body depends on the closeness with 

 •which the molecules are packed, or cohere together. But it also 

 depends on the relative weights of the molecules, for some are 

 'heavier than others; thus the molecules of quicksilver are, each 

 of them, much heavier than those of water. 



