146 HEAT AND LIGHT AS COMPANIONS 



self were moving outward, it would carry the chip with it, 

 but the water has no forward motion, and hence the chip as- 

 sumes the only motion possessed by the water, that is, an 

 up-and-down motion. Perhaps a more simple illustration is 

 the appearance of a wheat field or a lawn on a windy day ; the 

 wind sweeps over the grass, producing in the grass a wave like 

 the water waves of the ocean, but the blades of grass do not 

 move from their accustomed place in the ground, held fast 

 as they are by their roots. 



If a pebble is thrown into a quiet pool, it creates ripples or 

 waves which spread outward in all directions, but which soon 

 die out, leaving the pool again placid and undisturbed. If 

 now we could quickly withdraw the pebble from the pool, the 

 water would again be disturbed and waves would form. If 

 the pebble were attached to a string so that it could be dropped 

 into the water and withdrawn at regular intervals, the waves 

 would never have a chance to disappear, because there would 

 always be a regularly timed definite disturbance of the water. 

 Learned men tell us that all hot bodies and all luminous bodies 

 are composed of tiny particles, called molecules, which move 

 unceasingly back and forth with great speed. In Section 95 

 we saw that the molecules of all substances move un- 

 ceasingly ; their speed, however, is not so great, nor are their 

 motions so regularly timed as are those of the heat-giving and 

 the light-giving particles. As the particles of the hot and 

 luminous bodies vibrate with great speed and force they 

 violently disturb the medium around them, and produce a 

 series of waves similar to those produced in the water by 

 the pebble. If, however, a pebble is thrown into the water 

 very gently, the disturbance is slight, sometimes too slight to 

 throw the water into waves ; in the same way objects whose 

 molecules are in a state of gentle motion do not produce 

 light. 



