40 The Realm of Nature CHAP. 



colour comes from the light, not from the object, may be 

 illustrated by sprinkling salt on the wick of a burning spirit- 

 lamp. The sodium of the salt gives out light of one wave- 

 length only, producing the sensation of yellow. Objects 

 which reflect all kinds of light and those that reflect yellow 

 appear yellow, but such things as beetroots and grass absorb 

 all the yellow light and appear black, like charcoal, which 

 absorbs all light whatever, and the most brilliant painting 

 appears in tones of black and yellow only. 



65. Heat and Temperature. The action on matter 

 of radiant energy, particularly of the comparatively long 

 and slowly vibrating waves known as heat, is to make 

 the particles oscillate more rapidly. When the particles 

 of matter vibrate rapidly they send out waves of radiant 

 energy, and thus a heated body radiates heat. Two 

 bodies are said to be at the same temperature when 

 each communicates the same amount of heat to the other 

 as it receives from it. If one body by conduction ( 59) 

 or radiation ( 63) gives to another body more heat than 

 it receives from it, the former is said to be at a higher 

 temperature. The hand plunged into water (7) lets us 

 know whether the water is at a higher or lower temperature 

 than the hand. If the water is at a higher temperature, 

 heat passes into the hand which feels warmth, if the water 

 is at a lower temperature heat passes out of the hand which 

 feels cold. The amount of heat which gives a small body 

 a great rise of temperature imparts to a large body a much 

 smaller rise of temperature. Heat is the total amount of 

 molecular motion in the mass, while temperature depends 

 on the rate of that motion. The unit of heat used in this 

 volume is the amount required to raise the temperature of 

 I Ib. of water i F. Temperature is measured by the 

 thermometer ( 440). 



66. Capacity for Heat. Heat bears to temperature 

 exactly the same relation as volume of a liquid does to 

 level. When a large quantity of liquid must be poured 

 into a vessel to raise the level one inch, we say that the 

 vessel has great capacity ; while if only a few drops are 

 required to raise the level one inch, the vessel is said to 



