THE PEINCIPLES OF ENERGY 33 



Texture and Lustre. — This depends on the ways in which the 

 particles are, so to speak, laid alongside each other ; thus polished 

 steel has a smooth texture, but that of a fractured piece of steel is 

 rough and crystalline. Lustre is a kind of texture, the body 

 -exhibiting it being very smooth, so that it reflects the light that 

 falls on it. 



Colour. — This depends on the chemical nature of the mole- 

 cules, and on the ways in which they are arranged together. Gold, 

 for instance, when highly polished, has bright, metallic lustre, and 

 is yellow in colour, but when it is finely divided it may be purple. 

 Sunlight is a mixture of light of different colours, or wave-lengths, 

 and material bodies can act differently on this mixture. When 

 the proportions of the lights of different wave-lengths reflected 

 from a body are the same as the proportions in sunlight, the body 

 appears white (or grey) to our eyes. When all the light that falls 

 • on a body is absorbed by it, none being reflected, we say that the 

 latter is black. When some of the wave-lengths are reflected and 

 'others are absorbed, we say that the body is coloured. Thus 

 rouge absorbs all the light falling on it except that which has the 

 wave-length associated with what we call scarlet; it reflects this 

 kind of light, and so it appears coloured. In monochromatic 

 light — that is, light of one colour — all bodies look as if they had 

 the same hue, only brighter or duller. 



Smell and Taste. — Most bodies give off fine particles into the 

 ( air, and these become inhaled into our nostrils. If such particles 

 can dissolve in the liquid bathing the olfactory mucous mem- 

 brane, they can react upon or stimulate the nerve terminations in 

 the latter, and so they give rise to the sensation of smell. If 

 a substance placed on the tongue can dissolve and affect the 

 termination of the gustatory nerves, we have the sensation of 

 taste. 



Different Kinds o£ Molecules. — The molecules, or ultimate 

 particles of which material bodies are composed, are not all the 

 same; for instance, quicksilver consists of molecules of the 

 chemical substance mercury, and water consists of molecules, 

 each of which consists of three atoms (HgO), two of hydrogen and 

 one of oxygen. Molecules are therefore made up of atoms, and 

 'there are about 100 different kinds of the latter. All material 

 bodies, of whatever nature they may be, are therefore composed 



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