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34 to be set in motion by those of substances of a different  kind,   although  "vibrations never take place  between substances  of the  same  nature." By this means motion is propagated among the particles of neighbouring bodies, by what is called diffusion of heat, until each body is of the same temperature, as may be shown by testing with the thermometer. Thus, if we touch something, the temperature of which is higher than that of our bodies, an increased motion of our particles is set up, and we say it feels warm; whereas, if we touch something of a lower temperature, our own temperature is lowered by the contact, while that of the other body is raised. The vibration of the particles of bodies may be increased by percussion, friction, and combustion, or other chemical change.

Heat, like light, radiates from all bodies in straight lines, and the temperature of a body, on which radiant heat falls, is raised by transmitted motion, just as a string vibrates when a sound in unison with it is transmitted to it through the air. When heat passes by direct contact from particle to particle of a body, or from the particles of one body to another, it is said to be communicated by conduction, whereas, if a space intervenes it is said to pass by 'radiation. Thus the sun's heat is radiated on to the earth, the heat of a fire radiated