14 OUTLINES OF NATURAL PHILOSOPHY. 



31. A fluid is a body so constituted, that its 

 parts are all ready to yield to the action of the least 

 pressure. 



32. Elasticity is a power by which a body, when 

 its figure is altered by the action of any force, re- 

 sumes that figure as soon as the force has ceased to 

 act, 



This power is found both in solid and fluid bodies. 



33. Light is something which has the power of 



rendering objects visible to the eye. 



is? 

 Whether light is a substance, or merely a species of 



movement impressed on a certain substance, will be 

 considered hereafter. 



In respect of light, bodies are either luminous, opaque, 

 or transparent : Luminous, when they shine, or give 

 out light without having received it from other bo- 

 dies ; opaque, when they obstruct light ; and trans- 

 parent, when they transmit it. 



34. All bodies are subject, by expansion and 

 contraction, to change their magnitude or volume, 

 according to certain laws, and within certain li- 

 mits. When expanding, they produce in us (un- 

 der certain conditions) the sensation of heat ; and 

 when contracting, the sensation of cold. They 

 are therefore said to become hot or to acquire heat 



in 



