124 WELLS'S NATURAL PHILOSOPHY. (^^ 



Arc liquids cnm- 286. Liquids havG but a sliiibt dcCTec of 



pressible ana i o o 



elastic? compressibility and elasticity, as compaied 



with other bodies. 



.^ .„ 287. The elasticity of water maybe shown in various wars. 



What are illns- .^ ^ •' ■, y c ' c 



tratinns of the When a flat stone is thrown so as to strike the suriace oi 



elasticity of wa- ^-j^^pj. nearly horizontallv, or at a slight ande, it rebounds 

 ter r • • ' o o i 



with considerable force and frequency. "VTater also dashed 

 against a hard surface shows its elasticity by flying off in drops in angular 

 directions. Another familiar example of the elasticity of water is obser\'ed, 

 when we attempt to separate a drop of water attached to some surface for 

 which it has a strong attraction. The drop will elongate, or allow itself to bo 

 drawn out to a considerable degree, before the cohesion of its constituent par- 

 ticles is wholly overcome ; and if the separating force is at any time relaxed, 

 or discontinued, the elasticity of tlie water will restore the drop to very nearly 

 its original form and position. Mercury is much more elastic than water, and 

 rebounds from a reflecting surface with considerable velocity and violence. 

 Tlio exercise of both the elastic and compressive principle is, however, so ex- 

 tremely Umited in liquids, that for all practical purposes this form of matter is 

 regarded as inelastic and uncompressible ; or, in other words, the elasticity 

 and compressibility of water produce no appreciable effects. 

 .J, , . The compressibility of water is not so easDy demonstrated 



tent has water as is its elasticity, although the elasticity is a direct conse- 

 pressed*?'"' quent of the compressibility. An experiment of Mr. Perkins 



showed that water, under a pressure of 15,000 pounds to the 



square inch, waa reduced in bulk 1 part in 24. 



T , ^ 288. In Uquid bodies, as has been already shown (5S 3-t, 



In what man- ^ ' - \<.o ^ 



nerdothepar- 36), the attractive and repulsive forces existing between tho 

 move" upon particles are so nearly balanced, that the particles move upon 

 each other ? each other with the greatest facility. The particles wliich 



make up a collection of fine sand, or dust, also move upon 

 each other with great facility : but the particles of a liquid possess this addi- 

 tional quahty, viz., that of moving upon themselves without friction. Tho 

 particles of no solid substance, however line they may be rendered, possess 

 this property. 



289. From this is derived a great fundamental principle lying at the basis 

 of all the mechanical phenomena connected with liquid bodies, viz. : — 



parative purify, as follows ; Bain water must be considered as the purest natural watc", 

 especially that which falls in districts remote from towns or habitations ; then coiiios 

 river water; next, the water of lakes and ponds; next, spring waters ; and then tla 

 waters of mineral springs. Succeeding these, are the waters of great arms of the ocean, 

 Into which immense rivers discharge their volumes, as the water of the Black Sea, which 

 is only brackish ; then the waters of the ocean itself: then those of the Mediterranean 

 and other inland seas; and last of all, the waters of those lakes which have no outlet, as 

 the Dead Sea, Caspian, Great Salt Lake of Utah, etc. etc 



All natural waters contain air, and sometimes other gaseous substances. Fishes and 

 other marine animals are dependent upon the air which water contains for their respira- 

 tion and eiistence. It U owing to the preseuce of air in water that it sparkles and 

 bubbles. 



