THE THREE CONDITIONS OF MATTER. 23 



solids, as lead and iron, may be changed by heat to liquids ; 

 others, as iodine, may be apparently changed directly to 

 vapors ; still others, as ice, may be easily changed first to 

 the liquid, and then to the vapor form. It is probable that 

 any solid might be liquefied and vaporized by the applica- 

 tion of heat, and that the practical infusibility of certain 

 substances is due to our limited abilities in the production 

 of heat. 



(a.) Many vapors and gases, as steam and sulphurous anhydride 

 (SO 2 ,the irrespirable gas formed by burning sulphur), may be liquefied 

 by cold, the withdrawal of heat. The process is one of subtraction. 

 A still further diminution of the heat force would, in many cases, 

 lead to a solidifying of the liquid. It is probable that all gases 

 might be liquefied and all liquids solidified, if we had the power of 

 unlimited withdrawal of heat. In fact, it is claimed that the last of 

 the " permanent gases" has been liquefied already. 



60. What is a Fluid? A fluid is a body 

 whose molecules easily change their relative 

 positions. 



The term comprehends liquids, gases, and vapors. 



61. Optional Definitions. (1.) A body possess- 

 ing any degree of elasticity of form ( 45) is a solid ; a 

 body that possesses no elasticity of form is a fluid. 



(2.) A body that can exist in equilibrium under the 

 action of a pressure that is not uniform in all directions 

 is a solid ; a body that cannot exist in equilibrium under 

 such conditions is a fluid. 



(3.) A fluid that can expand indefinitely so as to fill any 

 vessel, however large, is an aeriform body ; a fluid, a small 

 portion of which, when placed in a large vessel, does not 

 expand at once so as to fill the vessel, but remains col- 

 lected at the bottom, is a liquid. 



