PHYSICAL FORCES 87 



occasions the assumption by water of its solid form, as 

 ice, and that heat will convert it into steam. By the 

 continued application of heat, a vessel of water may be 

 emptied the whole mass being boiled away into aqueous 

 vapour, while when that vapour passes into a cooler space 

 it becomes condensed into a cloud of particles of water 

 as above stated. 



A multitude of other substances are known to be 

 capable of similar changes as, for example, mercury. It 

 is notorious that lead can be easily made liquid by heat, 

 and iron also, though not as readily. There is little 

 doubt but that all substances can exist in these three 

 states. Various gases have been made both liquid and 

 solid, and Professor Dewar has recently liquefied the 

 gas oxygen and even the air we breathe, while many 

 components or rocks have been rendered both liquid 

 and aeriform. 



Heat then is evidently a very powerful agent in 

 effecting the change of state from solid to liquid, and 

 also that from liquid to vapour. Pressure has a con- 

 trary tendency, but it requires an enormous amount of 

 pressure to counteract the effect of a very little heat. 

 In mechanics, we have regarded bodies as incompres- 

 sible, but in fact they are all compressible, though in 

 very different degrees. Aeriform bodies are easily com- 

 pressed, and when released from pressure, spontaneously 

 expand ; but extraordinary force is required to com- 

 press water, and greater force still to compress solids, 

 which are almost incompressible. 



Liquids may spontaneously pass into the aeriform 

 condition. Such is the case with water, a thin layer of 

 which will (with different degrees of rapidity according 

 to circumstances) " dry up " by a spontaneous process, 

 called evaporation, by which it passes into the sta.te of 



