CHANGES OF STATE. 13 



formed into gaseous forms of matter). With some substances 

 such " changes of state " require only comparatively moderate 

 degrees of heat, whilst with others much higher temperatures are 

 requisite, such as are obtainable by condensing the heat of the 

 sun considerably by the aid of a "burning glass," or by the 

 application of fire, or by means of powerful electric currents, this 

 latter source of heat being the most powerful one yet known so 

 far as terrestrial phenomena capable of measurement are con- 

 cerned. 



Changes of State produced by Alterations of Pressure. 



Some very curious effects are producible under certain circum- 

 stances by the application of considerable amounts of pressure to 

 bodies of various kinds. As already stated, powdered substances 

 can often be thus converted into solid masses of considerable 

 strength when an intense pressure is used to bring the particles 

 sufficiently near to one another to enable them to cohere together ; 

 here, however, no change of state (i.e., from solid to liquid or vice 

 versa) occurs. If a block of ice, just at the temperature at which 

 ice melts (vide Expt. 28), be subjected to strong pressure, a 

 portion of the ice becomes liquefied to water; but on relieving 

 the pressure this melted ice instantly solidifies again; in this 

 case the application of pressure assists the effect of heat in con- 

 verting a solid into a fluid. Many other substances are affected 

 by pressure in quite a different way ; the pressure prevents their 

 melting, so that more heat must be applied to them when under 

 pressure before they liquefy than would be necessary under the 

 ordinary atmospheric pressure only. A simple rule is always 

 followed in such cases; some substances, like water, become 

 lighter on freezing, and heavier on melting, bulk for bulk, so 

 that the solid body floats on the liquid, as ice on water ; with 

 other substances the solid form is heavier than the liquid and 

 sinks therein. With the first class of bodies the effect of pressure 

 on the solids is to tend to compress them into a smaller volume, 

 and so render them heavier than before, bulk for bulk ; as this 

 is exactly what heat tends to do in such a case when it produces 

 melting, it results that the effect of pressure is to assist heat, and 

 therefore the greater the pressure the less heat will be requisite 

 to produce fusion. With substances of the second class the 

 case is just the opposite ; applying pressure to the solid as before 

 tends to squeeze it smaller ; but as this is now the opposite effect 

 to that produced by heat during melting, it results that with 

 bodies of this kind more heat must be applied to counteract the 

 effect of pressure in order to melt them. 



