46 CORRELATION OF PHYSICAL FORCES. 



If we compare the action of heat on the two substances, 

 water and mercury, alone, and throw out of our consideration 

 the ice, we shall be able to apply the same view : thus, if a 

 given source of heat be applied to water containing a mercu- 

 rial thermometer, both the water and mercury gradually ex- 

 pand, but in different degrees ; at a certain point the attrac- 

 tive force of the molecules of the water is so far overcome 

 that the water becomes vapour. At this point, the heat or 

 force, meeting with much less resistance from the attraction 

 of the particles of steam than from those of the mercury, ex- 

 pends itself upon the former ; the mercury does not further 

 expand, or expands in an infinitesimally small degree, and 

 the steam expands greatly. As soon as this arrives at a 

 point where circumambient pressure causes its resistance to 

 further expansion to be equal to the resistance to expansion 

 in the mercury of the thermometer, the latter again rises, 

 and so both go on expanding in an inverse ratio to their 

 molecular attractive force. If the circumambient pressure be 

 increased, as by confining the water at the commencement 

 of the experiment within a less expansible body than itself, 

 such as a metallic chamber, then the mercury of the ther- 

 mometer continues to rise ; and if the experiment were con- 

 tinued, the water being confined and not the mercury, until 

 we have arrived at a degree of repulsive force which is able 

 to overcome the cohesive power of the mercury, so that this 

 expands into vapour, then we get the converse effect ; the 

 force expends itself upon the mercury, which expands in- 

 definitely, as the water did in the first case, and the water 

 does not expand at all. 



Another very usual mode of regarding the subject may 

 embarras at first sight, but a little consideration will show 

 that it is explicable by the same doctrine. Water which has 

 ice floating in it will give, when measured by the thermo- 

 meter, the same temperature as the ice ; i. e. both the water 

 and ice contract the mercury of the thermometer to the point 



