HEAT. 33 



water, the substance we use as an indicator, i.e. mercury, does 

 not undergo the same physical change as do those whose 

 relations of volume we are examining. The force viewing 

 heat simply as mechanical force which is employed in loosen- 

 ing or tearing asunder the particles of the solid, ice, is ab- 

 stracted from the liquid, water, and from the liquid mercury 

 of the thermometer, and in proportion as this force meets with 

 a greater resistance in separating the particles of a solid than 

 of a liquid, so the bodies which yield the force suffer propor- 

 tionately a greater contraction. 



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 

 expand, but in different degrees ; at a certain point the 

 attractive 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, expends 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 molecular 

 resistance to expansion in the mercury of the thermometer, 

 the latter again rises, and so both go on expanding in an 

 inverse 'ratio to the resisting force. If the circumambient 

 pressure be increased, as by confining the w^ter at the com- 

 mencement of the experiment within a less expansible body 

 than itself, such as a metallic chamber, then the mercury of 

 the thermometer continues to rise ; and if the experiment 

 were continued, 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 indefi- 

 nitely, as the water did in the first case, and the water does 

 not expand at all. 



D 



