32 CORRELATION OF PHYSICAL FORCES. 



force, which produces expansion in the one case, produces a 

 correlative contraction in the other : thus, if two bladders, 

 with a connecting neck between them, be half-filled with air, 

 as the one is made to contract by pressure the other will 

 dilate, and vice versa ; so a bladder partly filled with cold air, 

 and contained within another filled with hot air, expands, 

 while the space between the bladders contracts, exhibiting a 

 mere transfer of the same amount of repulsive force, the 

 mobility of the particles, or their mutual attraction, being the 

 same in each body ; in other words, the repulsive force acts in 

 the direction of least resistance until equilibrium is produced ; 

 it then becomes a static or balanced, instead of a dynamic or 

 motive force. 



Let us now consider the case where a solid is to be changed 

 to a liquid, or a liquid to a gas ; here a much greater amount 

 of heat or repulsive force is required, on account of the cohe- 

 sion of the particles to be separated. In order to separate 

 the particles of the solid, precisely as much force must be 

 parted with by the warmer liquid body as keeps an equal 

 quantity of it in its liquid state ; it is, indeed, only with a 

 more striking line of demarcation, the case of the hot and cold 

 bladder a part of the repellent power of the hot particles 

 is transferred to the cold particles, and separates them in their 

 turn, but the antagonist force of cohesion or aggregation 

 necessary to be overcome, being in this case much stronger, 

 requires and exhausts an exactly proportionate amount of 

 repellent force mechanically to overcome it ; hence the dif- 

 ferent effect on a body such as the common thermometer, the 

 expanding liquid of which does not undergo a similar change 

 of state. Thus, in the example above given, of the mixture 

 of cold with hot water, the hot and cold water and the mercury 

 of the thermometer being all in a liquid state before, and 

 remaining so after contact, the resulting temperature is an 

 exact mean ; the hot water contracts to a certain extent, the 

 cold water expands to the same extent, and the thermometer 

 either sinks or rises the same number of degrees, accordingly 

 as it had been previously immersed in the cold or in the hot 

 solution, its mercury gaining or losing an equivalent of repellent 

 force. In the second instance, viz. the mixture of ice with hot 



