CORRELATION OF FORCES. 163 



tion of energy, has been advanced by Sir W. Thomson, 

 viz., that all the various forces or forms of energy in 

 nature are, sooner or later, transformed into heat ; and 

 as there is a tendency in all bodies to assume equality of 

 temperature, a time must sooner or later arrive when all 

 available energy due to difference of temperature must 

 cease. 1 



VIII. Coincidence of change of matter and its forces. 

 This principle affirms that every change in the molecular 

 structure of bodies is attended by a coincidental change of 

 its forces. Nearly all the physical and chemical properties 

 of bodies appear to arise from their atomic and molecular 

 structure. For instance, electric polarity and chemical 

 change are probably results of certain relative molecular 

 positions of two (or more) sets of particles, often coinci- 

 dent with a particular range of pressure and temperature. 

 As also the position of each set of particles amongst them- 

 selves varies with every change of temperature, electric 

 polarity .(and the electric and chemical attractions and 

 repulsions arising from it) may be caused or prevented by 

 sufficient change of pressure or temperature. An example 

 of this kind is seen in the union of a mixture of oxygen 

 and hydrogen gases by raising their temperature to that 

 of a red heat, and in the disunion of the elements of the 

 vapour of water thus produced, by raising steam to the very 

 much higher temperature of melting platinum. 



IX. The principle of correlation of forces affirms that 

 no one of the forms of energy which we term heat, light, 

 electricity, magnetism, chemical affinity, &c., can be dis- 

 turbed without effecting a change in some or the whole of 

 the others. 2 This principle (and the previous one) also 

 accords with the truth that each individual substance alters 



1 See Recent Advances in Physical Science, 2nd edit. p. 146, by 

 P. Tait. 2 See page 33. 



M 2 



