320 Prof. Liveing, On the measurement of kinetic [Oct. 26, 



If S L be some independent system with energy E t , or if the 

 nature or circumstances of 8 be such that it may have energy in 

 some form E v which cannot freely be transformed into any of the 

 forms of E, the potential of E 1 will be independent of that of E 

 but may be measured on the same scale if 8V is always in the 

 same ratio to d WjdE l . 



If the scales of measurement of any two forms of energy E 

 and E 1 coincide at one point they must coincide at all points. 

 Coincidence at one point may be determined by the consideration 

 that when E and E x exist in a system or in two communicating 

 systems in such circumstances that the one can be transformed 

 into the other, there will be no tendency to any such transforma- 

 tion when both are at the same degree of potential, for no work 

 could be got out of any such transformation. 



The zero point may be taken arbitrarily, and as a zero point 

 has already been assigned to the dynamic scale of temperature 

 the same point may be retained in all cases. It is plain that 

 E and V will vanish together whenever E can be directly trans- 

 formed into heat, and this appears to be the case whenever the 

 energy is strictly speaking the energy of the molecules. 



Molar energy does not exhibit differences of potential. 



When the energy is what is called energy of position, that is 

 when the energy is that of the field, it is only when there is 

 some difference in the energy of the field in different parts 

 occupied by the system that any dissipation of the energy of the 

 field can occur within the system. The potential of the energy 

 of the field, at that point within the system where it is least will 

 therefore coincide with the zero of our scale when the energy 

 of the field has to be brought into comparison with the other forms 

 of energy in the system. 



When the energy of the field is gravitation we can hardly 

 include in the field of any experiment a sufficient difference 

 of gravitational potential to bring the energy of the field into 

 comparison, in a measurable way with the molecular energies of 

 the materials of our experiments. Nevertheless in the large field 

 of nature differences of gravitational potential must, I should 

 think, be capable of producing such effects as chemical decom- 

 position. For example, the stable arrangement of a mass of 

 mixed hydrogen and oxygen under gravity is not one in 

 which the proportions of the two gases are constant at all 

 levels, and if we have a large mass of aqueous vapour extend- 

 ing to a considerable height above the earth, there must be 

 a tendency on account of gravitation towards a separation of 

 hydrogen and oxygen and an accumulation of hydrogen in the 

 upper part of the mass, and of oxygen in the lower part. Unless 

 the potential of the energy of chemical separation passes per 



