28 PRINCIPLES OF GENERAL PHYSIOLOGY 



tion, energy passes from the hot body to the cooler one, so that the difference 

 of temperature diminishes, and with it the free energy ; the reverse passage from 

 a cool to a hot body never occurs. This fact has various applications, as we 

 shall find later. It follows from it, for instance, that if a process resulting in 

 a diminution of free energy can take place, it will invariably do so. This principle 

 was applied by Willard Gibbs (1878, pp. 216, etc.) to the investigation of the 

 deposition of substances from solution on the surfaces of bodies immersed therein, 

 and will be discussed in the next chapter. 



Clausius, at the end of a fundamental paper (Pogg. Annalen, cxxv. p. 400, 

 1865), formulates the two laws of energetics as follows : 



I. The energy content of the universe is a constant quantity. 



II. The entropy of the universe is always striving to a maximum. The word 

 "entropy" is here used as having essentially the same meaning as the "bound" 

 energy of Helmholtz. The law is therefore equivalent to the statement that 

 " free " energy is always striving to a minimum. 



The fact, derived from universal experience, that free energy always tends 

 to diminish, if it possibly can, is sometimes known as the "principle of Carnot 

 and Clausius." It was also enunciated, about the same time as the publication 

 of the paper of Clausius referred to above, by Lord Kelvin (then Prof. William 

 Thomson) under the name of the " Dissipation of Energy." 



The principle has obviously a great practical, as well as philosophical, importance. It has 

 been made by Ostwald (1912) the basis of a general rule of conduct, which he calls the 

 "Imperative of Energetics." The rule may be translated thus: "Waste not free energy ; 

 treasure it and make the best use of it." As will be admitted, the admonition is an excellent 

 one, and, when applied, leads to interesting results, as may be seen from the collection of 

 essays under this name. To mention two subjects only, which are amongst those discussed, 

 the waste involvedin war and the value of a universal standard for the sizes of printed books. 



CAPACITY AND INTENSITY FACTORS 



Another property of energy will be made clear by the following consideration. 

 The work to be obtained from a stream of water depends not only on the height 

 from which it falls, but also on the quantity of water flowing. A mere trickle, 

 even from a considerable height, is of no practical use. Energy is composed, then, 

 of two factors, which are known as the " intensity " and " capacity " factors. 



In the above case the distinction is obvious, height being intensity, and quantity of water 

 capacity. In electrical energy, the intensity factor is difference of potential or electromotive 

 force, while the capacity factor is current. In heat, the intensity factor is temperature, what 

 the capacity is does not at once seem obvious. Sometimes the name "entropy" is used, as in 

 the 0<f> diagram of the engineer, where one co-ordinate is the absolute temperature (6), the other 

 (0) is the capacity factor, or "entropy," so that the area is the heat energy. It would be 

 better, perhaps, to limit the word "entropy" to its original definition as given by Clausius. 

 viz., the ratio of the "bound" energy to the absolute temperature. 



Energy, then, is equal to a capacity factor multiplied by its appropriate 

 intensity factor. 



It will be noticed that the intensity factors are what are called " strengths," 

 whereas the capacity factors are of the nature of spaces or masses, so that 

 the latter sum together when combined, while the former do not. 



If a litre of water at 50 be added to a second litre of water at the same temperature, the 

 energy content of the mixture will be twice that of a single litre, due to doubling the capacity 

 factor ; the intensity factor, temperature, on the other hand, is not altered. 



The distinction between capacity and intensity factors appears to have been first made by 

 Helm (1887). 



The considerations of this paragraph enable us to express the second law 

 of thermodynamics in a new way, viz. : in a closed isolated system transference 

 or conversion of energy can only occur when differences of the intensity factor 

 are present. 



THEORY OF QUANTA 



In ordinary cases of chemical combination, as is well known, additions are 

 made by not less than one atom at a time ; similarly, electric charges on ions 



