August 27, 1908] 



.AV7 TURE 



405 



merable degrees of freedom possessed by a molecule, and 

 was unwilling to accept the position that many of these 

 degrees of freedom were out of the running, so to speak — 

 were beside the mark, for the pur|)oses of gaseous theory, 

 inasmuch as it was only those which affected, and were 

 affected by, collisions that really mattered. Anything like 

 organised motion, such as that of the planets, is out of 

 the running, of course, and so is any internal motion of 

 the parts of an atom which collisions do not produce or 

 lessen or in any way affect. 



It may be said that some collisions, like those which 

 result in chemical combinations, do shake the parts of an 

 atom — as is known by the emission of light. That is 

 quite true, but then these collisions are e.xceptional, and, 

 moreover, energy so transferred is speedily radiated away. 

 The Boltzmann-Maxwell theory only applies to that which 

 remains a permanently constituent portion of the heat 

 energy of the substance — that is to say, the energy effec- 

 tive in producing pressure and the other manifestations of 

 temperature — the unorganised random collision energy ; it 

 is this alone which need ultimately distribute itself equally 

 among the parameters, through the agency of innumerable 

 encounters. It is probable, however, that Lord Kelvin 

 would not concur in the simplicity of this statement ; he 

 continued to be impressed by outstanding difficulties. 



Dissip.ATioN OF Energy. 



Of Lord Kelvin's work in connection with the dissipa- 

 tion of energy I shall not say much. I fancy that he him- 

 self, and certainly some of his disciples, have been at 

 times inclined to attribute to the law of degradation more 

 ultimate and cosmic importance than properly belongs to 

 it. Its significance is limited to the validity of the terms 

 " heat " and " temperature "; and if for any reason those 

 terms cease to have a practical meaning, then the dissipa- 

 tion of energy also ceases to be inevitable. The theory, 

 as originally staled by its author, was formulated as an 

 axiom beginning, " It is impossible by means of inani- 

 mate material agency," &c., which at once conveys a sug- 

 gestion that by some other means it may be possible. The 

 different availabilities of energy of various kinds must be 

 essentially a human and temporary conception, useful and 

 convenient for practical purposes, but not ultimate or 

 cosmic. What devices there are for thrusting aside the 

 inevitableness of dissipation, and so evading the goal of 

 ultimate stagnation, I do not know ; they have not yet 

 been discovered by us ; but there is nothing inconceivable 

 about them. .Maxwell's "demons" is one attempt in that 

 direction ; nitrifying bacteria have been suggested as 

 another. It is not at all certain what the influence of 

 " life " may be; and all these agencies have to be elimin- 

 ated if the uncompromising dissipation of energy doctrine 

 is to be accepted. It was not originally stated in quite 

 uncompromising form (see p. 514 of vol. i.). 



The conservation of energy is a very different thini* ; 

 that applies to every form, and is a comprehensive law ; 

 but the dissipation of energy has no meaning in circum- 

 stances when " heat " and " temperature " are obsolete 

 terms, — that is to say, when what we now consider to be 

 unorganised and intractable molecular motions can be 

 dealt with in an individual and organised way. Ultimately 

 and absolutely no operation need be irreversible. Irre- 

 versibility means only that things have got temporarily 

 be}ond our control, as a fire does sometimes. 



Absoli'te Measurement. 



To Lord Kelvin, more than to anyone else, we owe the 

 realisation of the system of absolute measurement applied 

 10 such intractable quantities as are found in electricity 

 and magnetism ; and if the world decides to call its com- 

 mercial electrical energy unit — now commonly spoken of, 

 in insular fashion, as a Board of Trade unit, or B.T.U. — 

 by the universally known and appreciated name of "a 

 Kelvin," such a procedure will be entirely appropriate. 



Counting, or the enumeration of discrete quantities, is a 

 very easy and natural operation ; but measurement, in the 

 sense of expressing the warmth of a day, or the brightness 

 of a light, or the strength of a current, or the field of a 

 magnet, or the resistance of a wire, or the transparency 

 of a window, or the elasticity of a metal, or the conduct- 

 ing power of a gas, in numerical fashion, is not by any 



NO. 2026, VOL. 78] 



means a simple thing ; it usually needs great ingenuity, 

 and sometimes can hardly be done. 



The invention of suitable units, and the mode of express- 

 ing currents and electromotive forces and resistances in 

 such units, is very far from being an obvious notion ; and 

 even now the full meaning of the idea of absolute 

 measurement is not in all quarters quite clear. In the first 

 instance it was not always quite clear, I venture to say, 

 even in the mind of Lord Kelvin himself ; and a certain 

 partial incompleteness was almost necessary in order to 

 reduce electric and magnetic quantities to simple me- 

 chanics. For, as a matter of fact, they cannot be reduced 

 to simple mechanics, or, at least, have not yet been so 

 reduced ; and it was by partially blinding ourselves to that 

 fact that the ideas of the ohm, the anip<;re, and the volt 

 were attained. We used to be told that resistance was a 

 velocity, and that electrostatic capacity was a length, also 

 that self-induction was a length, and so on. But, of 

 course, resistance is not a velocity, nor is self-induction or 

 capacity a length. Nevertheless, had it not been for this 

 partially erroneous simplification, the introduction of any 

 svstem of electric measurement would probably have been 

 seriously delayed. Incidentally, it may be noted that the 

 magnetic method of measuring resistance, or " determin- 

 ing the ohm," was devised by Weber. Kelvin's first 

 method was based upon Joule's law (see p. 502, vol. i.). 



.•\bsoh;te Temperature. 



One of the remarkable achievements of I.X)rd Kelvin has 

 been the conception and determination of absolute tem- 

 perature. The idea of an absolute temperature — that is to . 

 sav, of a temperature reckoned from a real and actual 

 zero, not a conventional one, and specified so as to be in- 

 dependent of the properties of any particular substance — 

 follows rather naturally from the second law of thermo- 

 dynamics, and from the fact that the eflfici'-ncy of a per- 

 fect or reversible heat engine is independent of the pro- 

 perties of the working substance — being dependent only on 

 the temperatures at which heat is supplied and with- 

 drawn. Absolute temperature is, in fact, the reciprocal 

 of Carnot's function, as K'-lvin showf-d in 1848 (p. 100, 

 vol. i., " -Math, and Phys. Papers ")■ And the absolute zero 

 is the temperature at which the working substance has 

 exhausted all its heat in doing work, so that there is none 

 to yield up as waste — the temperature, in fact, at which a 

 condenser or " cold body " becomes unnecessary. 



On a thermal diagram a scale of temperature can easily 

 be drawn, as the rungs of a ladder between two adiabatic 

 lines, such that the area of each space is the same ; and 

 in order to find the number of rungs, with a given-sized 

 degree, it becomes a matter of experiment to determine 

 the total heat obtainable from an isothermal operation 

 performed on the substance to which the adiabatic lines 

 belong. The measurement necessary can be made upon 

 anv substance — steam or anything else — but it must be 

 dependent on an actual operation (say an expansion) — 

 not a closed cycle of operations — and on a measurement 

 of the change of energy therefrom resulting. 



Lord Kelvin gives as the general expression for the 

 absolute temperature- of any substance whatever, the 

 internal energy of which is E, 



_,_/.. rfEWT </E </T ,., , 



For an ordinary gas — = K + r-— , where K is Laplace's 

 dv dv 



cohesion constant; and —- = c-^; so this expression (A) 



dp '<P . / > 



agrees with what we obtain below as equation (5). 



The actual determination, as hitherto experimentally 

 made, of the zero of absolute temperature, below which it 

 will be for ever impossible to cool bodies — since at tha' 

 temperature they possess no heat, and, therefore, cannot 

 have anv more removed — mav be said to depend (not neces- 

 sarilv or theoretically, but actually as the simplest method 

 in practice) on the conception of a perfect gas in the first 

 place ; — that is, one of the molecules of which act upon each 

 other and upon the surrounding walls solely by bombard- 

 ment, there being no cohesion whatever between the mole- 

 cules. The temperature at which the pressure of such a 

 1 '<.-e " EnrvrVj^sdla Uritannica." article '^ Heat." 



