594 



SCIENTIFIC THOUGHT. 



30. 

 Ix>rd 

 Kelvin. 



i.e., of its availability to do work. 1 The infinitesimally 

 small motions of an immense crowd may be exerted in 

 such a way as to total up to a finite movement per- 

 ceptible to our senses and accessible to our handling, or 

 they may so mutually annul each other as to present in 

 their finite sum and aggregate the appearance of rest and 

 inaction, however turbulent their behaviour might appear 

 to an observer gifted with powers of perception millions 

 of times more delicate than ours. Lord Kelvin intro- 

 duced the conception of the availability of energy, 2 

 Clausius that of entropy (or energy which is hidden 

 away), to measure this condition of any natural system. 

 Has the statistical view any conception to put at the 

 base of this remarkable property of natural phenomena ? 

 It has, and we must assign to Clerk - Maxwell 3 the 



1 See supra, chap. vii. p. 128, 

 &c. 



- Or of " motivity " (i.e., 

 " energy for motive power "), 

 this being "the possession, the 

 waste of which is called dissi- 

 pation." See supra, chap, vii., 

 p. 168 ; also Thomson (Lord 

 Kelvin), 'Popular Addresses,' vol. 

 i. p. 141. 



a The contributions of Clerk- 

 Maxwell to this topic are notably 

 two, independently of the larger 

 view which he took of statistical, 

 as compared with historical, know- 

 ledge, of which I treat farther on 

 in this chapter. First, in the con- 

 cluding remarks of his treatise on 

 the 'Theory of Heat' ("On the 

 Limitation of the Second Law of 

 Thermodynamics") he introduced 

 his famous conception of a " sort- 

 ing demon," the meaning of which 

 fanciful device was, to impress upon 

 the student of the dynamical theory 

 of heat, first the fact that the 



loss of availability of the energy of 

 molecular motion is owing to the 

 coarseness of our senses ; and second, 

 that the restoration of differences 

 of temperature, or of availability 

 of energy, is simply a matter of 

 arrangement or order, not of an 

 increase of the intrinsic energy 

 of the system. The subject has 

 been frequently referred to, notably 

 by Lord Kelvin, who says ("On the 

 Sorting Demon of Clerk-Maxwell," 

 Royal Institution, February 1879. 

 Reprinted in ' Popular Lectures 

 and Addresses,' vol. i. p. 137, &c. ) : 

 " Dissipation of energy follows in 

 nature from the fortuitous con- 

 course of atoms. The lost motivity 

 is essentially not restorable other- 

 wise than by an agency dealing 

 with individual atoms ; and the 

 mode of dealing with the atoms 

 to restore motivity is essentially a 

 process of assortment, sending this 

 way all of one kind or class, that 

 way all of another kind or class " 



