126 



NATURE 



[7une 5, 1879 



pteruges, and a Macroglossus, also a species of Scoto- 

 philus, probably S. nigrogriseus, Gould. 



The fishes were not numerous, but some interesting 

 species were obtained, respecting which the Hon. Wm. 

 Macleay read some remarks which will be published in 

 due time. 



The insects, about fifty species, will also we hope be 

 taken up by Mr. Macleay. Among the spiders are some 

 very interesting forms, apparently quite new. 



The collection of birds numbers about 350 specimens, 

 the mammals about 120. Spirit specimens about 100. 



We believe a large portion of this fine collection has 

 been secured by the curator, Mr. E. P. Ramsay, F.L.S., 

 for the Australian Museum. 



THE SORTING DEMON OF MAXWELL'^ 



'T'HE word "demon," which originally in Greek meant 

 ■*■ a supernatural being, has never been properly used 

 to signify a real or ideal personification of malignity. 



Clerk Maxwell's "demon " is a creature of imagination 

 having certain perfectly well-defined powers of action, 

 purely mechanical in their character, invented to help us 

 to understand the " Dissipation of Energy " in nature. 



He is a being with no preternatural qualities, and differs 

 from real living animals only in extreme smallness and 

 agility. He can at pleasure stop, or strike, or push, or 

 pull any single atom of matter, and so moderate its 

 natural course of motion. Endowed ideally with arms 

 and hands and fingers — two hands and ten fingers suffice 

 — he can do as much for atoms as a pianoforte player can 

 do for the keys of the piano — just a little more, he can 

 push or pull each atom in any direction. 



He cannot create or annul energy ; but just as a living 

 animal does, he can store up limited quantities of energy, 

 and reproduce them at will. By operating selectively on 

 individual atoms he can reverse the natural dissipation of 

 energy, can cause one-half of a closed jar of air or of a 

 bar of iron to become glowingly hot and the other ice- 

 cold ) cnn dirprt thp energy of the moving molecules of a 



basin of water to throw the water up to a height and leave 

 it there proportionately cooled (1° Fahr. for 772 feet of 

 ascent) ; can " sort " the molecules in a solution of salt 

 or in a mixture of two gases, so as to reverse the natural 

 process of diffusion, and produce concentration of the 

 solution in one portion of the water, leaving pure water 

 in the remainder of the space occupied ; or, in the other 

 case, separate the gases into different parts of the con- 

 taining vessel. 



" Dissipation of energy " follows in nature from the 

 fortuitous concourse of atoms. The lost motivity is 

 essentially not restorable otherwise 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. 



The classification, according to which the ideal demon 

 is to sort them, may be according to the essential cha- 

 racter of the atom ; for instance, all atoms of hydrogen 

 to be let go to the left, or stopped from crossing to the 

 right, across an ideal boundary ; or it may be according 

 to the velocity each atom chances to have when it 

 approaches the boundary : if greater than a certain stated 

 amount, it is to go to the right ; if less, to the left. This 

 latter rule of assortment, carried into execution by the 

 demon, disequalises temperature, and undoes the natural 

 diffusion of heat ; the former undoes the natural diffusion 

 of matter. 



By a combination of the two processes, the demon can 

 decompose water or carbonic acid, first raising a portion 

 of the compound to dissociational temperature (that is, 

 temperature so high that collisions shatter the compound 



' Abttract of Lecture at the Royal Institution, Friday, February 28, 1879. 

 by Sir William Thomson, LL.D., F.R.S. 



molecules to atoms), and then sending the oxygen atoms 

 this way, and the hydrogen or carbon atoms that way ; or 

 he may effect decomposition against chemical affinity 

 otherwise, thus : Let him take in a small store of energy 

 by resisting the mutual approach of two compound mole- 

 cules, letting them press, as it were, on his two hands, 

 and store up energy as in a bent spring ; then let him 

 apply the two hands between the oxygen and the double 

 hydrogen constituents of a compound molecule of vapour 

 of water, and tear them asunder. He may repeat this 

 process until a considerable proportion of the whole 

 number of compound molecules in a given quantity of 

 vapour of water, given in a fixed closed vessel, are sepa- 

 rated into oxygen and hydrogen at the expense of energy 

 taken from translational motions. The motivity (or 

 energy for motive power) in the explosive mixture of 

 oxygen and hydrogen of the one case, and the separated 

 mutual combustibles, carbon and oxygen, of the other 

 case, thus obtained, is a transformation of the energy 

 found in the substance in the form of kinetic energy of 

 the thermal motions of the compound molecules. Essen- 

 tially different is the decomposition of carbonic acid and 

 water in the natural growth of plants, the resulting mo- 

 tivity of which is taken from the undulations of light or 

 radiant heat, emanating from the intensely hot matter of 

 the sun. 



The conception of the " sorting demon " is purely me- 

 chanical, and is of great value in purely physical science. 

 It was not, invented to help us to deal with questions 

 regarding the influence of life and of mind on the motions 

 of matter, questions essentially beyond the range of mere 

 dynamics. 



The discourse was illustrated by a series of experi- 

 ments. 



PAOLO VOLPICELLI 



THIS eminent Italian physicist, whose death we recently 

 recorded, was born at Rome on January 8, 1804. 

 He lost his mother a few days after his birth ; his 

 father was Prof. Alexander Volpicelli, a member of the 

 Medical College of the Roman University. Paolo was 

 educated at the college of Veroli and the University of 

 Rome, where, in accordance with the wish of his father, 

 he commenced the study of medicine, but abandoned it 

 after the first year, declaring that medicine was not a 

 science. Of his own accord he applied himself seriously 

 to the course of mathematical philosophy, and four years 

 later received the degree of doctor ad honorem in that 

 faculty. It should be mentioned that doctorates ad 

 honorem are given to only two students each year, and 

 Volpicelli' s fellow-doctor was the eminent Professor Tor- 

 tolini, who followed the same course. Before leaving the 

 University his professors recommended him to the 

 Government for a scientific position. In fact. Prof. 

 Morichini wished to name him his successor to the Chair 

 of Chemistry in the University of Rome, but Volpicelli 

 preferred to succeed Dr. Barlocci, Professor of Experi- 

 mental Physics in the same University, and in 1845, on 

 the death of Barlocci, became titular professor. Volpi- 

 celli occupied this chair till 1873, when he was appointed 

 Professor of Mathematical Physics in the same Uni- 

 versity. In 185 1 he was made a member of the Philo- 

 sophical College, an honour accorded to only twelve 

 professors of the University of Rome. Besides his position 11 

 at this University, Volpicelli also filled that of Professor '' 

 of Mathematical Physics at the Roman Seminary, taught 

 geometry to the pupils of St. Michael's Hospital, and 

 founded at Rome the special School of Artillery, of which 

 he was director for thirty years. 



Vy'hen Pope Pius IX. revived the celebrated and histo- 

 rical Lincei Academy in 1847, Volpicelli was appointed 

 secretary, a post which he held for thirty years, when, as 

 his health was failing, the academicians made him secre- 



