NATURE 



\7an. 15, 1874 



The parts of the system are now at rest relatively to 

 each other, and are all at the same temperature and pres- 

 sure. No more work can be done by the system if it is 

 enclosed within a fixed boundary through which neither 

 matter nor heat can pass. We have exhausted its avail- 

 able energy. 



But there are two methods, both of them however un- 

 available to us, by which the energy of a system, even 

 when rendered in this sense unavailable, may be recovered. 

 One is by allowing the substances to expand into 

 infinite space ; the other is by conveying all the heat 

 through a perfect heat-engine into a refrigerator at the 

 temperature of absolute zero. Hence the importance of 

 excluding these two methods, by limiting the statement 

 to a system enclosed by a boundary through which 

 neither matter nor heat can pass. 



Now the doctrine of the dissipation of energy asserts 

 that by the mutual action of the parts of such a system 

 its available energy may be diminished, but can never be 

 increased. If there is difference of temperature, conduc- 

 tion of heat takes place, and this is always accompanied 

 by a diminution of the available energy. If there is visible 

 motion, friction occurs, and this renders a certain amount 

 of the energy unavailable. 



Here, then, we have an irreversible process always 

 going on, at a greater or less rate, in the universe. If, 

 therefore, there was ever an instant at which the whole 

 energy of the universe was available energy, that instant 

 must have been the very first instant at which the uni- 

 verse began to exist. If there ever shall come a time at 

 which the whole energy of the universe has become 

 unavailable, the history of the universe will then have 

 reached its close. During the whole intervening period 

 the available energy has been diminishing and tlic 

 unavailable increasing by a process as irresistible 

 and as irreversible as Time itself. The duration of 

 the universe according the present order of things 

 is therefore essentially finite, both a parte ante and 

 a parte post. 



But, according to pure dynamics, every motion of a sys- 

 tem may be performed in the reversed direction subject to 

 the same system of forces. If then at a given instant, every 

 particle of the universe were to have the direction of its 

 motion reversed so as to start anew with an equal but oppo- 

 site velocity, ever)thing would run backwards from the end 

 to the beginning. We might attempt a description of a 

 world thus recoiling upon itself — the rivers running up 

 into the hills, heat flowing from cold bodies to hot, and 

 men passing over the stage of life from their graves to 

 their cradles, ignorant of the past and remembering only 

 the future, as Shelley sings, in his musical delirium : — 



" We have passed Age's icy caves, 



And Manhood's dark and tossing waves, 



And Youth's smooth ocean, smiling to betray ; 

 Along the glassy gulfs we flee 

 Of shadow-haunted Infancy, 



Through Death and Birth, to a diviner day." 



But then we must remember that every characteristic of 

 the past is now transferred to the future, so that if this 

 reversal of nature were actually to occur, we would be 

 quite unconscious of it. 



" Thus 

 Our weakness somehow shapes the shadow, Time." 



Now why is this state of things, though dynamically 

 possible, physically absurd ? Simply because it requires 

 the exact reversal of the motion of every atom in the uni- 

 verse. If but one atom were to receive a velocity differing 

 infinitesimally from an exact reversal, that atom would 

 leaven the whole universe with that tendency to dissi- 

 pation of energy which actually exists, and things would 

 go on as they now do. 



We must now conclude, by thanking Dr. Balfour 

 Stev/art for bringing before the general public in so clear 

 and inteUigible a form some of the more intellectual 

 results of physical science. We hope, however, that, in 

 the next edition, the comparison between Euclid's rcductio 

 ad absurdiiDi and the experimental verification of the 

 results of a physical hypothesis, as given in Art. u8, will 

 be re-written, as it is one of the very few passages which 

 remind us of what is called the popular scientific style. 



IVEBERBAUER'S "FUNGI OF NORTH GER- 

 MANY" 



Die Pihc Nord-Deutschlaud iiiit besoiderer Bcriicksichti- 

 ^ung Silcsiens. Beschrieben von Otto Weberbauer 

 Heft I. mit sechs nach der Natur gezeichneten colorirten 

 Tafeln. (Breslau : Kern ; London : Williams and 

 Norgate.) 



THE mycologist has no reason to coYnplain that he 

 has not ampleopportunitiesforidentifyingthevarious 

 objects which fall into his hands, if he has but patience and 

 book-learning enough to enable him to avail himself of 

 all the various sources of information. There are not only 

 abundant collections of dried specimens, like those of 

 Rabenhorst, Fiickel, and others, on the Continent, with 

 others at home, but every day is bringing forward some 

 new publication of greater or less excellence, with figures 

 illustrative of obscure, or little known species, as well 

 as those which are of more general occurrence. In that 

 most difficult department, the Hymenomycetes, he has a 

 host of excellent figures in Krombholz, more recent copies 

 of which are, unfortunately, by no means equal to the 

 original, while the analyses, for the most part, are unsatis- 

 factory, and sometimes altogether deceptive. Eight 

 numbers have already appeared of the I cones by Fries, 

 which have all the advantage of coming from the author 

 himself of nearly half of the species which are contained 

 in the Epicrisis, a new edition of which is now in the 

 press, including all the more recent additions, and which 

 is proceeding with a rapidity which is somewhat wonder- 

 ful, since the Prince of Mycologists is at least an octoge- 

 narian. It would be easy to mention other important 

 works still in progress both in this country and abroad, but 

 amongst them not the least so is the one whose title is 

 given above, though from its nature the progress must, un- 

 fortunately, be somewhat slow. The first part now before 

 us contains figures and analyses of twenty-six species in 

 six plates, with descriptive letterpress, and two parts at 

 least are promised every year. 



Great care has evidently been taken in the identification, 

 and it is, we think, a great merit that the author has been 

 content to adopt the commonly received nomenclature, 

 without carelessly sanctioning every new name which has 

 been proposed by ambitious or shortsighted observers. 

 We are glad, moreover, that the measurements are given 



