January, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



17 



other bird, since this extends from 82° N. in summer to 

 74" S. or more in our winter. 



Hoopoe in Orkney. 



The Field, November 24, contains a record of a Hoopoe 

 which was picked up dead near the Loch of Harray, near 

 Stromness. It was in fcood plumage, but much emaciated, 

 and had apparently been either shot or struck by a hawl-:, 

 since blood was found upon the breast. 



Spotted Crake in Co. Dublin. 



.\ Spotted Crake (Fnr-ann ^[nrueHa) was shot on Novem- 

 ber 24 near Balbriggan, Co. Dublin, by Mr. G .B. Tunstall- 

 Moore, who records the fact in the Field of December i, 

 igo6. This bird is of rare occurrence in Ireland, and has 

 been obtained most frequently- in the autumn. 



Siberian Chiff-Chaff in Suliskerry. 



At the last meeting of the British Ornithologists' Club 

 (November 21) Mr. W. Eagle Clarke exhibited a Siberian 

 Chiff-Chaff (Phylloscopus tristis), which had been obtained in 

 Suliskerry, some 33 miles west of Orkney, on September 26, 

 1902. 



Red-rumped Swraliow at Fair Isle. 



At this meeting Mr. Eagle Clarke also exhibited a Red- 

 rumped Swallow (Hiriinda rufula) procured at Fair Isle on 

 June 2, 1906. During his stay at Fair Isle, where he was 

 engaged in studying migration, .Mr. Eagle Clarke obtained 

 many other rare birds, including fine specimens of Acanthis 

 hornemanni, 2Iuscicapa parca, PhyUiscopus superciliosus, 

 Ci/anecula suecica, Carpndacus erythrinus, Enuberiza pusilla, 

 E. horfulana, and Aerocephahts strcpcrus — a really remark- 

 able list, showing how much is yet to be done by careful 

 study of our migratory birds. 



Harcourt's Petrel at Hythe. 



Dr. N. F. Ticehurst at this meeting exhibited a female of 

 Harcourt's Stormy Petrel (Oceanodroma castro), which had 

 been shot near Hythe, Kent, on November 8, 1906. This 

 makes the second occurrence of this species in Great 

 Britain, the first having been killed near Littledale on 

 December 5, 1895. 



Greyheaded Wag-tail Breeding at Lydd. 



.V pair of tireyheaded wagtails {Motacilla borcalis), with 

 their nests, were taken by Mr. Bristow, of St. Leonards- 

 on-Sea, from a marsh between Rye and Lydd on June 20, 

 1906. These were exhibited at the November meeting of 

 the Club, and this is probably the first recorded instance of 

 the breeding of this bird in Great Britain, \^'e regret that 

 the birds were slaughtered, and are not overjoyed at re- 

 cording the taking of the eggs. 



PHYSICAL. 



By Alfred W. Porter, B.Sc. 



The Writings of J. Willard Gibbs. 



The publication of the collected papers of the late J. W. 

 Gibbs is drawing renewed attention to his important 

 theoretical work on the borderland of chemistry and 

 physics. .Most of these are now available to the En"-Iish 

 reader for the first time. His paper on the Equilibrium of 

 Heterogeneous Substances, first published in the Trans- 

 actions of the Connecticut Acadcmv in i«;8, has indeed 

 been transited into German bv Professor Ostwald 'and into 

 French by Professor Le Chatejier ; but not until now has it 

 been made generally accessible. This reticence is as re- 

 niarkablc as the paper itself is of importance in the study of 

 physical chemistry. 



. ^,"^.?,[ "^^ earliest of his papers is on Graphical Methods 

 in the Ihermoclynamics of Fluids. Evervone is familiar with 

 the indicator-diagram of a steam-engine, the ordinates of 

 which represent the pressures of the steam at successive 

 instants, while the abscissa; represent the corresponding 

 volumes I he area enclosed by the diagram represents 

 the work done while the piston goes through a complete 



cycle of positions. This is the only diagram which had 

 been in use up to his day. Gibbs discusses all the various 

 diagrams which might possibly be useful. One of these is 

 the diagram in which the entropy of the steam is plotted 

 against temperature. This diagram, which was independ- 

 ently suggested in Britain by J. MacFarlane Gray, is now 

 in general use in connection with engines. It is probably 

 not possible in this note to convej- to anyone unfamiliar 

 with entropy what is denoted by that term ; but the follow- 

 ing remarks may indicate how the usefulness of the notion of 

 entropy arises. 



If we reckon the heat that flows into or out of the steam 

 during one complete cycle of changes (considering it as 

 positive or as negative, according as it enters or leaves the 

 steam respectively), the total amount is not usually zero, as 

 is evident from the law of the conser\-ation of energy. For, 

 as a rule, energy at the same time passes out of the steam 

 in the form of external work, and it is the total energy of all 

 kinds which comes back to its initial amount when the 

 steam is brought back to its original state. It is meaning- 

 less, therefore, to talk of the heat in a body. Heat passes 

 in, it is true, but a fraction of it is at once transformed into 

 work. If we spoke of the " heat in a body " as the algebraic 

 sum of the amounts that had flowed in or out, its value for 

 a pound of steam, for example at looo C, would depend 

 upon how many times it had been made to pass in any 

 inter\-al through a cyclic series of changes. But it was 

 shown by Clausius that if you divide each small amount of 

 heat which enters by the absolute temperature at which it 

 enters, the sum of these quantities is zero for any complete 

 cycle of changes in the steam (or other substance), provided 

 the changes are such that they could be made to take place 

 in a reverse way without making more than a minute 

 change in surrounding bodies. The meaning of the last 

 clause depends on the fact that heat will only flow from hot 

 bodies to cold ; hence, if the surroundings from which heat 

 has been drawn while the changes are taking place in the 

 direct way are at a much higher temperature than the 

 steam, it would be necessary- to make a considerable change 

 in their temperature before the steam could be made to give 

 up the heat to them again ; such a case is said to be an 

 irreversible one, and is to be excluded from the above state- 

 ment. The quantity thus obtained, which depends onlv 

 upon the state of the substances, is what is known as the 

 eniropy of the substance. If its value for each state be 

 plotted against the temperature, areas on the diagram so 

 obtained represent heat, and such a diagram is "now in 

 frequent use, especially in connection with turbines. 



Gibbs shows that diagrams can be obtained in which all 

 the standard lines for a perfect qas are straight. Thus, if 

 log P be plotted against log V the following lines are all 

 straight :— Lines of (i.) constant pressure ; (ii.) constant 

 volume; (iii.) constant temperature; (iv.) constant energ>- ; 

 and (v.) constant entropy. The first arc vertical, the second 

 horizontal, the third slope downwards from left to right at 

 450, and so do the fourth; the fifth slope downwards from 

 left to right at an angle whose tangent is 7 where 7 is the 

 ratio of the specific heats of a perfect gas. 



Optical Properties of Carbon Films. 



Pure carbon, which has often been regarded as " a per- 

 fectly black body," that is to .say, a perfect absorber of all 

 kinds of rays incident upon it, is, in reality, transparent in 

 sufficiently thin films. The transparency decreases, on the 

 whole, in the infra-red as the wave-length of the incident 

 radiation diminishes ; but it does not do so in a perfectiv 

 regular manner. R. W. Wood, experimenting on carbon 

 prisms, measured the refractive index for various lights, 

 and found that red rays are more retarded than the blue; in 

 other words, the prisms showed anomalous dispersion. 

 Greater retardation, it must be borne in mind, implies a 

 greater refractive index, and the law for normal dispersion 

 is that the greater index corresponds to the shorter wave- 

 length. This anomaly is what would result if there were a 

 strong absorption band in the yellow. In the case of lamp- 

 black a maximum of absorption was actually found by 

 Nichols at wave-length 520 ntJ., and in the case' of asphalle 

 a similar band is found, but somewhat nearer the red end. 

 A series of observations has recently been made bv H. .\. 

 Clark (Physical Pevicw, November", 1906), in which the 



