Feb. 25, 1886J 



NATURE 



391 



count of a new Entozoon from the coeca of Strtithio 

 Dwlybdophanes, Rchw. This Nematoid [Sderostoma 

 struthioiiis) is figured on Plate S. 



A title-page and table of contents and index accom- 

 pany the October part. 



Solutions of Weekly Problem Papers. By the Rev. J. J. 

 Milne, M.A. (London: Macmillan and Co.,, 1SS5.) 



In our notice of the " Weekly Problem Papers " we gave 

 an account of the contents, and accorded to the collection 

 a warm welcome. In the meanwhile we have gone 

 through the major part of the papers, and have found the 

 questions to be well suited for their purpose : here and 

 there may occur occasional apparent exceptions, but if 

 these are exceptions, then were they also in the original 

 papers in which they occur. 



The solutions are neat, and in several cases elegant, 

 and in all the object of " increasing a student's stock of 

 mathematical knowledge, and of teaching him to put it to 

 a practical use" is well kept in view. Mr. Milne is 

 prodigal in his solutions, as not unfrequently he gives 

 two solutions of the same question. To compensate for 

 the non-insertion of solutions already printed in the col- 

 lections of Answers to the Tripos questions of 1S75 and 

 187S, Mr. Milne appends a collection of an equal number 

 of questions of a similar character, with their solutions ; 

 these, with two appendices, occupy about one-fifth of the 

 whole volume. The two notes are on the geometrical 

 theory of envelopes and geometrical maxima and minima. 



We congratulate Mr. Milne on having so successfully 

 carried out his design. We believe it was originally in 

 his plan to trace each question up to its original proposer, 

 but this very difficult task he has apparently abandoned, 

 at any rate he makes no sign in this first edition. 



The two books together will be of great service to 

 students who cannot afford to read with a private tutor, 

 as the solutions are written out in proper style, and are as 

 full in explanation as a student could expect them to be. 

 There is a long list of errata, but of these the greater part 

 are due to the original papers. We have noticed several 

 more, but this is not the place for indicating them, as the 

 student will in most cases be able to detect them 

 himself. 



LETTERS TO THE EDITOR 



[ The Editor Joes not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



\^The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containingintertstin^ and no7>elfacts.'\ 



Heat Quantities 



It would be a great advantage if some definite and generally- 

 accepted meaning were attached to the words "capacity for 

 heat," "thermal capacity," and "specific heat." As it is I 

 find that scarcely two text-books give the saine definitions of 

 these quantities. The French writers, Verdet, Jamin, Fourier 

 (Freeman's translation), and Deschanel (Everett's translation), 

 seem to agree in considering specific heat a quantity of heat ; so 

 also do B. Stewart and Daniell. Prof. Everett, in " Units and 

 Physical Constants," defines thermal capacity as a quantity of 

 heat, and on p 79 says that specific heat is also a quantity of 

 heat, while on p. 80 he points out that it is of zero dimensions. 

 In Maxwell's and Garnett's books it is pointed out that specific 

 heat is a ratio, but in the latter the term is used loosely later in 

 the book. Glazebrook and .Shaw, in their " Practical Physics," 

 make it a ratio. 



To avoid all this confusion, I would propose the following 

 definition";, on the analogy of the accepted ones for density and , 

 specific gravity : — 1 



" Capacity for Heal. — The quantity of heat required to raise 



the temperature of any body, under given conditions, by i° C, is 

 called its ' capacity for heat ' under those conditions. 



" Thermal Capacity. — The quantity of heat required to raise 

 the temperature of unit mass of any material, under given con- 

 ditions, by 1° C, is called its 'thermal capacity' under those 

 conditions. 



" Spicifc Heat. — The ratio of the thermal capacity of any 

 material to the thermal capacity of some standard material is 

 called its ' specific heat.' This standard material is usually water 

 at o' C. or 4° C." 



If some such definitions as these were generally accepted, a 

 good deal of confusion would be avoided, and the analogy with 

 density and specific gravity would be accentuated. At the same 

 time the word "specific" would be defined to mean that the 

 quantity to which it was applied was a ratio, and of zero 

 dimensions. H.^rry M. Elder 



Wellington College, Wokingham 



Permanent Polarity of Quartz 



The second note in Nature (Feb. 4, p. 325) contains an ac- 

 count of an "important discovery" by Dr. O. Tumlirz, which 

 is wrong in all essential particulars. The permanent polarity 

 discovered by that gentleman is not diamagnetic, but paramag- 

 netic. Nor is it correct that "Dr. Tumlirz appears to think 

 that these facts negative Becquerel's theory of diamagnetism," 

 but on the contrary, he takes some trouble to show that his 

 experiments are in complete agreement with that theory. 



" Becquerel's theory (1850) " seems to be a name for the one 

 discussed, though not adopted, by Faraday in his second paper 

 (1S45) on the subject, that all matter is magnetic, the medium 

 occupying an intermediate position between so-called para- 

 magnetic and so-called diamagnetic bodies. 



Arthur Schuster 



[Dr. Tumlirz's own words are : " Der Bergkrystall ist hiermit 

 der erste diamngnelische Korper der eine dauernde Polaritat 

 zeigt." — Ed.] 



Variegated Iridescent Halo 



About noon of J.anuary 28 the sky w.as overcast by a hori- 

 zontal cloud, and an iridescent halo appeared directly beneath 

 the sun. The length of the arc was not measured, but appeared 

 to be nearly or quite 90°. The most of the halo was of a bright 

 yellow colour, but not far from the middle of the arc was a spot 

 of green separated on each side from the yellow arcs by a short 

 neutral tract. The green spot was nearly round, and its breadth 

 (as measured from the sun) was nearly the same as that of the 

 yellow arcs, and the angular distances from the sun of both the 

 yellow and green arcs were approximately equal. Iridescent 

 lialos are seen here every year, usually as complete circles about 

 the sun, and often showing all or several of the prismatic colours, 

 each forming its own definite ring at different distances from the 

 sun. Such halos appear through a thick haze, or quite thin 

 cloud. This variegated halo appeared through a cloud so dense 

 that the position of the sun could barely be made out by the eye, 

 and it was of very variable density. The borders of the coloured 

 arcs were not clearly defined, and apparently the light had been 

 somewhat scattered by reflection upon the lower cloud particles. 

 The temperature was several degrees below the freezing-point of 

 water. Twice before during the present winter short mono- 

 chromatic arcs of halos have appeared, once a yellowish-green, 

 once a bright red. 



The iridescent clouds recently seen by so many of your cor- 

 respondents in Scotland appear to be, in part at least, the same 

 phenomenon as here recorded, only in this case there was 

 enough of the arc visible to definitely show that the colour was 

 due to an iridescent halo. G. H. SroNE 



Colorado College 



White Rainbows 



Probably A. E. E. will find some answer to his questions in 

 the undermentioned articles: — R. Smethurst, "Account of a 

 White Solar Rainbow" (Mem. Phil. Soc, Manchester, iii. 

 pp. 176-78, 1819) ; Aug. Bravais, " Observations sur le Pheno- 

 mene de I'Arc-en-Ciel blanc " [Proc. Verb. Soc. Philom., 

 pp. 33-35, 1846 ; .tun. de Chimie, xxi. pp. 348-61, 1847) ; 

 F. Moigno, " Nouvelle Theorie de I'Arc-en-Ciel blanc" 

 Moigno, Cosmos, ii. pp. 107-11, 1852-53) ; G. J. Symons, "On 



