Dec. 27, iS8:,l 



NA rURE 



'95 



was included in the general investigation, made by the 

 writer of the present review/ by which the phenomenon 

 of thermal transpiration was discovered, and although it 

 still appears that these are the only experiments on this 

 subject, yet they conclusively prove that the difference of 

 the pressure on the two sides of the plate is proportional 

 to the square roots of the absolute temperatures. So far 

 then it would seem that the crucial experiment has been 

 made and that the verdict is against the vortex atom 

 theory ; but this is not so, for, although the experiment 

 Mr. Thomson suggests has been made, it is definitely and 

 experimentally shown in the same investigation that the 

 action of the porous plug is entirely different from that 

 which Maxwell calls thermal effusion, being due entirely 

 to the tangential action of the walls of the passages, and 

 further this tangential action is in strict accordance with 

 the present dynamical theory of gases. This experiment 

 with the porous plug, then, aftbrds no te=t whatever in the 

 way suggested by Mr. Thomson. Mr. Thomson has, we 

 think, been unfortunate in his choice of tests ; and we 

 would suggest the velocity of sound as affording a crucial 

 test for which the experimental worlv is already done. It 

 appears to be an almost obvious deduction from the 

 vortex atom theory that the velocity of sound must be 

 limited by the mean velocity of the vortex atoms ; and 

 since Mr. Thomson has shown that this mean velocity 

 diminishes with the temperature, while experimentally it 

 is found that the velocity of sound increases as the square 

 root of the temperature, it appears that the verdict must 

 be against the vortex atom theory. However the vortex 

 atoms are very slippery things, and we should like to hear 

 Mr. Thomson's opinion before adopting one of our own. 



Besides discussing the theory of gases, Mr. Thomson 

 goes somewhat fully into a vortex atom theory of chemical 

 combinations ; in this he raises many points which will 

 doubtless be of great interest should the hypothesis 

 survive the crucial test by the theory of gases which this 

 essay now for the first time renders possible. 



Of the mathematical interest of the essay we can only 

 say that to those who can appreciate it this will be found 

 to be very great. OSBORNE Reynolds 



OUR BOOK SHELF 



Krystallographische Uiitcrsuchungen an homologen und 

 isomer en Reihen. Von Dr. A. Brezina. I. Theil. 

 Methoden. (Wien, 1884.) 

 This very useful volume forms an introduction to the 

 author's crjstallographic investigations which earned the 

 prize of the \'ienna Academy. It deals exclusively with 

 the principles and the methods employed in those inves- 

 tigations, and constitutes a complete storehouse of the 

 formula required in the study of crystals, and of the best 

 means of applying those formula'. The following subjects 

 are successively treated : the optical principles involved 

 in the goniometer ; the practical use of the instrument, 

 and the errors to which it is liable ; the criticism of 

 probable errors of observation ; stereographic projection ; 

 all possible cases of trigonometrical calculation, including 

 the method of least squares ; and a slight sketch of the 

 use of the polarising apparatus. 



An important feature of the book is the illustration of 

 methods by the actual measurement of seven crystals of 

 a triclinic substance. The readings of the goniometer 

 scale are first given, and from these the reader is led 



' " Certain Dimensional Properties of Matter in the Gaseous State," Phil. 

 Trans. 187&, Part II. 



through the entire series of processes : stereographic 

 projection, assignment of indices, calculation of ele- 

 ments, and recalculation of angles, each given in its 

 place as an example of the principles and formulte em- 

 ployed. This practical illustration is a far more effectual 

 means of recommending the methods to the reader than 

 mere verbal description. 



It will probably be found that these methods of calcu- 

 lation are the most valuable part of the book ; they are 

 so systematically arranged and tabulated that the various 

 steps may be distinguished at a glance, and any numeri- 

 cal error must be detected at once, while much labour is 

 saved by the methodical order in which the operations 

 are conducted. 



It is to be presumed that the laborious process of cal- 

 culating the angle between each pair of faces from the 

 elements by means of the general formula is given as an 

 e.>cercise in the method of least squares rather than as 

 an example of the course to be actually adopted in any 

 but rare cases. 



One subject, however, of some importance is barely 

 touched upon ; namely, the criticism of images obtained 

 from crystal faces on the goniometer, and their interpre- 

 tation. Both in the descriptive paragraphs and in the 

 above-mentioned illustration, all measurements of the 

 same angle upon different crystals are assumed to be 

 equally good, so that their arithmetic mean is adopted as 

 the observed value, whereas the difficulties presented by 

 multiple images seem to deserve treatment in a boo'.c 

 which deals so exhaustively with the practical side of the 

 subject. It is to be regretted also that the discussion of 

 optical properties and measurement has been almost 

 crowded out of the work. H. A. M. 



LETTERS TO THE EDITOR 



[ The Editor does not hold hi)nse}f responsible for opinions expressed 

 by his eorrespondcnis. 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 cort espondents to keep their letters 

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

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel /acts.'\ 



The Remarkable Sunsets 



Speaking of Virginia City, the great silver mining centre of 

 Nevada State, I said, in "An Engineer's Holiday," that it "lies 

 among the foothills of the Sierra, at an elevation of 6200 feet, 

 on the eastern face of Mount Davidson . . , surrounded by innu- 

 merable interlocked mountains, conical iu outline, red-browu in 

 colour, and perfectly bare of all vegetation. These stretch, as 

 far as the eye can reach, to where the snowy tops of the Hum- 

 boldt peaks stand against the sky, and the terrible sterility of 

 the scene is enhanced rather than relieved by the thin meander- 

 ings of the Carson River, whose cour. e is marked by a narrow 

 green line. This is the only sign of water visible in the arid 

 panorama, whose bare, red cones are steeped all day in dust- 

 haze, and lighted for a few minutes at sunset by an ' Alpen- 

 glow ' which dyes the countless peaks in as countless gradations 

 of rosy light." 



It certainly did not occur to me, when I wrote the above 

 three years ago, that the finer and higher particles of the drst; 

 haze which obscures the dry air of the American desert may 

 have been concerned in producing the splendid sunset effects 

 which I witnessed at Virginia City ; but this, after our recent 

 experiences, seems very probable. D. PiDGEON 



Holmwood, Putney Hill, December 22 



I HAVE received a letter, dated December 5, from Mr. Joseph 

 Moore, of New Garden, North Carolina, U.S.A., in which he 

 informs me that "the phenomena at both sunset and aunrise have 

 been unusual in more than a dozen instances here during the 

 autumn. Only ihe night before last we had an extraordinary 

 sunset. The sky bore all the tints of which you speak, but I do 



