Jan. 24, 1884] 



NATURE 



283 



manual, whose raison d'etre is justified by its having 

 reached a fourth edition. If our readers are " philo- 

 sophers," they will not require an account of what logar- 

 ithms are (see Mr. Glaisher's excellent description in the 

 " EncyclopiEdia Britannica," vol. .\iv.) ; if they are not, 

 with Mr. Squeers we say, " Then I am sorry for you, for 

 I sha'n't be able to explain them." 



The tables, are in the main, five-figure tables, except 

 that the logarithms of looi to iioo are given to seven 

 places, and in the case of the logarithms of numbers ex- 

 tend to the logarithm of loooo. The other tables are 

 logarithms of sines and tangents to every minute of the 

 quadrant, and Gauss's sum and difference logarithms. 

 Besides, there are a capital introduction, tables of useful 

 constants with their logarithms, and solutions, by trigono- 

 metrical tables, of quadratic and cubic (.i"^ ± px + q = 0) 

 equations. There are no tables of natural sines and tan- 

 gents. We have no hesitation in commending these 

 tables to a still wider public than they have already 

 reached. R. T. 



Frmciples of Theoretical C/ieinistry. By Ira Remsen. 



(Philadelphia ; H. C. Lea's Son and Co., 1883.) 

 IJNFORTUN..VTELV for some years past we have been 

 treated with an immense number of "books" on che- 

 mistry in England of a most mechanical type : books in 

 which no reasoning theory is apparent. A dry epitome 

 of facts in a most unpalatable shape, embellished here 

 and there with formulae of various kinds, graphic, sym- 

 bolic, empiric, or glyptic, but in very rare cases any 

 attempt at showing the learner, easily, how these ideas of 

 chemical constitution, represented by formuke, are clearly 

 arrived at. If a student is unable to see, m his mind, 

 how the formula H^SOj represents a knowledge of the 

 constitution of sulphuric acid, he had much better only 

 know its percentage composition, as it may otherwise 

 lead him wrongly. 



From the style of the present work, and some others 

 we have recently seen from the other side, our cousins 

 are taking up chemistry in a more philosophic manner 

 than ourselves. And it is easy to see whence this view 

 comes. Considering that we own a Dalton it is strange 

 that the development of chemical theories is so lightly 

 treated in English text-books. Are English students so 

 superficial or so under the domination of Exams, that a 

 work like Kopp's " Entwickelung" is too much for them? 



This very condensed little work, just over 200 pages, is 

 intended for somewhat advanced students who have a 

 basement of facts to build upon. It commences with a 

 general discussion of atoms and molecules, which is con- 

 tinued in a very simple and clear manner, with the excep- 

 tion of a few nauish words like chemism. The chapters 

 on atoms and molecules and on valency are about as 

 clear and simple as they can be made, and the same may 

 be said in regard to the opening chapter on carbon com- 

 pounds. The author has evidently a reasonable notion 

 of the value and permanence of a chemical theory, and 

 no exception can be taken to the manner of discussion or 

 expression. Speaking, for instance, of Avogadro's hypo- 

 thesis, the author says : " It is at present almost uni- 

 versally accepted by chemists, some, indeed, going so 

 far as to speak of it as a law." It is certainly one of the 

 best additions to the list of j'/wa// chemical books that has 

 been made for some time. 



Studies in Micrographic Petrography. (Ady and Hen- 



soldt, 7, Muchell Road, Nunhead, S.E.) 

 The growing interest taken in this country in the study 

 of petrography is well shown by the rapidly increasing 

 facilities offered for the prosecution of this branch of 

 science. The most recent of these has just appeared under 

 the foregoing title. It is to consist of the issue of two dozen 

 microscopic slides of characteristic minerals and rocks 

 prepared by Mr. Hensoldt of Wetzlar, with illustrative 



drawings and descriptive text by Mr. J. E. Ady, who is 

 already favourably known for his microscopic prepara- 

 tions of British rocks. The first number of the " Studies " 

 is devoted to " Eozoo/i, Led Beg, Sutherland." It con- 

 tains two lithographic plates illustrative of the so-called 

 eozoonal structure of a limestone in the north of Scot- 

 land, and four pages of descriptive text. The author 

 gives a brief reference to the literature of the subject, 

 and an account of the microscopic structure of some 

 portions of the limestone in question, which he regards 

 as akin to that of the Canadian Eozoon, but as bemg of 

 inorganic origin. We are afraid his sketch is too slight 

 to have much weight in the controversy regarding Eosoon. 

 His effort to extend the opportunities of petrographical 

 investigation, however, and to popularise this fascinating 

 but difficult branch of geology is praiseworthy, and we 

 hope that his " Studies" may meet with such success as 

 may induce him to continue them. 



LETTERS TO THE EDITOR 



[T/ie Editor does 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 con espondents 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 comii, unications containing interesting and novel facts ."l 



The Remarkable Sunsets 



The numbers of Nature for October, which are the latest to 

 be seen here at \\\i<- date, contain in the correspondence accounts 

 of the green appearance of the sun in India. Some solar phe- 

 nomena observable at present and during the whole of the past 

 month are probably related to these, and yet are sufficiently dis- 

 tinct to deserve a separate description. They have, indeed, 

 attracted the attention of everybody here by their novelty and 

 spectacular magnificence, and to some have an ominous signifi 

 cance in connection with recent seismic disturbances. 



In November and December we have in this part of Japan a 

 remarkably clear atmosphere, and this year has proved no ex- 

 ception. The ijreat snow-capped muuntain, Fujii, some ninety 

 miles away to the west, is beautifully defined to view both at 

 sunrise and sunset on most days, although during the greater 

 part of the year — the warmer part that is — it is rare to catch a 

 glimpse of ir. 



The phenomena of which I wish to record a description occur 

 everv day before and after sunset and sunrise, and serve to ma- 

 terially lengthen our day. In this latitude, although not in the 

 tropics, the shortness of the twilight is very noticeable as com- 

 pared with that of England, but at present at least an hour and 

 a half elapses between the moment of sunset and that of the dis- 

 appearance of the last of its rays, and this, with the same time 

 betu een dawn and sunrise, causes our day to be very appreciably 

 lengthened. 



On some d.iys there is round the sun, even while it is still 

 high, a considerable area of silvery glare, 40" to 50" in diameter, 

 and bordered by a lurid reddish-brown or purplish-brown halo. 

 A similar lurid turl)idity lies in the horizon, and as the sun 

 descends the halo blends with this below, while above the sun it 

 attenuates and disappears, the silvery glare remaining undi- 

 minished. When the sun sets there is still a nearly circulir area 

 of this intense glare with a diameter of about 12°. On other 

 days there is before sunset only a thin silvery light round the sun 

 diffusing away from it, and only about and after the setting is 

 the more defined area of strong light strikingly visible, and on 

 these days the horizon also shows little of the dull redness men- 

 tioned aljove. Besides the above peculiarities, the sun preserves 

 its whiteness much more than usual, so as to be only golden 

 orange when setting. 



Now follow the more remarkable phenomena. The white 

 glare, or patch of .silvery light, gradually sets, spreading out 

 along the horizon as it does so, and passing through the sunset 

 colours until little more than a red line one or two degrees det p 

 remains. This happens at about twenty minutes after .sunset. 

 At this moment, on the gray curtiin of twili^;ht appears a white 

 luminosity, which rapidly intensifies over the sunset, and shades 

 away over almost half the visible hemisphere. The brightne 



