October 5, 1905] 



NA TURE 



557 



tlic first place, the student has never been taught 

 arithmetic in relation to actual measurements, but 

 has been exercised in fictitious transactions with 

 oranges and nuts, rods, poles or perches, and vats 

 into which liquor flows at the rate of so many gallons 

 a minute and out of which it flows (notwithstanding 

 the dwindling pressure) at another exact and steady 

 rate. The result is that the student has no idea of 

 the relation of magnitude to measurement, and no 

 opinion whatever on the subject of significant figures ; 

 he cannot use logarithms or a slide-rule^ and is un- 

 practised in contracted methods of computation. In 

 the second place, it is very likely that he has no sound 

 idea of proportion. Given a student in this condition 

 — and it is still the common case — the teaching of 

 what is called chemical arithmetic becomes a serious 

 |)art of the duties of a teacher of chemistry. The 

 fundamental numbers of chemistry — the atomic 

 weights — are proportional numbers, and it may be 

 --aid without exaggeration that the failure to realise 

 this and the inability to see how proportional numbers 

 may be used for the calculation of absolute weights, 

 locate the real pons asinorum of elementary chemistry. 

 In these circumstances any well considered attempt 

 to expound the elements of chemical arithmetic is 

 to be welcomed, and Prof. Wells has certainly suc- 

 ceeded in writing something on the subject which is 

 likely to be very useful. He does not quite descend 

 to the meanest capacity, but he deals in a very clear 

 way with the meaning of figures and the limits of 

 accuracy in measurement and computation. He also 

 gives a good survey of the chief types of chemical 

 problems, including all kinds of analyses and the cor- 

 rections of gas volumes. Great pains are taken to 

 impress the student with the importance of using 

 common .sense and judgment whilst performing arith- 

 metical operations, and to this end set rules and 

 stereotyped formulae are avoided. .An appendix to the 

 book contains tables, including a well printed set of 

 five-figure logarithms. Altogether the work is one 

 that may be warmly recommended to the notice of 

 English teachers. A. S. 



The Physics and Chemistry of Mi\u\ig. By T. H. 

 Byrom. Pp. xii+i6o. (London: Crosby Lock- 

 wood and Son, 1905.) Price 3^. 6d. net. 



This elementary class-book supplies information re- 

 quired for such examinations as the Board of Educa- 

 tion principles of mining, stage i. The idea is a good 

 one, as the principles of pure science upon which 

 mining practice is based are apt to receive scant 

 attention in mining classes. The author, who is 

 chemist to an important collierv company, has, as 

 lecturer at the Wigan Technical College, become 

 acquainted with the needs of students, and he gives 

 in concise form much useful information regarding 

 the atmosphere, the laws relating to the behaviour of 

 gases, the diffusion of gases, the composition of the 

 atmosphere, water, carbon, fire-damp, combustion, 

 coal dust, explosives, the composition of coals, the 

 analysis of coal, the strata adjoining the Coal- 

 measures, magnetism and electricitv. The language 

 is simple, and chemical symbols are sparingly used. 

 There is, however, a want of uniformity in nomen- 

 clature that might confuse the beginner. The terms 

 " carbonate of magnesium " (p. 96) and " magnesia 

 carbonate " (p. 125), " iron oxide and aluminia " (p. 

 46) and " iron peroxide and alumina " (p. 125) are 

 examples. The author, too, should not have included 

 Cumberland haematite among the ironstones, nor 

 granite among the strata adjoining the Coal- 

 measures. 



NO. 1875, VOL. 72] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neitlier can he undertalie 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part o/Natukk. 

 No notice is taken of anonymous communications. \ 



On the Absorption Spectrum of Benzene in the Ultra- 

 violet Region. 



In the Transactions of tile Chemical Society for August 

 Messrs. Baly and Collie, referring to the previous work of 

 Baly and Desch (Trans. Chem. Soc, 1904, Ixxxv., 1029, and 

 1905, Ixxxvii., 7t)6) on the absorption spectrum of acelyl- 

 acetone and its derivatives and the conclusions arrived at, 

 namely, that the absorption band is caused by dynamic 

 isomerism, or rather isodynamic changes, are led to infer 

 from the occurrence of bands in the spectrum of benzene 

 that these also are caused by the making and breaking of 

 the carbon bonds in the molecule of the substance. I have 

 given a similar, but not identical, explanation of the cause 

 of the bands in the spectra of uric acid, murexide, and the 

 ureides, and have pointed out that there is but little difficulty 

 in accepting a like explanation in order to account for 

 the bands in aromatic hydrocarbons, seeing that this would 

 harmonise with Kekul6's view of the constitution of 

 benzene. The particulars are contained in two papers 

 communicated to the Chemical Society on May 17, but as 

 they are still unpublished I cannot refer to them in detail. 



Messrs. Baly and Collie consider all the possible phases 

 in change of linking between the six carbon atoms in 

 benzene, and assign a band to each phase. In doing this 

 they feel justified in assuming that an even number of 

 carbon atoms is concerned in each individual process, and 

 in accordance with chemical evidence it could scarcely be 

 imagined otherwise. They argue that there are only seven 

 different makings and breakings of bonds possible, to 

 which seven different absorption bands should belong, and 

 on investigating the spectrum of benzene they find only 

 seven bands. .Seven bands were photographed (Phil. Trans., 

 1S79), as they remark, by Hartley and Huntington, but 

 no measurements are given. The wave-lengths of lines 

 in the ultra-violet had not been determined at that time 

 (1878), with the exception of the principal lines of cadmium 

 measured by Mascart, hence the reason for the absence 

 of measurements. 



In a subsequent observation (Hartley and Dobbie, 

 "Notes on the Absorption .Spectrum of Benzene," Trans. 

 Chem. Soc, 1898, Ixxiii., 695) seven bands were photo- 

 graphed and measured, but one of these appeared to differ 

 from the others in constitution, and it was indicated as 

 doubtful ; it is also a feeble band. The general character 

 possessed by the first six bands was most distinctly marked 

 in the four strongest ; each was stronger and generally 

 sharper towards the side where the rays of shorter wave- 

 lengths lie, and was weakened in the opposite direction, as if 

 the bands were composed of groups of lines occurring closer 

 together and being stronger towards the more refrangible 

 edge. Baly and Collie appear to have overlooked some 

 points of importance in this communication, since they 

 state that Hartley and Dobbie found only si.x bands, and 

 that the measurements of the actual heads of the bands 

 are not given. They give a series of numbers derived from 

 Hartley and Dobbie's measurements which for comparison 

 with their own are printed in a parallel column. The gist 

 of the paper by Hartley and Dobbie was to show the 

 structure of the benzene absorption spectrum partly by 

 measurements and partly by the aid of a photograph. 

 The bands which distinctly showed the structure were 

 numbered, but unfortunately the manner in which the 

 photograph was reproduced failed to render delicate details 

 which were visible on the original plate. The statements 

 contained in the paper appear, however, to have been 

 clearly and fully understood by W. Friederichs, who photo- 

 graphed the vapour of benzene with a Rowland grating. 

 He found fifty-six bands of absorption in its spectrum in 

 the ultra-violet, which are arranged in eight groups, and 

 he compared the principal lines of each group with the 



