NA TURE 



{May i, 1884 



which we find treated at considerable length. As to the 

 arrangement of the chemical part, the method adopted in 

 " Milter's Chemistry " of arranging the elements under the 

 terms metals of the alkaline earths, &c, has been adopted, 

 which is a very excellent method of arrangement for 

 teaching purposes, as it allows of elements with similar 

 propertfes being compared. There is evidently through- 

 out the whole of the book a tendency to condense far too 

 much into a small space. It would be an exceedingly 

 difficult book indeed to be put before an absolute 

 beginner. The explanatory part is reduced apparently as 

 much as possible, although a great many facts are 

 crammed in, certainly in good order ; but still a beginner 

 requires very much more explanation of facts than is to 

 be found in this book. On that account, and being more 

 an epitome of facts than explanations, especially in the 

 chemical portion, it is scarcely possible to criticise it. 

 The arrangement is very excellent and the details are 

 well up to date. We notice that ozone has been put in 

 in the form of an addendum : surely its position is closely 

 in connection with oxygen. It is very liable in this posi- 

 tion to be overlooked, or at any rate neglected, by a 

 student. As there is such a considerable amount of 

 attention given to the rare metals, especially vanadium, 

 many of its compounds being detailed, it is somewhat 

 surprising that davyum, though perhaps not yet abso- 

 lutely settled, is not mentioned along with them. On 

 looking carefully through the book, a number of points 

 occur in which more explanation, or even an explanation 

 of formula;, would be very advantageous ; but on the 

 whole Mr. Watts is to be complimented on having 

 produced a very complete, though certainly not quite 

 elementary, manual on the science. 



Arithmetical Chemistry. By C. J. Woodward, Birming- 

 ham and Midland Institute. (London: Simpkin, 

 Marshall, and Co., 1S84.) 

 THIS is almost a book of questions selected from the 

 Cambridge and Oxford Local, University of London, 

 Science and Art Department, and other examination 

 papers. It is divided into headings on laboratory calcu- 

 lations where, after an example of a volumetric or gravi- 

 metric analysis, a number of exercises and questions 

 follows, and gas analysis with corrections of gases for 

 pressure, &C, and determinations of vapour densities, 

 specific and atomic volume, specific heat, calorific power, 

 calorific intensity, kinetic theory of gases, and diffusion. 

 The explanations are in most cases short and to the 

 point, but the immense number of examples and exercises 

 given tend to make it a " getting-up " book for examina- 

 tions rather than a book to work with in the laboratory. 

 Experimental Chemistry. By J. Emerson Reynolds. 

 Part III. (London: Longmans, Green, and Co., 

 1884.) 

 Prof. Reynolds, in the first and second parts of this 

 little work, has departed somewhat from the usual method 

 adopted in chemical books for junior students. The first 

 and second pans deal entirely with the non-metals and 

 their compounds, acids, &c. : while the third part is de- 

 voted to metals. It is divided into numbered experi- 

 ments for the student to perform in rotation, and should 

 be exceedingly valuable to medical and other students 

 who have only a short time at disposal for practical che- 

 mistry. There is no tempt at systematic analysis, but 

 the experiments an sufficiently logically arranged to 

 enable a student who gives his attention to them to be 

 able to perform any simple qualitative analysis. At the 

 same time each experiment is very fully explained, and 

 the reactions expressed in most cases with equations. 

 Part III. is supplemented by a series of analytical tables 

 it the end, which, however, are not very clear. They are 

 certainly somewhat too complex for the class of student 

 for which the book is intended. On the whole, however, 

 it is a very excellent work. 



LETTERS TO THE EDITOR 



[ The 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 correspondents to keep their la as 

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Reply to Mr. Grubb's Criticisms on the Equatorial 

 Coude of the Paris Observatory 



I have just received the last number of the Scientific 

 Transactions of the Royal Society of Dublin, containing a 

 description of an instrument analogous to the equatorial con- 

 structed on a principle which I published in 1 87 1 m the 

 Camples Rendus. Since it seems to me indispensable after I 

 have studied Mr. Grubb's memoir to reply to it publicly, I beg 

 you to insert this note in Nature. In order to give greater 

 importance to the modifications imagined by himself, the author, 

 Mr. Grubb, submits the instrument imagined by me to a very 

 severe criticism, and attributes very severe defects to it. There 

 is no doubt that there is a considerable difference between the 

 instruments in question. One of them, in fact, as experiments 

 carried on during two years have proved, lends itself to the exe- 

 cution of all possible astronomical researches under the best 

 possible conditions for securing precision, while the other, 

 according to the description given in the publication above cited, 

 renders impossible a very great part of the researches to which 

 an equatorial is specially destined. Further on I shall insist on 

 this difference, but I wish first of all to reply to Mr. Grubb's ob- 

 jections and to show that they are in all points contrary to the 



fact - ,-. , c 



To give weight to his argument Mr. Grubb examines a 

 case of the construction of an instrument of 27 inches aperture, 

 and he anticipates in the construction the following difficulties, 

 which he considers insurmountable :—( 1 ) The optical difficulty 

 of constructing a large plane mirror. (2) The practical difficulty 

 of procuring a disk of the necessary dimensions. _ Mr. Grubb 

 affirms that there is no glass-works capable of making a disk of 

 •la ■ 50 large. (3) The difficulty of moving a mirror of which 

 the weight, according to Mr. Grubb's calculations, will be very 

 nearly half a ton. (4) The dearness of the instrument, which 

 would cost more than an ordinary equatorial, plus dome and 

 observatory. . 



I will discuss these points one by one. (1) The construction ot 

 plane mirrors is a settled question nowadays. Many astronomers 

 have been able to convince themselves that in the equatorial 

 coude of 11 inches aperture the introduction of a plane mirror 

 of 16 inches in no way marred the definition. The brothers 

 Henry, who constructed this mirror, have just made another 

 of 40 inches diameter, which leaves absolutely nothing to 

 be desired in this respect. This difficulty, moreover, is _ so 

 little felt or feared by our opticians that the price of a similar 

 mirror for an instrument coude is only about a quarter of the 

 price of an object-glass, in spite of the great difference of their 

 respective surfaces. Thus, for an object-glass of 27 inches, 

 the price of which is 70,000 francs, that of a plane mirror of 38 

 inches is 18,000 francs. For an object-glass of 40 inches, price 

 200 000 francs, the corresponding mirror of 58 inches only costs 

 40,000 francs. I admit, nevertheless, that in this respect Mr. 

 Grubb may have had apprehensions. In the past, in fact, serious 

 difficulties have been met with in the production of plane mirrors, 

 but my own personal experience has enabled me to realise the 

 real cause of this want of success. Up to the present time, to 

 satisfy preconceived ideas it was believed that to establish rapidly 

 an equilibrium of temperature it was necessary that the thick- 

 ness of the mirror should be small. Then, under the influence 

 of a ti<mtenin<*, however slight, or only a flexion, the mirrors 

 were deformed unequally, and consequently produced an 

 obvious alteration in the beauty of the images. I he brothers 

 Henry, studying the same question by different processes, 

 have arrived at the same conclusions. In giving to the disk a 

 sufficient thickness, the production of a plane surface is not more 

 difficult than any optical surface whatever. The means of verifi- 

 cation are so delicate that in a minor of 40 inches diameter an 

 error of 1/50,000 of a millimetre can easily be determined and 

 eliminated'. So if there be a sphericity in the minor, theradmsol 

 curvature will have at least 1600 leagues, that is to say. about 



