Oct. 3, 1878] 



NATURE 



587 



In the next portion of the work, comprehending 

 Chapters X. to XVIII., devoted to the description of the 

 metals, Mr. Tidy has introduced some very useful tables, 

 in which he gives, under the more important metals, lists 

 of their salts, with the different acids, as well as the 

 common and constitutional formulae of the compound, 

 and in many cases the molecular weight, specific gravity, 

 and percentage of metal in the salt. This tabular form 

 has not usually been systematically adopted in text-books, 

 and has many advantages, more especially when the 

 work is used for consultation. 



In connection with this part, however, which embraces 

 also the general consideration of acids, bases, and salts, 

 it is to be regretted that the definition of an acid comes 

 so late in the volume as p. 252 ; we should have expected 

 to have found some description at least of an acid a little 

 earlier. With respect also as to what "an acid is," the 

 index of the work is misleading ; thus, for instance, on 

 consulting it with regard to the above point we are re- 

 ferred to p. 527, where we find the definition of an 

 organic acid ; the term acid in its ordinary sense being 

 only found under the term salt. It may be urged that 

 an acid is a salt of hydrogen, but it is rather too much to 

 expect one who may be reading a chemical text-book for 

 the first time to know all this, more especially as he 

 meets with no explanation of the matter as far as we have 

 found till p. 252. 



The last part of the work, comprising Chapters XIX. 

 to XXXI., is occupied with the consideration of the 

 organic division of the science. We regret that space 

 does not permit us to enter very fully into a review of this 

 portion of the book, but as far as we can judge the infor- 

 mation contained in it is accurate and well arranged. 

 Here we are glad to see, as in the chapters describing 

 the metals, tables giving the names, formulae, specific 

 gravities, boiling points, &c., of the different substances 

 arranged in their respective groups or series. These 

 tables we have no doubt will prove of very great use to 

 the student. An appendix contains an account of the 

 recent experiments of Pictet on the liquefaction of 

 oxygen, &c., and a description of Mendeleeff's ** Law of 

 periodicity of the chemical elements." Great clearness 

 in style is given to the book by the tables just referred to, 

 and to the methodical manner in which Mr. Tidy has 

 arranged the individual consideration of each element 

 under several ^eads, as " (i) History, (2) Natural History, 

 (3) Preparation, (4) Properties — (a) sensible, (/3) physical, 

 and (y) chemical," and so on ; but notwithstanding this 

 arrangement, which adds to the value of the book, we regret 

 the order in which the author has placed the non-metallic 

 elements, which we cannot help regarding as defective in 

 the case of a text-book designed for the use of students. 



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 oj anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his spcue is so great that it 

 is impossible otherwise to ensure the appearatue even of com- 

 munications containing interesting and novel facts.^ 



On the Proposed Observatory on the Summit of 

 Mount Etna 

 Two years ago (September 22, 1876) Prof. Tacchini, of 

 Palermo, read a paper before the Accademia Gioenia of Catania 



" Sulla Convenienza ed utilita di erigere suU' Etna una Stazione 

 Astronomico-Meteorologica " (Nature, vol. xv. p. 262). 

 He proposed herein that an observatory should be erected at 

 the Casa Ingles i, which is situated at the foot of the cone of 

 the great crater 9,652 feet above the 'sea. In it daily obser- 

 vations, both astronomical and meteorological, should be taken 

 during six months of the year, and the telescope should then 

 be removed to Catania and the observations continued. 



No more has been heard of this scheme but we sincerely trust 

 that it will not fall to the ground, and that, if need be, our own 

 astronomers will come forward to promote so good a work. No 

 one who has not witnessed a cloudless starlit sky on a per- 

 fectly calm night from an elevation of two miles, can realise the 

 difference between it and the same sky seen from the surface of 

 the earth. When I ascended Etna in August, 1877, I was par- 

 ticularly struck by the extraordinary briUiancy of the midnight 

 sky. It was one blaze of brilliant light. Myriads of stars 

 which I had never seen before were visible, and the whole sky 

 was studded with stars of every magnitude, colour, and bright- 

 ness. The meteors which flashed across the sky were too 

 numerous to coimt, and the stars themselves shone ^vith extra- 

 ordinary scintillations. I specially noticed a curious effect for 

 which it is not easy to account, viz., the apparent lowness of 

 the sky. It appeared to be almost pressing down upon one's 

 head, and the larger stars seemed to be suspended below the 

 sky. A good telescope brought to bear upon such a sky would 

 reap a harvest of results. Tacchini noticed that Venus cast 

 shadows, and Sirius appeared to rival Venus. 



The observatory on Etna should be constructed on the most 

 sheltered side of the mountain. It might be placed a little to 

 the west of the Torre del Filosofo, the traditional observatory of 

 Empedokles. It could be built of lava collected on the spot, 

 and it would not be difficult to sink the foundations to a depth 

 sufficient to ensure steadiness. It should be telegraphically con- 

 nected with the observatory at Catania, and barometric and 

 thermometric readings should be taken at the same instant of 

 time at the two stations. It should be provided with a good 

 8 or lo-inch refractor, the lenses of which could be transported 

 to a duplicate mounting in the observatory of Catania during 

 the winter months. Moreover, it should possess a complete 

 set of self -registering seismological instruments similar to those 

 employed by Palmieri, and now exhibited in the Paris exhi- 

 bition. Good spectroscopes should be provided, and a set of 

 instruments for magnetic observations. 



We are quite confident that considerable results would accrue 

 to many of the sciences if systematic observations were carried 

 out under the proposed conditions which have never yet been 

 attempted, and we trust that astronomers both at home and 

 abroad will not allow the subject to fall to the ground. 



G. F, RODWELL 



Compound Lightning Flashes 



In Nature, vol. xviii. p. 67, an instance is given of several 

 flashes of lightning following in the same path, and information 

 concerning similar observations is asked for. 



In almost every tropical thunderstorm the phenomena may be^ 

 seen ; to best advantage when the storm is distant. Three, four, 

 and even more discharges may take place, the second and 

 remaining flashes following in rapid succession through the 

 identical path taken by the first. The intervals between the 

 flashes vary ; one may follow another so rapidly as to seem 

 merely hke a bright pulsation in the first, or there may be an- 

 appreciable interval of darkness ; but it is certain that, if the 

 eye can be trusted, these secondary flashes follow the exact 

 course of their primary. The reason of this may be looked for 

 in the heating effect of the lightning. The partial vncuum 

 caused by the first discharge offers a line of comparatively small 

 resistance to succeeding currents. 



The singular part of the phenomenon is the rapidity with 

 which the electricity must form or collect to admit of several 

 discharges taking place at the same spot, for I do not think the 

 secondary flashes can be regarded as merely residual. 



During a severe storm at Mangalore on the 28th of last 

 April two military buildings were struck by lightning, and, 

 from the numerous paths taken by the electricity through the 

 buildings, in its passage to the earth from the points struck, I 

 was led to think the damage might be the result of the>e com- 

 pound flashes; for it was conceivable that the destruction caused 

 by each discharge might increase the resistance of the path 

 taken, leaving succeeding flashes t'^ follow in fresh directions of 



'^ 



