264 



NA TURE 



[August 2, 1877 



greatly diminishes the electric conduction-resistance which 

 would be produced by polarisation, due to layers of liquid 

 of opposite electrical nature, collecting in contact with the 

 electrodes " (p. 344). 



" Potential and /(V/wd^w.— Previous to the completion of 

 the circuit and formation of an unimpeded current, the 

 free ends of the polar wires attached to the two metals 

 are charged with the two kinds of tlectricity in an 

 accumulated or free static condition, and are in a state of 

 electric potential, i.e., possessing a capability of doing 

 electric work. These accumulated electricities in the 

 wires may be detected by means of a very delicate 

 electroscope. The free electricities are also in a state 

 of tension, constantly tending to escape and unite ; and 

 their degrees of tension may be measured by means of an 

 electrometer" (p. 71). 



From which it would appear that the difference between 

 potential and tension lies in the fact that the one is to be 

 detected by an electroscope, and the other measured by 

 an electrometer. It would be just as satisfactory a 

 distinction, and would besides have the merit of being 

 true, to say that " potential " is the shibboleth of the 

 electrically unlearned, while " tension " is their refuge at 

 all times. 



Over and over again we find such phrases as these : — 

 " If the current to be measured is one of low electro- 

 motive force " (p. 73) ; " a current of less quantity and 

 greater electromotive force " (p. 338) ; and after we have 

 been expressly told on p. 72 that there is no difference 

 between currents except as regards their quantity per 

 minute, it is surprising to learn that " as a general rule, 

 the greater the electromotive force, and the smaller the 

 quantity of the current, the harder and brighter is the 

 deposited metal " (p. 344). 



But it is needless to multiply examples. We have given 

 enough to show how much Dr. Gore has done to mar a 

 really good book by adopting a precedent which, however 

 well followed, is of very doubtful utility. In the present 

 instance it may, perhaps, serve the good purpose of acting 

 as a warning to future practical writers. 



We have noticed comparatively few typographical 

 errors. Is it the author, or the printer, or the author's 

 evil genius, or the printer's devil that we have to thank 

 for this bewildering statement on p. 182 ? — 



" Silver may be cleaned in water in which potatoes 

 have been boiled, and a superior polish is thus imparted 

 to them." 



OUR BOOK SHELF 



Enumeracion lic las Plantas Europeas que se Italian como 

 silvestres en la Provincia de Buenos Aires y en Pata- 

 gonia. For Cdrlos Berg. (Buenos Aires, 1877.) 



This is a very interesting list of European plants intro- 

 duced by various means into the two above-mentioned 

 countries. It gives the relative abundance of each species 

 and the conditions under which it is found. Altogether 

 116 Dicotyledons, 30 Monocotyledons, and S Cryptogams 

 are mentioned. Of these no less than loS are common 

 to Britain. As might be expected, the natural orders 

 Compositas and Gramineas, each with 20 species, and 

 Caryophylleas with 12, are the strongest in point of number 

 of species. Many notes are scattered through the twenty- 

 four pages, from which we learn that under such extremely 

 different conditions some of our British plants attain 

 extraordinary dimensions. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undatate to return, 

 or to correspond with the writers of, rejected maiinsaipts. 

 No notice is taken of anonymous communuatiotis. 



The EdUor urgently rei]uests correspondents to keep their littei's as 

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

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts. '\ 



Optical Spectroscopy of the Red jEnd of the Solar 

 Spectrum 



Being now, with my wife, on tlie return voyage from a private 

 spectroscopic experiment on sular light in Lisbon, there appear 

 to be two or three reasons why I should request your leave to 

 send this preliminary note to Nature belore attempting to 

 publish anywhere a full account of what was seen. 



As the first of these reasons, I may mention that the continual 

 assistance kindly afforded us by M. Oom, the Astronomer-Royal 

 of Portugal, and the several lacilities obligingly granted to us, 

 through his intervention, by the Portuguese Government, render 

 an early and hearty acknowledgment imperative. All the more 

 so, too, as the last and most successful series of observations, 

 through four successive days of blazing sunshine, without the 

 smallest speck or suspicion of a cloud anywhere from morning to 

 evenmg each day, was made in a new suite of rooms recently 

 prepared for the local astronomer's residence in the Royal Park 

 of the Ajuda. 



The second reason is the pleasing one to confess, that out o( 

 four prismatic arrangements tried in the same spectroscope — the 

 one which had the highest dispersion (viz. , 32" from A to H) 

 gave also the best and most satisfactory definition, showing 

 thereby such wondrously fine and minute detail amongst close 

 lines as to cause it to be almost invariably employed, and that 

 prismatic arrangement, I am happy to say, was lately made for 

 me by Mr. Adam Ililjjer, of 192, Tottenham Court Road, 

 London, on his own long-approved plan of three powerful and 

 symmetrical compound prisms, while the eye-piece of the 

 telescope, also by him, was of rock-crystal, and fitted with his 

 peculiar reference line for micrometer mensuration. 



The third reason is the total contradiction given by the best 

 of these observations to some conspicuous features of the Royal 

 Society's last pubhcation on tlie red end of the solar spectrum, 

 when seen at a high altitude with their second and most improved 

 " Indian Spectroscope." 



Our late Lisbon measures, though made at a station close to 

 the sea-level, were yet, near the noon of each day there, and 

 on a midsummer sun in that latitude, taken through almost the 

 very same thickness of atmosphere, as the Royal Society's, and 

 Mr. Hennessey's high-sun series on the Himalaya Moun- 

 tains. But those Indian observations having been printed in 

 the Phdosophical Transactions so long ago as 1S74, 1 should be 

 glad to know whether either the Royal Society or anyone else 

 has published further particulars of the extreme red end of the 

 solar optical spectrum since then. PiAzzi Smyth, 



Astronomer-Royal for Scotland 



The Cretaceous Flora of America 



Near the close of his very interesting lecture "On the Tropical 

 Forests of Hampshire," published in Nature (vol xv. pp. 229, 

 25S, 279) Mr. J. S. Gardner says : — ■" I have great doubts, how- 

 ever, as to the correct position of many of the foreign so-called 

 cretaceous beds. Those of America, from which most of the 

 list of dicotyledons of this period is derived, appear to me, 

 from the character of their fauna, to be rather lower eocene, 

 or at most, filling in the gap between our chalk and London 

 clay. Most of the shells have a marvellously eocene-like aspect, 

 and I take it that the presence of an ammonite, and some few 

 other forms of shells which in England do not reach above the 

 chalk, sliould not be taken as conclusive evidence of the antiquity of 

 the bed, as, although migrated from our seas, they may very well 

 have lived on in other regions. It is inconsistent to assume that 

 no ammonite lived on in any part of the world to a more recent 

 period than th.it of our chalk." 



I'Vom these remarks it is evident that Mr. Gardner is not fully 

 informed in regard to the evidence which exists on the question 

 he has raised ; and as the subject is one of great interest, and 

 one which it is necessary should be carefully understood by those 



