March 17, 1881] 



NATURE 



457 



aim, being fairly easily conducted, and at the same time 

 definite and trustworthy in their results. 



The principal chemical differences between metals and 

 non-metals are illustrated by experiments on hydrogen 

 and oxygen ; the meaning of the terms "acid,"' "base," 

 " salt," &c., are clearly demonstrated by experimental 

 evidence. The clearness of the enunciation of the fun- 

 damental assumptions of the modern atomic theory ; the 

 method, experimentally illustrated, of determining mole- 

 cular and atomic weights ; the experimental proof of the 

 splitting of elementary molecules in chemical changes ; 

 the method of determining the atomic heat of a metal ; 

 the proof of the gaseous laws ; the detciTnination of the 

 volume of unit weight of hydrogen, and the application 

 of this determination to the calculation of the weights of 

 gaseous volumes generally ; these and other experiments 

 and deductions are all admirably described. 



The author is certainly to be congratulated on the pro- 

 duction of this book ; the care and trouble bestowed on 

 it are doubtless not to be measured by the small number 

 of pages which it contains ; the result is most satisfac- 

 tory. No better guide to the study of chemical science 

 could be placed in the hands of the beginner than this 

 modest little volume of Prof. Reynolds'. M. M. P. M. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondiuts. 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 letters as 

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

 is impossible othenuise to ensure the appearance even oj com- 

 munications containing interatin^ and noi'el facts.'] 



Barometric and Solar Cycles 



I SEE that Prof. Hill regards tlie barometric evidence as 

 favourable to the hypothesis that the sun is most powerful when 

 there are fewest spots on his surface. Perhaps I may therefore 

 be allowed to stat« the reasons which have induced me to enter- 

 tain a contrary opinion, which are, I imagine, the same as have 

 also occurred to others. I quite agree with Prof. IIUl that the 

 true relation between the variations of sun-spot area and baro- 

 metric pressure will ultimately be discovered by means of the 

 admirable weather-maps of the United States. Nevertheless, 

 we must wait until these have been produced in sufficient 

 number before we attempt to generalise. 



I do not think therefore that Prof. Hill is warranted in 

 drawing any conclusion from a single map, however important, 

 such as that for July, 187S — a time of minimum sun-spots. 



Referring to your article (Nature, vol. xxi. p. 567), I find 

 the evidence from this map to be summarised as follows ; — 



" It may be vvotth remarking that this increased pressure over 

 the oceans and diminished pressure over the land of the northern 

 hemisphere is in accordance with what might be expected to 

 result from an increased solar radiation ; whilst on the other 

 hand the increased pressure over Southern and Central Asia, 

 and diminished pressure in the southern hemisphere, is not in 

 direct accordance with this supposition." 



It thus appears that this evidence is after all of a very mixed 

 nature. 



Regarding the unequal distribution of barometric pressure as 

 without doubt caused by the sun, we may with much justice 

 imagine that whenever the sun is most powerful these peculiarities 

 of distribution will be greatest and most apparent. If we now 

 look at a map of isobaric lines {Buchan, " Handy Book of 

 Meteorology") we shall find that the Indo-Malayan region is 

 one that for the mean of the year has a barometric pressure 

 probably below the average. Now during years of powerful 

 solar action we should imagine that this peculiarity would be 

 increased. But this is precisely what all the Indian observers 

 have found for years with most sun spots. On the other hand, 

 Western .Siberia in the winter season has a pressure decidedly 

 above the average, and we should therefore imagine that diuring 

 years of powerful solar action the winter pressure in Western 

 Siberia would be particularly high. This again is the state of 

 things that Mr. Blanford has found in his discussion of the 



Russian stations (Nature, vol. xxi. p. 479) to correspond with 

 years of most sun spots. 



It therefore appears that the barometric evidence, as far as it 

 goes, is favourable to the belief that years of maximum sun-spots 

 are years of greatest solar power. Balfour Stewart 



Bi-Centenary of Calderon 



I am requested by H. E. Don A. Aguilar, Secretary-General of 

 the Royal Academy of Science of Madrid, to beg you will have 

 the goodness to insert in your journal the inclosed notice from 

 that body, offering a prize for an essay on the works of 

 Calderon de la Earca. I am aware that the other Aca- 

 demies (History and Spanish) have already offered prizes for 

 similar works, but this being intimately associated with science, 

 the Academy in that branch has thought it desirable to offer a 

 separate and special one. 



I trust I may count on your kind hospitality for a foreign 

 colleague if not trespavsing too far on your valuable space. 



F. J. Ricarde-Seaver 



Conservative Club, St. James Street, S.W., March 11 



Royal Academy of Science, Madrid. 

 Programme [adopted by the Council) for the adjudication of a 

 Prize in Commemoration ef the Bi-centenary of Calderon 

 delaBarca, 1681, May 2.<„ 1881. 



The Royal Academy of Science of Madrid being desirous 

 amongst others of commemorating the bi-centenary of the great 

 Spanish dramatic poet Don Pedro Calderon de la Barca, offers 

 a prize for public comjretition on the following theme : — 



"The conception of Nature and her laws deducible from 

 the works of Calderon, as the expression of the standard of 

 scientific knowledge amongst individuals at that period who, 

 without specially professing science, excelled in the cultivation 

 of letters. An analysis of the works of contemporary poets in 

 support of their theme being optional with competitors." 

 Conditions. 



Article i.— The author of the successful essay will receive a 

 prize consisting of a bronze medal with the legend of the Royal 

 Academy of Science and the sum of 500 pesetas (20/.), as also 

 200 copies of the prize essay printed and bound at the cost of 

 the Academy. 



Article 2.— The competition shall remain open from this date 

 up to the loth May next. 



Article 3. — The essays must be written in Spanish or Latin. 



Article 4.— These must be delivered or forwarded to the 

 Secretary of the Academy (H. E. Don A. Aguilar, 2, Plaza de 

 la Villa, Madrid) before the above date, with a distinctive 

 endorsement on the outer cover, so as to be easily recognised, 

 but without further notes or indication whatever. 



Accompanying the essay the author must transmit a sealed 

 letter bearing the same endorsement as the essay itself, and 

 containing inside the name and address of the author. 



Further conditions may be learned from 



A. Aguilar, Secretary-General 



2, Plaza del.la Villa, Madrid, Februaiy 12 



The Photophone 



Three years ago, w hilst experimenting on the action of radiant 

 heat and light on the electrical resistance of substances, I was 

 induced to believe that coating selenium with varnish or lamp- 

 black would largely increase its sensibility to light. I therefore 

 annealed a stick of selenium about 2 cm. in length and 5 cm. m 

 diameter, having previously meltetl into each end a platinum 

 wire, and thus oMained a specimen which, though of very high 

 resistance, was exceedingly sensitive to the action of light. The 

 effect of diff'iised daylight was tested in the following manner :-- 

 The specimen was placed in a glass box and connected directly 

 with two Leclanche cells and a very delicate Thomson's galvaiio- 

 meter having a resistance of 6000 ohms ; a deflection of, as far 

 as I now remember, about 300 divisions of the scale was pro- 

 duced and the light was then brought to zero by means of the 

 adjusting magnet; a dark blind which had previously been 

 drawn down was now pulled up, and the result was a deflection 

 of about ICO divisions in the same direction as before. The glass 

 box was placed three yards in front and a little to one side ot 

 the window, which was closed, and the sun at the time (about 

 4 p m July, 1877) was on the other side of the house. The 



