March 23, 1876] 



NATURE 



407 



Long. 

 179° 51' E. 

 168 19 W. 

 160 49 W. 

 140 46 \v. 



Lat. Long. Lat. 



2S^43'S. ... 139^51' W. ... 6^S3'S. 



19 52 S. ... 137 44 W. ... 6 24 S. 



15 49 S. ... 119 52 ^- •• 5 51 S. 



7 8 S. ... 108 12 W. ... 7 51 S. 



By a direct calculation for a point 140'' 46' W., 7° 8' S. in 

 the longitude of the Marquesas, totality is found to com- 

 mence at oh. 39m. 303. local mean time, duration 5m. 15s., 

 the sun at an altitude of 64°. 



The Minor Pl.\xets. — The four older minor planets, 

 Ceres, Pallas, Juno, and Vesta, are now in pretty favour- 

 able positions for observation, and Vesta, which will be 

 in opposition on the 28th inst., is very little below an 

 average sixth magnitude in brightness, and may probably 

 be detected without the telescope by those who are gifted 

 with strong sight and are acquainted with the planet's 

 position with respect to stars in the vicinity. On the 

 28ch it will be a very little to the left of the line joining 

 8 and e Virginis, and nearly equi-distant from these stars, 

 which are of the third magnitude. 



Niobe, like Euphrosyne, is occasionally situate at a 

 coasiderable declination. At the beginning of November 

 next she nearly attains 53° N. in the constellation Camelo- 

 pardus. An observer who may chance to meet with a 

 small star which he has not before seen at a great dis- 

 tance from the equator, must not too hastily conclude 

 that it belongs to the list of variables. 



No. 160 was discovered by Prof. Peters at Clinton, 

 U.S., on the morning of Feb. 21 ; it has been observed at 

 Marseilles by M. Bonelly. 



ON THE ACTION OF LIGHT ON SELENIUM 



ON the 18th of last month Dr. C. William Siemens 

 gave a lecture to the members of the Royal Insti- 

 tution on the above subject. 



Commencing with a general reference to light as a 

 natural force, he showed how little the potential action of 

 light made itself evident to our senses, as with the dis- 

 appearance of the Ught its effects seemed entirely to 

 vanish ; he then showed a temporary effect of light by 

 its action on phosphorescent salts, which continue to 

 glo V for a long time after the source of hght is removed, 

 and drew attention to the permanent effect produced by 

 the decomposition of the salts of silver in photography. 

 He next referred to the radiometer, Mr. Crookes' little 

 machine for illustrating light effects, which he brought 

 forward for the purpose of showing the motive power of 

 light, and closed his introduction by a reference to some 

 experiments which -he had made on a fungus that 

 lives in the dark, and which, on analysis, gave no evi- 

 dence of containing carbon, thus helping to favour the 

 hypothesis that it is not heat, but tlie ray of light which 

 breaks tip carbonic acid in the leaves of plants in order to 

 separate the carbon. 



Selenium was discovered in 181 7 by Berzelius, as a bye 

 product from the distillation of iron pyrites. It is fusible, 

 combustible, and similar to sulphur, phosphorus, and tel- 

 lurium. If melted (at 217= C.) and cooled rapidly, it 

 presents a brown amorphous mass of conchoidal fracture, 

 and is a non-conductor of electricity ; if heated only to 

 loo"" C, and retained for some time at this temperature, 

 it becomes crystalline, and is a slight conductor of elec- 

 tricity, the conductivity increasing with battery power, 

 and vary-ing according to the direction of the current, as 

 lately observed by Prof. Adams. 



It was on the 12th of February, 1873, that the Society 

 of Telegraph Engineers received a communication from 

 Mr. Willoughby Smith, one of its members, of an obser- 

 vation made first by Mr. May, a telegraph clerk at Valentia, 

 viz. that a stick of crystalline selenium offered considerably 

 less resistance to a battery current when exposed to the 

 light than when kept in the dark ; this was corroborated 

 by the Earl of Rosse, who clearly proved the action to 



be due solely to light, and who also showed the effects of 

 the light of different portions of the spectrum, and after- 

 wards by Lieut. Sale, and conjointly by Messrs. H. N. 

 Draper, F.C.S., and R. J. Moss, F.C.S. 



About twelve months ago the matter was again taken 

 up by two independent observers. Prof. Adams in this 

 country, and Dr. Werner Siemens in Germany, and 

 it was to the results obtained by the latter, and which 

 have been communicated to the Academy of Sciences of 

 Berlin, that the lecturers remarks were chiefly confined. 



The sensitive selenium plates made by Dr. Werner 

 Siemens, with which experiments were made, are formed 

 of two spirals of platinum wire, laid upon a plate of mica, 

 in such a manner that the two wires run parallel without 

 touching ; upon the plate a drop of molten selenium is 

 allowed to fall, and before solidifying, another plate of 

 mica is pressed down ; the two protruding ends of wire 

 serve to insert the selenium element in a galvanic circuit. 

 Araorphus selenium when thus tested produces no deflec- 

 tion of the galvanometer, either in hght or darkness. If, 

 however, a selenium disc which has been kept for some 

 time at 100^ C. and then cooled is inserted, a slight deflec- 

 tion of the galvanometer takes place when it is under the 

 influence of light, but none in dxrkness. If now a selenium 

 disc which has been kept for several hours at a temperature 

 of 210*^ C, a point below that of the fusion of selenium, 

 and which has been gradually cooled, is substituted for 

 the other, a considerable deflection under the influence of 

 light will be observed, whilst a hardly perceptible deflec- 

 tion takes place in the dark. 



It was also explained, as the result of an experiment, 

 that amorphous selenium did not conduct up to 80° C. ; 

 from this temperature to 210° C, its conductivity gradu- 

 ally increased, after which the conductivity again dimin- 

 ished ; in cooling it followed the same law, but in a 

 different ratio. The modification prepared by heating to 

 100° C. only is Dr. Werner Siemens' ist, or electrolyte 

 modification, whilst the other, prepared by heating to 

 210° C, is his 2nd, or metallic modification. In the ist, 

 the conductivity increases with rise of temperature ; in 

 the 2nd it decreases j the 2nd is a much better conductor, 

 but is less stable, and its conducti%'ity increases with the 

 intensity of the light, as shown from the following table, in 

 which is given the effects of different intensities of light 

 on selenium (Modification II.) obtained by Dr. Obach in 

 Mr. Siemens' laboiatory at Woolwich on the 14th February, 

 1876:- 



Relative conductivities. , 



Selenium in 



! Resistance iaOluns. 



Dcfl;Ctions. Ratio. 



From these experimental results an extension of Helm- 

 holtz's theory is derived, viz., that the conductivity of 

 metals varies inversely as their total heat (Helmholtz 

 having only the sensible heat in view), and the influence 

 of light upon selenium is explained by a change in its 

 molecular condition near the surface, from the first or 

 electrolj'te into the second or metallic modification, or, 

 in other words, by a liberation of specific heat upon the 

 illuminated surface of the crystalline selenium. 



In testing the sensitive selenium plate in the different 

 parts of the spectrum, it was shown that the actinic ray 

 exercised no sensible effect, that the effect increases as 

 we gradually approach the dark red, and that beyond that 

 point the effect again decreases, reaching almost zero 

 in the heat rays ; the value of the material then for pur- 

 poses of photometry is apparent. 



