Aug. 12, 1875] 



NATURE 



291 



LETTERS TO THE EDITOR 



[l^he Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the ruriters of, rejected manuscripts. 

 No notice is taken of anonymous communications.'] 



Properties of Selenium 



In a letter headed "Anomalous behaviour of Selenium," 

 which appeared in Nature (vol. xii., p. 187), Mr. Gordon 

 states that "it has lately been observed that the electrical 

 resistance of selenium is greater in light than in the dark." I 

 am anxious to learn where an account of this remarkable obser- 

 vation is to be found. 



Mr. Gordon afterwards announces the discovery that a bar of 

 granular selenium belonging to the Cavendish Laboratory ex- 

 hibits a decrease of resistance under the influence of light. This 

 phenomenon was well-known outside the Cavendish Laboratory 

 more than two years ago. Mr. Gordon also states that the very 

 high resistance of a certain medal of selenium did not sensibly 

 alter under the influence of light; and concludes that "the 

 physical form of the metal " seems to have some influence on its 

 electrical properties. From his description of the medal it would 

 appear that it is made of vitreous selenium. I am therefore 

 surprised that its resistance was so low, A conducting form of 

 selenium having the appearance of black-lead is certainly a 

 novelty. 



It is perhaps not generally known that the electrical properties 

 of selenium are very variable. In a paper by Mr. Henry Draper 

 and myself which appeared in the " Proceedings of the Royal 

 Irish Academy" (vol. i. ser. ii. (Sci.)p. 529), we have shown that 

 there is a granular variety of the element which is, at ordinary 

 temperatures, apparently as good a non-conductor as the vitreous 

 variety. Unlike the latter, howevor, it cannot be rendered elec- 

 trical by friction. Another granular modification of the element 

 was found to conduct electricity comparatively well in darkness, 

 and scarcely any better under the influence of light ; while there 

 is an intermediate state of the element which appears to possess 

 a molecular structure so susceptible of change, that light is 

 capable of converting it temporarily into the form which con- 

 ducts comparatively well. Some bars which we prepared of 

 this sensitive variety exhibited an increased conductivity of 

 100 per cent, under the influence of sun-light. In appear- 

 ance there is not the slightest difference between this and 

 the non-conducting granular variety, both exhibiting a gray 

 granular fracture resembling that of the metal cobalt. In 

 the course of our experiments JNIr. Draper and I prepared a 

 large number of bars and plates of various shapes and sizes, but 

 we have not observed any unusual connection between the shape 

 of the bars or plates and their resistance. There is a great diffi- 

 culty in making observations witli reference to this point, as we 

 are as yet unable to produce two or more bars of the sensitive 

 variety possessing the same electrical properties. Thin plates 

 are generally more sensitive to light than cylindrical bars, but 

 we have occasionally prepared bars as sensitive in proportion as 

 a plate measuring 75 x 15 mm., and only 0*5 mm. in thickness. 



I have not as yet been able to learn the contents of Trof. 

 Adams's recent paper on this subject, but Mr. Gordon says that 

 he has shown that the phenomenon is a purely optical one. I 

 may state that Mr. Draper and I have long since shown that, so 

 far as the effect of heat on electrical resistance is concerned, 

 some forms of granular selenium conform to the metallic type. 

 This was demonstrated by placing a plate of selenium inside a 

 spiral of platinum, at a distance of about 4 mm. from the wire. 

 The usual decrease of resistance took place when the plate was 

 exposed to light ; but on heating the surrounding platinum wire 

 by passing a current of electricity through it, the resistance of 

 the selenium increased considerably. The effect of light is 

 therefore partially counterbalanced by the effect of the heat 

 which usually accompanies it. This partly explains the increase 

 of resistance that is known to follow prolonged exposure to light. 

 A portion of this increase being doubtless due to the slight eleva- 

 tion of temperature that must result from the passage of the cur- 

 rent through the selenium. The opposite action of light and 

 heat is very remarkable, especially as the longest light undula- 

 tions are those that cause the greatest decrease of resistance. It 

 is remarkable, also, that a thin film of non-conducting vitreous 

 selenium transmits these red rays, while an equally thin film of 

 granular selenium is perfectly opaque to them. 



KiCHARD J. Moss 



Mr. Darwin and Prof. Dana on the Influence of Volcanic 

 Action in preventing the growth of Corals 



In his critique on the new edition of Mr. Darwin's work on 

 Coral Reefs (Nature, vol. x., pp. 408-410), Prof. Dana adduces 

 four examples of islands in which he thinks comparatively recent 

 volcanic action has prevented the formation of extensive coral 

 reefs. One of these is Savaii, the largest island of Samoa. 



Some time ago I read Prof. Dana's " Corals and Coral 

 Islands," while on a tour on Savaii, and on the margin of page 

 302 I noted this very point now brought forward by the author 

 in his paper in Nature, intending, at some future time, to show 

 that his view respecting this island is based upon imperfect know- 

 ledge, and is altogether incorrect. 



I do not intend to enter here into all the details respecting 

 Prof. Dana's incorrect statements, but will confine myself to the 

 one point on which his views and those of Mr. Darwin are at 

 variance. In his work (p. 302) Prof. Dana says : " Savaii 

 abounds in extinct craters and lava streams, and much resembles 

 Hawaii in character ; it bears proof in every part of being the 

 last seat of the volcanic fires of the Samoan Group. Its reefs 

 are consiquently few and small.'" In Nature (vol. x. p. 409), he 

 says : " Savaii has coral reefs on its wtstern (eastern) and 

 northern shores, while elsewhere without them. 1 failed to find 

 evidence in the case of either of these volcanic regions that they are 

 situated zvithin areas of elevation rather than subsidence. Only 

 ten miles west (this should be east) of Savaii lies the large island 

 of Upolu, having very extensive reefs — on some parts of the 

 north side three-fourths of a mUe wide ; and it has not seemed 

 safe to conclude that while Upolu thus bears evidence of no 

 movement or of but little subsidence, Savaii was one of elevation; 

 or that the north and west {east) sides of Savaii have differed in 

 change of level frojii the rest of thi island." 



In the above passage Prof. Dana has reversed the relative 

 positions of Savaii and Upolu. Savaii is west of Upolu, and its 

 reefs are on the eastern end next to Upolu, and extend for some 

 distance on its north-eastern side. Its south, west, and north- 

 west sides are free from coral reefs except in bays, where they are 

 verj' narrow. 



Now what Prof. Dana did not consider it "safe to conclude," 

 viz., that part of Savaii had "differed in change of level from 

 the rest of the island," is nevertheless a fact. And more than 

 that, those parts of the island which present unmistakable evi- 

 dence of upheaval are destitute of a coral reef on their shores, 

 except the narrow fringes above mentioned. 



The elevated portions of the island commence at the south- 

 eastern point, in a line with three small islands which stand in 

 the straits between Upolu and Savaii, and which doubtless indi- 

 cate the line of fissure. I have traced the upheaval for many 

 miles along the southern coast. In some places there are old 

 water-worn cliffs from twenty to thirty feet above the cliffs which 

 at present form the coast line, and which are themselves fronr 

 twenty to thirty feet above high-water mark. These old cliffs 

 are usually within two or three hundred yards of the present 

 coast line, but are sometimes more distant. I have not at pre- 

 sent traced this upheaval around the entire western end of 

 Savaii, but I have observed the point at which it commences on 

 the northern side, as well as at the south-eastern extremity. 



How this fact tells on the point on which Prof Dana's view 

 differs from Mr. Darwin's, I may leave to those who are familiar 

 with the subject to decide. My own conviction is, that instead 

 of furnishing proof of the correctness of Prof. Dana's view, 

 Savaii supplies a remarkable example of the correctness of that 

 of Mr. Darwin, that, ceteris paribus, the extent of coral reefs is 

 chiefly determined by the depth of water on the coast. 



I have visited and examined a good many intertropical islands 

 of the Pacific belonging to the three orders : i. Volcanic islands 

 with fringing coral reefs, such as Samoa, the New Hebrides, lic. 

 2. Atolls, such as the Low Archipelago, Ellice, Gilbert Islands, 

 &c. 3. Upraised coral islands, such as Nine or Savage Island, 

 part of the Friendly, the Loyalty Islands, &c. I have studied 

 their structure with Mr. Darwin's " Coral Reefs " as my text- 

 book ; and the further I have gone the more firmly have I been 

 convinced of the correctness o.f his theory. 



Prof. Dana is, without doubt, correct in his opinion that sub- 

 marine or littoral volcanic action would destroy living corals 

 which came within its influence ; and it might for a time, even 

 after the volcano became quiescent, prevent the spread of corals 

 within the area affected by it. But the fact that in some of the 

 areas where extensive reefs are not found, narrow coral fringes 

 exist in bays (as at Savaii), where the slope of the shore is less 



