ynne 13, 1872J 



NATURE 



131 



normal dispersion of the solvent ; and even if no ray can then 

 acquire an index of refraction less than unity, or be completely 

 isolated from the rest, the succession of colours in the dispersion - 

 spectrum of the solution will be sensibly different from that 

 wlrich is commonly observed. 



The method employed by Kundt for the observation of ab- 

 normal dispersion in solutions of bodies with surface-colours 

 (suggested by a memoir published by Christiansen of Copen- 

 hagen), is as follows : — A drop of the concentrated solution to 

 be examined is placed on a glass plate, and over this drop is 

 fixed, at an angle of about 25°, a sharp edge of a second glass 

 plate of the same breadth as the first. At a small distance from 

 the line of contact of the two plates, the liquid prism formed by 

 capillary action is usually opaque ; but close to the line of con- 

 tact there is a prismatic layer of liquid, scarcely broader than a 

 hair, which is transparent for most of the colours. Tlie dis- 

 persion may be recognised by viewing through this layer a 

 narrow and bright flame or an illuminated slit. The observations 

 of course require a certain amount of practice, and the 

 observer must be on his guard against deception arising from 

 phenomena of reflection or refraction. The observation is 

 generally made by carrying the liquid prism to the place of the 

 glass prism in the spectroscope of Bunsen and Kirchlioff, the 

 most favourable angle to give to the liquid prism being deter- 

 mined by trial. 



Christiansen has observed that the index of refraction of a 

 strong alcoholic solution of fuchsine increases from the line B to 

 D, then diminishes rapidly as far as G, and increases again 

 beyond that line ; and the recent observations of Kundt, made 

 as above described, have demonstrated the generality of this 

 anomalous dispersion for surface-coloured bodies in the state of 

 solution ; that is to say, that in the dispersion-spectra of tliese 

 solutions the order of the colours is not the same as in the 

 solar spectrum, or in the dispersion-spectra of ordinary solutions. 

 Such was found to be the case, not only with fuchsine, but with 

 all specimens of aniline-blue and aniline-violet, with aniline- 

 green (Hofmann's iodine green), indigo (dissolved in fuming 

 sulphuric acid), indigo-carmine, carthamin, murexid (dissolved 

 in potash), cyanine, potassium-permanganate, and carmine. 



All the bodies in this list refract red more strongly than 

 violet light ; and in bodies for which the green forms the principal 

 part of the surface-colour, and can be distinctly recognised in 

 the transmitted spectrum, the green rays are the least 

 deviated. Cyanine, aniline-violet, and anfline-blue, as well 

 as indigo-carmine, give, therefore, in their dispersion- 

 spectra the following series of colours : green, blue, red, the 

 green being the least deviated. Cyanine is particularly well 

 adapted for producing the abnormal spectrum, and exhibits 

 the following series of colours : green, light blue, dark blue, 

 a dark interval, red, and traces of orange. The dispersion 

 varies however with the concentration of the solution; in 

 dilute solutions all the bodies above enumerated exhibit 

 normal dispersion. Potassium-permanganate and carmine exliibit 

 reversed spectra only when their highly concentrated solutions 

 are mixed with fine solid particles so as to form a sort of pulp, and 

 the two glass plates are pressed strongly together. A change in'the 

 dispersion is also observed accoi'ding to the angle of incidence. 



Soret pours the solution under examination into a hollow prism 

 having an angle of about 30°, and places this prism in a glass 

 trough with parallel sides, filled with the liquid which serves as 

 solvent. By this arrangement the reverse spectrum may be ob- 

 tained with solutions less concentrated and therefore more trans- 

 parent than when the liquid prism is merely surrounded by air. 



If the ordinary prism of a spectroscope be replaced by a hollow 

 prism filled with a concentrated solution of fuchsine, the reversed 

 spectrum will be seen without the aid of the trough above de- 

 scribed, provided the light is veiy strong and passes veiy close to 

 the edge of the prism. With a less concentrated solution the 

 spectrum is normal, and with a solution of intermediate concen- 

 tration the spectrum is reduced to a single red line. In this case 

 the anomalous dispersion due to the fuchsine is entirely compen- 

 sated by the nonnal dispersion produced by the alcohol, and the 

 result is deviation without dispersion. If now the prism con- 

 taining this last-mentioned solution be immersed in a trough 

 containitig alcohol, the deviation of the rays produced by the 

 alcohol will be almost wholly destroyed, while the abnormal dis- 

 persion of the fuchsine will remain, the red being more strongly 

 deviated than the violet. With this arrangement it is no longer 

 necessary to employ so strong a light, or to make the rays pass so 

 close to the edge of the prism. 



With the prism in air the deviation of the red rays is about 

 11° 30'; but when the prism is immersed in alcohol, the violet is 

 scarcely deviated, the red by fifteen minutes, and the orange by 

 twenty-three minutes. 



Similar results were obtained with aqueous solutions of aniline 

 violet and potassium-permanganate. 



MR. BENTHAM'S ANNIVERSARY ADDRESS 



TO THE LINNEAN SOCIETY* 



(Conclude./ front fi. 114.) 



TO Griseb.-ch's notes on the connections of the tropical African 

 flora with that of other countries I shall have but few obser- 

 vations to add. The intergrafting with the South African flora 

 along the eastern side of the Continent may well be attributed to 

 climate and other present physical conditions. The European 

 character of the higher mountain vegetation of Abyssinia and 

 the Cameroons may be indicative of the remains of that western 

 flora, the mysteries of whose distribution north and south of the 

 tropics I have on several occasions alluded to. The supposed 

 evidences derived from the vegetable kingdom of a once existing 

 connection between west tropical Africa and east tropical 

 America through an ancient Atlantis gradually disappear on 

 further investigation. No traces of a western Atlantic or 

 American vegetation were met with by Mann in the mountains 

 of Fernando Po and the Cameroons, nor by Dr. Hooker in the 

 western Atlas of Morocco. The tiopical American races found 

 in Western Africa are chiefly confined to the coast region ; they 

 are more generally identical than representative species, and they 

 may have been brought over in the course of ages by some of 

 those means of transport which even now may occasionally occur, 

 such as the Gulf Stream, as mentioned by Grisebach. You may 

 recollect, for instance, a short notice by Dr. Dickie inserted in 

 our Journal (Botany, vol. xi. p. 456) of a green floating ma=?, 

 twelve to fourteen miles broad, crossel by Captain Mitchell in 

 the Atlantic, within 300 miles of the mouth of the Gambia, 

 W'hich had evidently, as Dr. Dickie believes, come from some 

 part of America within the influence of the Gulf Stream, 

 probably passing between the Cape Verde Islands and the 

 mainland of Africa. Besides alga;, the portions of this mass 

 picked up by Captain Mitchell and examined by Dr. Dickie 

 contained, amongst other substances, fruits, seeds and " seed- 

 ling plants several inches long, ali with a pair of cotyle- 

 dons, roots, and terminal bud, quite fresh, "t With regard to 

 those American genera represented chiefly in eastern tropical 

 Africa, to which I called your atttention in my paper on Com- 

 posita;, there are various considerations, requiring too much de- 

 tail for me now to enter upon them, tending to show a greater 

 probability of an ancient interchange having taken place far 

 south of the tropics, or eastward over lands long since sub- 

 merged, than across the tropical Atlantic. A prevailing eastern 

 element in the tropical African flora has, indeed, been frequently 

 pointed out. An interchange with continental India is so well 

 marked north of the equator as to have been generally admitted ; 

 but south there are many distinct types represented only in 

 Madagascar, Ceylon, Malacca, the Archipelago, or Australia. 

 This would lead one into speculations put forward also by 

 naturalists in other branches as to a vast continent once bridging 

 over the Indian Ocean, and extending even far to the eastward 

 into the Southern Pacific. Similar views derived from zoology 

 have been recently put forwaid by Grandidier in a most interest- 

 ing sketch of the physical geography and natural history of 

 Madagascar, contained in No. 46 (May 11) of this year's Frjue 

 Sticnlijiijtit:. This island, whose evident antiquity and long 

 isolation, aided by its broken surface, has enabled it to become 

 the seat or centre of preservation of a very large number of 

 endemic monotypes, shows also in its vegetation, besides African, 

 many Archipelago and even Australian types. Grandidier be- 

 lieves that ill zoology the more distant eastern connection is at 

 least as evident, if not more so, than that with the almost adja- 

 cent African continent. In plants the African connection is de- 

 cidedly predominant. 



I shall not attempt to follow Grisebach in discussing the pecu- 

 liarities of the remainder of his regions. We may observe 

 throughout the same careful investigation of the climatic conditions 

 and their influence on their vegetative character of the individual 



* Delivered Friday, May 24, and abridged. 



t It may require, however, as suggested by Dr. Hooker, some further 

 evidence to show that this green mass might not as well have been brought 

 down from some African as from some American river. 



