62 



NATURE 



[May 20, 1897 



proved that crystallisation is only caused by the introduction of 

 a crystal of the substance, or of a strictly isomorphous substance. 

 For example, fused salol (melting point 39°*5) cannot, at 

 ordinary temperatures, be induced to crystallise by any of the 

 usual means ; but if a fine thread of glass be lightly drawn over 

 a crystal of salol, it acquires the power of inducing crystallisa- 

 tion in the liquid ; it loses it again by exposure to the air for a 

 few minutes, by wiping with soft sheet india rubber, or by warm- 

 ing above 40°. There is, however, only a limited range of 

 temperature below the melting point in which spontaneous 

 generation of crystals is impossible ; the liquid is here in stable 

 equilibrium, except with respect to a ready-formed crystal. 

 Ostwald proposes the name tnetastable for this condition. At 

 still lower temperatures, crystals form spontaneously and with- 

 out the presence of ready-formed nuclei ; the equilibrium is here 

 really labile. The analogy between the phenomena observed 

 during the passage from the liquid to the solid condition, and 

 those observed in the passage from gas to liquid, is pointed out. 

 Notwithstanding the very minute quantity of substance required 

 to start the crystallisation. Prof. Ostwald has succeeded in show- 

 ing that it has a lower limit. The two methods employed 

 (successive dilution with an indifferent solid substance in the way 

 practised by the homoeopathists, or evaporation of minute drops 

 of successively more and more dilute solutions of the solid on a 

 platinum wire, and introduction of the residues into the super- 

 saturated liquid) gave practically identical results. With sodium 

 chlorate solution, for example, containing 107 parts of the salt 

 to 100 of water, the smallest quantity of solid, which would still 

 induce crystallisation, was from a millionth to a ten-millionth of 

 a milligram. The fact that a very minute quantity of ammonia 

 alum induces crystallisation of a solution of potassium alum, 

 instead of being itself dissolved, may be explained by supposing 

 that the dissolved salt diffuses into the solid particle as soon as 

 it comes in contact with the solution ; a nucleus of the solid 

 potassium alum having been thus formed, it continues to in- 

 crease. This explanation is in agreement with the facts that 

 only truly isomorphous salts are capable of forming solid solu- 

 tions, and also that they alone are capable of mutually inducing 

 crystallisation. For the statement and discussion of a pro- 

 position, which may be paraphrased as follows, the paper must 

 be consulted. When a system passes from any given condition 

 to a more stable one, it will not pass into the state which, under 

 the circumstances, is the most stable, but into that which is 

 nearest to the original state. 



The additions to the Zoological Society's Gardens during the 

 past week include a White-throated Capuchin (Cebus hypo- 

 ieucus) from Central America, presented by Sir Henry A. 

 Blake, K.C.M.G. ; a Pig-tailed Moxike-y {Afacaats nemestrinus) 

 from India, presented by Mr. W. B. Orme ; an Egyptian 

 Jerboa {Dipus mgyptuis) from Egypt, presented by Mr. S. 

 Whitehouse ; a Kinkajou {Cercoleptes caudivolvulus), a Sharp- 

 nosed Crocodile {Crocodilus acutiis) from Venezuela, a Rough- 

 eyed Cayman {Caiman sclerops), two Tuberculated Iguanas 

 {Iguana tuberculata), a Black-pointed Teguexin {Ttipinambis 

 nigropunctahis), a Chequered Flaps {Elaps lemniscctus), an 



Anaconda {Eunectes murittus) from Trinidad, two Geckos 



(Thecadactylns rapicauda), a Cuvier's Scolecosaurus {Scoleco- 

 satirus cuvieri), an Agile Lizard {Mabuina agilis), three Thick- 

 necked Tree Boas {Epicrates ceuchris), four Common Boas {Boa 

 constrictor), five Cooke's Tree Boas ( Coraliits cookii), a Mocassin 

 Snake {Tropidonotus fasciatns), a Boddaert's Snake {Dryfiiobins 



boddaerti), a. Snake {Coronella calligaster), a Snake 



{Helicops angiilatus) from the West Indies, presented by Mr. 

 R. R. Mole ; an Anaconda {Etmectes murinus) from Trinidad, 

 presented by Mr. F. W. Urich ; an Indian Pigmy Goose {Net- 

 topus corornandelianus) from India, a Laughing Kingfisher 

 {Dacelo gigantea)ixoxa Australia, a Temminck's Snapper {Macro- 

 NO. 1438, VOL. 56] 



clemmys teimnincki) from the Southern United States, deposited ; 

 six Mexican Quails {Callipepla sqtiamatd) from Mexico, pur- 

 chased ; two Egyptian Weasels {Mttstela subpalmata), eight 

 Sh&w's GtxhiWes {Gerbillus shawi), an Egyptian Jerboa (ZJ^^Mi- 

 agyptius), three Long-eared Hedgehogs {Erinacetis auritiis), a 

 Grey Momior {Varanus griseus), nine Egyptian Cobras {Naia 

 haje), eight Cerastes Vipers ( C^raj-to cormitiis), a Rough-keeled 

 Snake {Dasypeltis scabra), three Clifford's Snakes {Zamenis dia- 

 dema), two Hissing Sand Snakes {Psammophis sibilans), ten 

 Ocellated Sand Skinks {Seps ocellatus), four Vinaceous Turtle 

 Doves {Tiirtur vinaceus), two Lesser Pin-tailed Sand Grouse 

 {Pterocles exustus) from Egypt, received in exchange. 



OUR ASTRONOMICAL COLUMN. . 

 Resolving Power of Telescopes and Spectroscopes. 

 — In the current number of the Memorie della Societct Degh 

 Spettroscopisti Italiani (vol. xxvi., 1897), Prof F. L C. 

 Wadsworth discusses the question of the theoretical resolving 

 power of optical instruments, distinguishing between four dif- 

 ferent cases. According to Rayleigh, the theoretical angular 

 resolving power of any instrument having an aperture of width 



where o is the angle between two fine lines or points which 

 can just be separated (a close double, for instance) : A the mean 

 wave-length of the light employed, b the linear aperture of the 

 instrument, and m a constant, varying according as the aperture 

 is rectangular or circular. The spectral resolution of separa- 

 tion of a spectroscope can be determined from this formula by 

 considering the dispersing train of prisms or gratings as a series 

 of spectral images of the slit of the spectroscope. The four 

 cases which are minutely dealt with are: (i) The resolving 

 power (theoretical) of a spectroscope train for an infinitely 

 narrow slit and monochromatic radiations, i.e. infinitely narrow 

 spectral lines. (2) The resolving power (also theoretical) for a 

 wide slit and monochromatic radiations. (3) The resolving 

 power (limiting) for an infinitely narrow slit, but for lines of 

 finite width A A. (4) The resolving power (practical) for a 

 wide slit and non-monochromatic radiations, ranging for each 

 line over a small value of A A, as in (3). This quantity repre- 

 sents the practical resolving power or purity of the spectrum. 



The expression for the spectroscopic resolution for the second 

 case differs from that obtained in the first by the presence of 

 a new factor in the denominator of the former. The existence 

 of this necessitates, as Prof. Wadsworth says, a considerable 

 modification of certain statements based on the old formula of 

 purity. Instead of a continual decrease with increase of slit 

 width, the purity of the spectrum actually increases up to a 

 certain point, and is equal to the theoretical resolving power of 

 the instrument. On a further widening of the slit, the purity 

 begins to diminish, but not so rapidly as previously supposed. 

 This modification of the old idea requires, as he points out. a 

 correction in Schuster's remarks on the practical purity of a 

 bright-line spectrum, which gives the purity as 50 per cent, of 

 the resolving power, and not 75 per cent., as Prof Wadsworth 

 now finds it must be. Other points of equal interest result 

 from this new discussion, and are dealt with in this paper. 



Photographs of Metallic Spectra. — An investigation 

 of considerable utility in astrophysics {Siizungsberichte der 

 Konig, Preuss, Akad. d Wiss. zu Berlin, March 4, 1897) has 

 recently been concluded by Dr. O. Lohse in Potsdam. This 

 consists in the determination of the wave-lengths of the lines in the 

 spectra of cerium, lanthanum, didymium, thorium, yttrium, 

 zirkonium, vanadium, and uranium, for the violet region 400 fx.^ 

 to 460 /Uyu. The spectra were obtained by photography with 

 a spectroscope fitted with a prism filled with zimmtaethyl, 

 the length of the resulting spectrum between the above wave- 

 lengths measuring 180 mm. Spark spectra alone were investi- 

 gated, and by means of a gas motor and dynamo a considerable 

 strength of current was obtained. During the experiments it 

 was found that the heat affected to an appreciable extent the 

 refractive and dispersive power of the fluid in the prism, although 

 it was not sufficient to be measurable with delicate thermo- 

 metric instruments. The definition of the lines was therefore to 



