May i8, 1:^93] 



NA TURE 



61 



refraction, calculated from ihe meaQofall these determinationi, 

 is 2 '2 1. As the simplest type of nitrogen compound ammonia 

 gas is next considered, in which the three affinitiesof the nitrogen 

 are attached by single linkage to the three hydrogen atoms. 

 The molecular refraction of ammonia calculated from its refrac- 

 tive index is 5 "65. Now, if it is admitted that the hydrogen in 

 this simple compound possesses the same atomic refraction (I 05 

 for sodium light) as in the free state and in other ordinary com- 

 binations, an admission in support of which Prof. Briihl has 

 previously adduced a considerable amount of experimental 

 evidence, then the atomic refraction of the nitrogen in ammonia 

 is 2 '50. The compounds of nitrogen with oxygen are next dis- 

 cussed. The atomic refraction of oxygen for sodium light is 

 2 05, the molecular refraction of the free gas Oj being 4 '09. If 

 one calculates the molecular refraction of nitric oxide, NO, by 

 adding together the atomic refractions of the gaseous elements 

 2*21 and 2'05, the number 4'26 is obtained. It is interesting to 

 find that the molecular refraction of nitric oxide, calculated from 

 the values obtained experimentally by Dulong and by Mascart 

 for the refractive index of the gas, is very nearly the same, 4'47. 

 Hence in nitric oxide both elements retain about the same re- 

 fractive power as in the free state. The case of nitrous oxide, 

 N.jO, however, is quite different and leads to an interesting con- 

 clusion. Its molecular refraction calculated from the observed 

 refractive index of the gas is 7-58. The value, however, obtained 

 by summation of the values of its components, 2 x 2"2i and 

 2-05, is only 6-47. The very considerable increase of I'li is 

 due to the fact that we are here dealing with a case of double 



N=N 

 linkage, \/, the two nitrogen atoms being mutually attached 



by two of their affinities. Indeed the increase is probably more 

 than this, forthe atomic refraction of oxygen in organic compounds 

 of this type has been found by Prof. Briihl to be less than the 

 value above ascribed to it. The atomic refraction of the nitrogen 

 in N2O is therefore at least 277. It is thus found that nitrogen 

 as singly linked in ammonia possesses an atomic refraction of 

 2-50, when doubly linked, as in nitrous oxide, 277, and when 

 trebly linked, as it probably is in the free gas, 2-21. The value 

 therefore increases with double linkage, but curiously enough di- 

 minishes again with treble linkage, unlike that of carbon, which 

 still further increases with treble linkage, and showing that there 

 is some very essential difference between the nature of the two 

 elements. Prof Briihl concludes his interesting paper by dis- 

 cussing the various values of nitrogen when combined with car- 

 bon. When it is attached with only one of its valencies to a 

 carbon atom, as in the tertiary amines, its atomic refraction is 

 found to be 2-90, a very high value, higher than that of the 

 diazo nitrogen in nitrous oxide. When doubly linked to carbon, 

 C : N, as in the oxims, there is a much larger increase still, the 

 exact amount of which Prof. Briihl prefers to state after carrying 

 out further determinations on a larger number of compounds. 

 In case of cyanogen gas, N ; C • C i N, where triple linkage of 

 nitrogen occurs, there is also a very considerable increment 

 (l 52) in refraction. In the case of hydrocyanic acid, however, 

 the molecular refraction corresponds almost exactly with that 

 calculated from the empirical formula HCN, showing that the 

 cyanogen in this compound and in cyanogen gas are quite 

 different in molecular structure, a point which Prof. Briihl hopes 

 further to elucidate by observations of the refraction of the nitriles 

 aftd other allied organic nitrogen compounds. 



Erratum.— \n our chemical note of last week (p. 39) SOblj 

 and Ilbl should read SOClj and HCI. 



Notes from the Marine Biological Station, Plymouth.— 

 Last week's captures include the Anthozoa Gorgonia verrucosa 

 and Caryophyllia Smithii, the Nemertine Drepanophorus 

 rubrostrialus, the Mollusca Sepia rupellaria (= hiserialis), 

 NO, I 229. VOL 48 i 



Gahiiii tricolor and Antiopa cristata, and the Ascidians Corelta 

 larvcFj'ormis and Fragarittm clegans. Several swarms of the 

 medusa Obelia lucifera, lull-grown and mature, were taken in 

 the townets during the latter half of the week. Polychsete 

 larvae, so abundant earlier in the year, are now very scarce. 

 Zoasae of Porcellana, on the other hand, have increased in 

 numbers, and every towneiting contains a variety of Decapod 

 larva: in different stages of development. The Hydroids Eu- 

 dendrium capillare and Antennularia antennina, and the Poly- 

 chaete Sabellaria spinulosa are now breeding. 



The additions to the Zoological Society's Gardens during the 

 past week include two Red-winged Parrakeets (Aprosmictus 

 erythropterus, ? 9 ) from Australia, presented by Mr. H. Good- 

 child ; two Ravens {Corvus corax) British, presented by Mr, 

 Philip A. Wilkins ; a Ducorp's Cockatoo {Cacatua ducorpsi) 

 from the Solomon Islands, presented by Mr. R. Armitage ; a 

 Changeable Lizard (Calotes versicolor) from Ceylon, presented 

 by Mr. H. L. Gibbs ; a Vervet Monkey {Cercopitkecus lalandii^ 

 from South Africa, a Common Peafowl {Pavo cristalus, 6 ) from 

 India, deposited ; a Yellow-cheeked Lemur {Lemur xantho- 

 mystox) from Madagascar, eleven Green Lizards {Lacertaviridis) 

 South European, purchased ; a Senegal Touracon {Corythaix 

 persa) from West Africa, received in exchange ; a Japanese 

 Deer {Cervus sika) born in the Gardens. 



OUR ASTRONOMICAL COLUMN. 



The Greatest Brillianxy of Venus. — Dr. G. Miiller, 

 whose work on the brightness of the major and some of the 

 minor planets we referred to in this column two weeks ago 

 (p. 15) contributes to Astronomischen Nachrichten, No. 3162, 

 some interesting results with reference to the greatest brilliancy 

 of Venus. That this planet does not appear brightest at the 

 time of conjunction, but some days before or after, has been 

 shown by the work of Halley, Lambert, &c., and the values, as 

 obtained from their formula, are : — 



The greatest brilliancy occurs at 

 Formula Angle of E'onffation No. of days before Greatest 



used. phase. * or after inf. conj. brilltancy. 



Halley ... 117 56 ... 39 43 ... 36 ... 4-263 



Lambert .. 103 46 ... 44 38 ... 51 ... 2"I26 



Bremiker ... 115 15 ... 40 52 ... 39 ... 2772 



Seeliger ... Ii6 o ... 40 33 ... 38 ... 3-018 



Referring to the curves of the observed and computed bright- 

 nesses, as here set forward by Dr. Miiller, several important 

 points may be noticed. In the former the maximum brightness 

 takes place at a phase angle of 119°, decreasing very gradually 

 to 140°, and after that more rapidly. At the maximum the 

 curve is moderately flat, only a very small variation being 

 noticed between position angle 100° and 140", a period of 36 

 days. 



Dr. Miiller remarks that the statements of epochs given in 

 the astronomical ephemerides have no practical interest. As 

 an example showing the deviations of the values therein stated 

 from those computed by his formula he works out the next 

 epoch of the greatest brilliancy of Venus, which will be in in- 

 ferior conjunction on February 15, 1894. The values for the 

 brightness and the corresponding times result as follows : — 



Jan. 9 ... oh. G.M.T. ... A = -4-3776 



10 .. ,, -4'3798 



11 ... -4'38o9 



12 ... „ „ ... -4"38o9 



13 ••• .. -43802 



14 ••• -4"3782 



which give for the epoch of greatest brilliancy January 11, 

 iSh. M.T.G. The times of epoch, as given by the ephemerides, 

 are : — 



Berliner Ast. J ahrhiich Jan. 8 ... i6h. M.T.G. 



Nautical Almanac II ... 2h. ,, 



Connaissaiue des Temps 12 ... ih. ,, 



Finlay's Periodic Comet. — This comet, which was dis- 

 covered by Finlay in 1886, is one, if not the only one, of the 



V 



