570 



NA TURE 



\_Ap)-ll 14, \\ 



ASTRONOMICAL PHENOMENA FOR THE 

 IVEEK 1SS7 APRIL \-]-ni 



/pOR the reckoning of time the civil day, commencing at 

 ^ ^ Greenwich mean midnight, counting the hours on to 24, 

 is here employed.) 



At Greenwich on April 17 

 Sun rises, Sh. 2m. ; souths, iih. 59m. 33"93. ; sets, i8h. 57m. ; 

 decl. on meridian, 10° 28' N. : Sidereal Time at Sunset, 

 8h. 39m. 

 Moon {New on April 23) rises, 3h. 3m. ; souths, 7h. 53m. ; 

 sets, I2h. 49m. ; decl. on meridian, 14° 2' S. 

 Planet Rises Souths .Sets Decl. on meridian 



Mercury at greatest elongation from the Sun, 



27° west. 

 Mercury in conjunction with and 0° 31' north 



of the Moon. 

 Jupiter in opposition to the Sun. 



Variable Stars 

 R..\. Decl. 



The most interesting shower of the week is that of the Lyrids, 

 April iS-20, R.A. 268°, Decl. 33' N. Other showers are as 

 follows : — Very slow meteors from a radiant between Bootes and 

 Virgo, R.A. 213°, Decl. 9° N. ; very swift meteors from near -n 

 Herculis, R.A. 255°, Decl. 37° N., and from Vul. ecula, R.A. 

 299°, Decl. 24° N. 



VALENCY AND RESIDUAL AFFINITY^ 

 I. 

 T N my address to the Chemical Section of the British 

 -'■ Association at Aberdeen, I specially called attention to 

 the "affinity" of negative e\t\XitvA% — chlorine, o.tygen, sulphur, 

 &c. — for negative elements ; and I fought to show that the 

 formation of so-called molecular componiids is largely, if not 

 entirely, an outcome of this peculiarity of negative elements. I 

 also ventured to suggest "that in electrolysing solutions, 

 the friction arising from the attraction of the ions for each other 

 is perhaps diminished, not by the mere mechanical interposition 

 of the neutral molecules of the solvent — in the manner suggested 

 by F. Kohlraufch — but by the actual attraction exercised by 

 these molecules upon the negative ion in virtue of the affinities 

 of the negative radicles." In this passage I but vaguely hinted 

 at a modification of the current theory of electrolysis which had 

 occurred to me : as further consideration of the question, espe- 

 cially of Ostwald's electro-chemical studies, has strengthened my 

 views, I am led to think that it may be justifiable to submit them 

 for discussion. 



It is usual to divide bodies into thi-ee classes according to the 

 mode in which they are acted on by an electromotive force : 

 metals forming one class, electrolytes a second, and dielectrics a 



' Revision and extension of a paper by Prof. H. E. Armstrong, F.R.S. , 

 ted to the Royal Society last year. 



third. In making this division, perhaps the fact is not sufli- 

 ciently borne in mind that some compounds — silver chloride, for 

 example — axe per iv electrolytes, while others — such as hydrogen 

 chloride and water — are individually dielectrics, but behave as 

 electrolytes when conjoined. On this account, it appears to me 

 desirable to distinguish between — 



(a) Metals. 



(b) Aot/Zc' electrolytes — compounds like silver chloride which 

 in the pure state are electrolytes. 



(c) AivWd - dielectrics — compounds like water, hydrogen 

 chloride, and sulphuric acid, which behave as dielectrics when 

 pure, but as electrolytes when mixed with other members of their 

 own class. Conducting mixtures of members of this class may 

 conveniently be {exmsA composite electrolytes. 



{d) Dielectrics. 



Simple El ctrolytes. — It is undoubtedly a fact that only a 

 limited number of binary compounds are simple electrolytes ; 

 and it is especially noteworthy that, with the single doubtful 

 exception of liquefied ammonia, no hydrogen compound — 

 whether binary or of more complex composition — can be classed 

 with the simple electrolytes. Indeed, all the simple electrolytes 

 with which we are acquainted are either compounds, such as the 

 metallic chlorides; or metal ic salts — nitrates, sulphates, &c. 

 Including metallic chlorides and their congeners and the corre- 

 sponding oxides and hydroxides among sails — regarding water 

 as an acid, in fact — and denying the title of salts — hydrogen salts 

 — to the acids, Hittorf's proposition {Wicd. Ann., 1878, iv., 

 P' 374)1 "Electrolyte sindSaIze" maybe safely upheld. But 

 only some of the binary metallic salts are electrolytes : beryllium 

 chloride, for example, belongs to the class of " pseudo-dielec- 

 trics " (Nilson and Petterson, IVied. Ann., 1878, iv., p. 565; 

 Humpidge, Phil. Trans., 1883, p. 604) ; and in the case of those 

 elements which readily form two classes of salts — so-called oiis 

 or/z-o/ij-salts and ic or/tv-salts, the oiis compotinds alone appear 

 to be electrolytes. 



It is highly remarkable that whereas fused silver chloride is 

 easily decomposed on passage of a current of low electromotive 

 force, hydrogen chloride is a "pseudo-dielectric" which forms 

 when coupled with the " pseudo-dieIev;tric " water a reulily 

 conducting " composite electrolyte " ; while mercuric chloride 

 conducts with great difficulty — possibly not at all when pure 

 — not only in the fused state, but even when coupled with 

 water. No explanation of these facts seems to be afforded by 

 thermo-chemical data. 



The consideration of these and other similar cases, I think, 

 can but lead to one conclusion : that electrolysability is con- 

 ditioned both by the nature of the elements in the compound 

 and its molecular structure. 



The remarkable difference in the electrical behaviour of two 

 compounds of the same element, such as stannous chloride, in 

 which the ratio of tin to chlorine atoms is as I to 2, and stannic 

 chloride, in which Sn : CI = i : 4 — the one being a simple 

 electrolyte, the other a pseudo-dielectric, if indeed it be not a 

 dielectric — would appear almost to justify the conclusion that in 

 the case of per-salts such as stannic chloride the metal is, as it 

 were, enveloped in a non-conducting sheath of the negative 

 radicle. But whether this be so or not, if — as appears to be the 

 case — all simple electrolytes are metaUic compounds, and if only 

 proto-salls are electrolytes, may it not be that electric conduc- 

 tion in simple electrolytes is of the nature of ordinary metallic 

 conduction, differing from it only in the circumstance that the 

 compound is decomposed as a consequence of the passage of the 

 current ? 



This would lead to the conception of an electrolyte as being a 

 metallic compound of such elements, and so constituted, that 

 electric conduction may take place through its mass in a manner' 

 similar to that in which it takes place through a mass of metal : 

 in fact, through the agency of its metallic atoms. On this vie w, 

 it is essential that the metallic atoms in the molecules comprising 

 a mass of an electrolyte should be in proximity — as they pro- 

 bably are in proto-salts, but not in many per-salts. The con- 

 ductivity of two-metal alloys is in many cases much less than 

 that of either of the contained metals : for ex.ample, the conduc- 

 tivity of the alloy SnCuj is about one-fourth that of tin and 

 about one-thirtieth that of copper. The specific conductivity of 

 metals may, therefore, be much reduced by association with one 

 another ; and this being the case, it appears probable that the 

 specific conductivity of a metal would be still more reduced by 

 association with a uon-metal, and that if the metal were one of 

 low specific conductivity, it might thus practically become 

 altogether deprived of conducting power : perhaps the " exccp- 



