October 13, 192 1] 



NATURE 



211 



electrolytes on dilution. These difficulties, if they are 

 real, might be removed by reverting to the idea of 

 electrons acting as binding material in compoimds 

 capable of ionisation instead of supposing that they 

 have passed completely and irreversibly over from 

 the positive to the negative atom and have been in- 

 corporated into the sheath of the latter. This is not to 

 confuse electrovalency with covalency, for the essen- 

 tial difference between the two kinds of valency is 

 still that electrons are transferable in the one case 

 and not in the other. 



It is apparent, however, that Dr. Langmuir regards 

 ionisation in a wider sense than is usual from his 

 statement that " compounds without covalency must 

 consist of positively and negatively charged ions"; 

 and that among such compounds he places SF,, 

 because the sulphur atom is sufficiently positive to 

 yield to the powerful attraction of the fluorine atoms 

 for electrons. 



In this connection I would like to direct attention 

 to the unique characteristics of the elements of the 

 first short period as compared with those of the second 

 short period and following periods, and to inqufre 

 whether Dr. Langmuir's theory can dfford an explana- 

 tion of them. 



The hydrides of the first short f)eriod are inert com- 

 pared with corresponding hydrides in subsequent 

 periods. Compare, for example, CH4 with SiH^. 

 Why is CH, stable and inert, while SiH, is rather 

 tinstable and undergoes metathesis with some salts? 

 Presumably H is joined to C in methane by covalency, 

 while the hydrogen of silane is "ionised." Why is 

 this so? Comparison of HF with HCl shows an 

 analogous difference. Why is HF so weak an acid? 

 Certainly not for lack of electronegativeness on the 

 part of fluorine. If we are to regard HCl as always 

 "ionised," does covalency in the case of HF gradually 

 give place to electrovalency as its aqueous solution is 

 diluted and sjjecific conductivity increases, and, if so, 

 why? Comparison of CCl, with SiCU is equally 

 interesting, because CCl, is analogous to CH4 in inert- 

 ness and SiCl, to SiH^ in reactivity. Why, then, does 

 covalency obtain with one chloride and electrovalency 

 with the other? 



These few questions point to the fundamental 

 problem of the gradation of properties of the elements 

 in the periodic classification ; and further develop- 

 ments of his theory, which Dr. Langmuir promises, 

 will be looked for with great interest. 



R. M. Caven. 



Royal Technical College, Glasgow, 

 September 26. 



The Dushman Equation for the Velocity of a Mono- 

 molecular Reaction. 



In view of the discussion at the Faraday Society 

 on September 28, it is perhaps of Interest to direct 

 attention to the Dushman equation for the mono- 

 molecular velocity constant (Journal A.C.S., 1921, 



X/tv 



vol- 43. P- 397)- This equation, K = ve ^^^ ^ where A, R, 



T, and v have the usual significance and e '^^ repre- 

 sents the number of active molecules, is found to hold 

 fairly accurately for certain monom6lecular reactions 

 that have been investigated. Of these the chief are 

 the decomposition of phosphine and nitrogen pent- 

 oxide. Much dispute has arisen as to the meaning 

 of V. Is this related to the frequencies of the 

 activating radiation, or is it a frequency characteristic 

 of some degree. of freedom in the decomposing mole- 

 NO. 271 1, VOL. 108] 



cule? Judging from the constancy of hv over a range 

 of temperatures, the latter would appear to be the 

 more reasonable assumption. The relationship 

 between v and the absorbed and emitted frequencies 

 would thus be left open. 



On this hypothesis it is possible to ascribe a simple 

 meaning to the Dushman equation, for the "period 

 of existence " of the molecule in the active condition 

 will be i/v, i.e. the {>eriod of time corresponding to 

 one molecular vibration. This equation could not 

 hold if the rate of chemical decomposition of the active 

 molecule was in any way different from its rate of 

 decay, due to the emission of quanta to its environ- 

 ment. For if there is any difference between the two 

 rates, the position of equilibrium will be disturbed, 

 and the number of active molecules will not be given 

 by the Maxwell relation. In general, the time of 

 association of an activating quantum (of a lower 

 frequency than v) with a gaseous molecule will be 

 greater than i/v, so that the effect of chemical de- 

 composition will lower the concentration of the active 

 molecules at the steady state and decrease the velo- 

 city constant, k would in these circumstances be less 

 than the value obtained from the Dushman equation. 

 Tolman (Journ. A.C.S., 1921, vol. 43, p. 269) finds 

 for nitrogen p>entoxide that k (calc.)= 19X 10-*, while 

 K (obs.) = 4-87X 10-'. The change in « is thus in the 

 required direction. The exact agreement obtained by 

 Dushman in the case of phosphine would thus appear 

 to be accidental. W. E. Garner. 



University College, London. 



The Duration of Sunrise and Sunset. 



Mv attention having been accidentally directed to 

 this matter, I have made observations for the purpose 

 of comparing the observed duration of sunrise and 

 sunset with the duration computed from geographical 

 position and Nautical Ahyianac data, and have now 

 seventy-nine complete observations, made mostly with 

 marine horizons on Manila Bay, the China Sea, the 

 open Pacific, Vineyard Sound, and Buzzard's Bay, 

 with a few made with land horizons. The view- 

 points have been from sea-beach to 15 12 metres 

 elevation. 



There seem to be two types of marine sunrise and 

 sunset ; one, type A, about twice as frequent as the 



l92|in30A 

 —Type A— 





Oo 



4 5 



1921 ffl JOP 



— Type B — 



Fig. I.— Types of marine sunrise and sunset. 



Other and accompanied by horizon mirage. The 

 diagrams (Fig. i) accompanying this letter, sketched 

 from photographs obtained at Woods Hole, show the 

 characteristics of the two types, which are described 

 with historical details in Popular Astronomy (vol. 29, 



