December 1, 1898.] 



KNOWLEDGE. 



233 



respects agree with the faculas which are visible to the eye 

 when near the sun's limb. In many cases small spots, 

 which are clearly visible to the eye in direct observation 

 or in photographs taken at the focus of a telescope in the 

 ordinary manner, are completely hidden in spectro-helio- 

 grams by bright patches in which H and K are reversed. 

 Hitherto the name " facula " has been applied indifferently 

 to the visible and invisible phenomena, but it seems 

 desirable that a distinction be made. The true faculse 

 give a continuous spectrum in addition to the doubly- 

 reversed H and Klines. In the invisible " faculiB " the 

 continuous spectrum is either very faint or altogether 

 lacking. 



As you have been kind enough to request me to discuss 

 the nature of faculie more at length in a paper for 

 Knowledcie, I need not pursue the subject further at the 

 present time. Very truly yours, 



George E. Hale. 



EXPLOSIONS IN THE SUN. 

 To the Editor of Knowledge. 



Dear Sir, — I am much obliged to Prof. Smithells for 

 his interesting criticism of my letter on the above subject, 

 which was written chiefly to point out that explosive 

 combinations of gases cooling from above their dissociation 

 temperatures would not necessarily take place. I quite 

 agree that it would be absui-d for anyone to argue the 

 impossibility of such combinations, seeing that we have no 

 experience to guide us as to the behaviour of the elements 

 under solar temperatures. 



At the same time I think it is permissible, by reasoning 

 upon the little we do know, to see how far the observed 

 solar phenomena can be explained ; and I must maintain 

 that our chemistry fails at present to account for the 

 fact of solar explosions in the same sense that it accounts 

 for terrestrial explosions. Even if we make the assumption 

 suggested by Prof. Smithells, that the gases become super- 

 cooled before combining, it is still difficult to believe that 

 the explosive force would be anything approaching in 

 intensity that of (say) an explosion of oxygen and 

 hydrogen at ordinary terrestrial temperatures ; for if we 

 suppose the dynamical theory of gases to be a fair approxi- 

 mation to the truth under solar conditions, it will follow 

 that the absolute temperature at dissociation would have 

 to be reduced by super-cooling by as much as one-half 

 to produce an expansion which would even double the 

 volume with equal pressures. For whatever assumption 

 we make as to the law of increase of chemical affinity with 

 decrease of temperature, in no case can the tempera- 

 ture of the resulting compound rise above the dissocia- 

 tion point ; combination may be incomplete, but the 

 volume (under constant pressure) cannot increase in a 

 greater ratio than the dissociation temperature bears to 

 the combination temperature. 



Taking as an example the case of dissociated ox5'gen 

 and hydrogen cooling uniformly and under constant 

 pressure, for the sake of argument, the sequence of 

 phenomena would be as follows : both temperature and 

 volume of the mixture decrease uniformly until a certain 

 temperature has been reached below the dissociation point 

 (supposing the super-cooling to be a fact). Then, either a 

 portion or the whole mass will suddenly combine, the 

 amount depending on the quantity of heat withdrawn in 

 super-cooling ; and, apart from the energy due to chemical 

 affinity, an amount of heat will be evolved (latent heat of 

 dissociation) exactly counteracting the contraction due to 

 the combination, the heat of chemical aflSnity alone being 



effective in producing an expansion, and this, as explained 

 above, cannot be greater than the ratio of the absolute 

 temperature at combination to that at dissociation. 



To return, however, from theoretical arguments to a 

 consideration of the phenomena actually observed on the 

 sun. Prom my own personal acquaintance with the 

 chromosphere, I am mclined to doubt whether the evidence 

 relied on to prove the superficial origin of the " explosions" 

 is conclusive, or even real ; and I think that too little 

 attention has been paid to certain phenomena which indicate 

 that these outbursts may often originate at a consider- 

 able depth below the ])hotospheric cloud layer, and there 

 fore in a region presumably more or less shielded from the 

 effects of cooling by radiation. Thus, in looking over my 

 own observations, which extend over a period of five years 

 and comprise more than five hundred separate drawings 

 of the chromosphere, I can find no single instance of 

 an eruption, great or small, showing the principal motion 

 inclined more than about 40" from the vertical ; in general, 

 these visible motions differ but little from a radial direction. 

 It is true that displacements of the bright lines at the 

 solar limb, indicating tangential motion, are frequently 

 seen in active prominences, but this is often found to be 

 only a fractional component of the real direction of motion. 

 Another fact bearing on this question is the enduring 

 nature of the centres of eruption. Many metaUic and 

 eruptive prominences have been seen to reappear alter- 

 nately east and west after intervals equal to one or more 

 semi-rotations. These are usually associated with spots, 

 and have been found to even outlive a large spot. Thus, 

 the great group of February, 1892, was heralded by an 

 eruptive prominence on February 4th, and subsequently, 

 similar violent eruptions were seen by various observers in 

 the same latitude on February Ibth and 19th, March 3rd 

 and 4th, March 18th, May 11th and 12th, and finally, 

 June 5th, all of which can be referred to the same helio- 

 graphic position, the last-mentioned alone being observed 

 two days before it was theoretically due on the west limb. 

 The recurring nature, or rather longevity, of the quieter 

 high latitude prominences I have found to be still more 

 marked, and from this one is led to believe that many of 

 these supposed transient phenomena are the outcome of 

 forces as deep-seated and of as enduring a nature as the 

 spots themselves. Yours truly, 



J. EvERSHEl). 



[While concurring with Mr. Evershed in thinking that 

 the long-enduring solar prominences and suuspots prob- 

 ably have a deep-seated origin, it seems to me improbable 

 that structures like the tangential rays of the corona and 

 the numerous prominences which exhibit an inclination to 

 the radial, have their origin at a great depth, and on 

 reaching the level of the photosphere are deflected to 

 continue their course in approximately straight Imes. 



The differences of rotation period of the photosphere 

 in different latitudes, as well as of the absorbing material 

 which gives rise to the Fraunhofer lines at various dis- 

 tances from the solar equator, seem also to point to a 

 continual radial circulation outward as well as inward 

 from the photosphere. Whether such radial motions 

 in the outer parts of the sun are due to explosions at 

 a high or low level, or to fierce winds due to the rapid 

 fall of cooled material under the powerful influence of 

 .solar gravity, is a matter on which I at present keep an 

 open mind ; but I should like to point out that the lateral 

 motions observed are not inconsiderable, and that the 

 effect of perspective cannot increase any apparent inchna- 

 tion to the radial. A ray which is inclined to the radial 

 may be seen in projection so that it appears radial, but 

 any apparent inclination to the radial must be real. 



