492 



NATURE 



{March 21, 1889 



just as fresh matter is hourly brought to the earth by 

 meteors, it steadily loses during its orbit in the solar system 

 some of its gaseous constituents ; and the near mainten- 

 ance of a status quo during ages partly depends upon 

 the circumstance that gain and loss balance each other, 

 and partly also upon the gain and loss, during the record 

 of mankind, being so infinitesimally small in proportion 

 to the gaseous and fluid matter surrounding the litho- 

 sphere of the earth. But during the geological ages even 

 this " little " may be appreciable : long periods may have 

 passed when the gain has been in excess, which has 

 caused a rise in the volume of the sea ; whilst at others 

 the loss has predominated, whereby a gradual fall of the 

 sea-level has taken place. 



That the atmosphere sometimes decreases through loss 

 in space it is of course impossible to demonstrate by 

 direct observation ; but as we at present know fairly well 

 the forces acting upon a gas or dust molecule in the 

 outer strata of the atmosphere, it seems that we may, 

 from a theoretical point of view, be able to obtain an 

 answer to the question raised here. However, we must, 

 to avoid erroneous and hasty conclusions, here take into 

 consideration so many factors difficult to estimate that 

 the definite answer probably for a long time will give rise 

 to much controversy. 



Therefore, although the problem of the risin g of the 

 land on the coasts of Sweden and Finland may now be 

 said to have, in the main, been decided, the old question 

 about the diminishing of the sea-level, and that just in 

 the general form in which it was presented by Swedenborg 

 and Linn£Eus, still remains an unsolved riddle of immense 

 importance to the history of our earth. 



A. E. NORDENSKIOLD. 



VARIABLE STARS AND THE CONSTITUTION 



OF THE SUN. 

 J/ARIABLE STARS.— The theory of variable stars 

 set forth by Dr. Brester in a recent essay ^ some- 

 what resembles those suggested by Zollner in 1865,- and 

 by Dr. Lohse in 1877.^ Briefly, Zollner's theory regards 

 variability as being due to the formation of scoriae on the 

 photospheres of the stars and their subsequent dispersion 

 by the heat due to chemical combinations which take 

 place in virtue of reduced temperature. Dr. Lohse sub- 

 stitutes absorption by the cooled atmospheres of the stars 

 for the reduction of light by scoriae as in Zollner's theory, 

 although both agree as to the cause of the removal of the 

 light-obstructing agencies. The theory suggested by Dr. 

 Brester is a little more ambitious than either of these, 

 and, to give a translation of his own statement, "All the 

 phenomena that variable stars present to us are the varied 

 effects of one cause — the intermittent chemical combina- 

 tion at the cool external layers of that which had 

 previously been dissociated by heat" (p. i). All the 

 explanations are based on the assumption that the stars 

 are stratified spherical agglomerations of gaseous matter, 

 the difl"erent layers having different compositions according 

 to their distances from the centre. 



Most stars are too hot to allow of the formation of 

 compounds as we know them, but the stars most subject 

 to variability (the red stars) are sufficiently cool, in their 

 outer layers at least, for the formation of such compounds 

 as hydrocarbons. It is in the obstruction of light by these 

 compound vapours that Dr. Brester finds his explanation 

 of variability. He aptly compares the cooling of a star to 

 the running down of a clock-spring, and the intermittent 

 chemical combination to the escapement which regulates 

 it. These changes pass unnoticed in the hottest stars, 

 because the periods elapsing between the coolings is very 



' '' Es.sai d'une 'I'heorie du Soleil et des Etoiles vari.ibles," p.ir A. Brester, 

 D.Sc. ( elft : J. Waltman, 1889). 



^ " Photometrische Untersuchungen," p. 252. 



3 Monaisl'er. der Akad. der IVissenschaften, p. 826. 



great, and the combinations formed exert no very great 

 absorbing influence ; but in the cooler stars only smalt 

 changes of temperature are necessary, and the periods 

 are correspondingly short. 



Again, although the temperature may be low enough 

 for a combination to take place, the combining substances 

 may be so diluted by other matter that the combination is 

 impossible, just as a mixture of oxygen and hydrogen will 

 not explode if admixed with more than 7i volumes of air 

 (Bunsen). This condition Dr. Brester describes as a 

 state of surdissociation. This state does not last long, 

 because, as the combining molecules get nearer the 

 centre, they get more concentrated, while the substances 

 which prevent their union diminish in proportion. When 

 the combination does take place, there is an "eruption 

 of heat " and the clouds in the outer cool layers are 

 dispersed, the brightness of the star consequently 

 increasing. 



So much for the general theory, which Dr. Brester 

 believes to be competent to explain every description of 

 variable star, even such diverse ones as Algol and/3 Lyras. 

 He rejects the eclipse explanation of the Algol type on 

 the ground that it is impossible to conceive such large 

 obscure bodies to travel at such an enormous rate, and 

 that it is in contradiction to the recent work of Chandler 

 and Sawyer, showing irregularities in the periods, espe- 

 cially in the case of U Ophiuchi. 



Secondary maxima, such as occur in j3 Lyras, he 

 believes to be due to double combinations : the first 

 substances which combine by the fall of temperature do 

 not produce sufficient heat to reproduce the first maximum, 

 whereas the next combination does, and these taking 

 place alternately, the jS Lyrae type receives explanation. 

 Irregularities in the variability are, according to this- 

 theory, due to disturbances brought about by very rapid 

 rotations. 



" New stars " are believed by Dr. Brester to be pro- 

 duced by the sudden dispersion of the obscuring clouds 

 which formerly surrounded the star, by heat due to a 

 new chemical combination. On this supposition they 

 must be at a very low temperature before they burst out. 

 It is not easy to understand, however, how any such 

 action as this could raise a star from the ninth to the 

 second magnitude, as was the case with T Coronje. The 

 spectroscopic difficulty is a still greater one. How the 

 spectrum of a new star just before its disappearance 

 could, on Dr. Brester's view, be like that of a planetary 

 nebula, is not easy to explain. For the present it seems 

 more consistent with the facts to regard " new stars " as 

 being due to the clashing together of two meteor-swarms 

 in space. 



In this theory Dr. Brester has attempted too much. 

 Most astronomers are agreed that more than one cause 

 of variability is at work, and it is certainly too much to 

 expect one theory to explain all the various types. Dr. 

 Brester does not seem to be aware that Algol is one of 

 the hottest stars in the heavens, and that a recent photo- 

 graph by Prof Pickering shows the spectrum to be the 

 same at maximum as at minimum. If one hot star be 

 variable, why not all.'' Again, if variability is only 

 manifest in the cooler stars, why .does not every cool 

 star give indications of variability 'i Further, the theory 

 assumes that all variable stars are cooling, whereas Mr. 

 Lockyer's i-ecent work has shown that those of the Mira 

 type are increasing in temperature. Dr. Brester's only 

 objection to Mr. Lockyer's theory of variability is its 

 limited application, but it was not set forth as being 

 universally applicable. If Dr. Brester's theory had been 

 limited to the variables of Group VI. (Vogel's Class III(^), 

 it would be more reasonable, but even then it would not 

 be easy to understand why all the stars of the group do- 

 not exhibit variability. 



The Sun. — The second part of the essay attempts to- 

 explain the various phenomena presented to us by the: 



