564 POPULAR SCIENCE MONTHLY. 



concerning them if they had remained unseen except during eclipses, we 

 can echo our author's opinion that success in revealing the mysterious 

 corona in a similar manner " is certainly devoutly to be desired." 



Prof. Young has so long been a conspicuous and brilliant figure in the 

 field of spectroscopic investigation that one naturally turns to his revised 

 chapter on The Spectroscope and the Solar Spectrum with pleasing antici- 

 pations which are not disappointed. The results of the immense work 

 accomplished in the last decade are here ranged in order with an expert 

 hand, and the remaining gaps in the line of acquired knowledge are made 

 clearly apparent. The list of elements known to exist in the sun has been 

 largely extended in the past few years, but still many that go to constitute 

 a great portion of the crust of the earth have not been recognized in the 

 solar spectrum. Why are they absent ? Is it simply a failure to show 

 themselves, or do they not exist there at all ? Prof. Young indicates his 

 preference for the view that the missing elements are not really absent 

 from the sun, but only nonapparent, although he points out that the answer 

 to the question is not easy. And then he goes on to marshal some most 

 interesting facts and considerations relative to this subject, and discusses 

 briefly but luminously such topics as the multiple spectra of certain ele- 

 ments, Lockyer's revival of the old " pantogen " speculation and his theory 

 of " basic lines " common to the spectra of different substances, the later 

 work on the question of solar oxygen, etc. 



While this book is a record of facts and achievements rather than of 

 theories and attempts at the interpretation of mysteries, yet the great 

 questions still remaining to be answered are, of course, discussed, and in a 

 masterly manner. A fine instance of the author's method of dealing with 

 such subjects is shown in the chapter on The Sun's Light and Heat, where 

 he succinctly reviews questions like these: ''How is the sun's heat main- 

 tained?" "How long has it lasted already ?" "How long will it con- 

 tinue ? " u Are there any signs of either increase or diminution ? " It is 

 undoubtedly true, as Prof. Young remarks, that to such questions, * in the 

 present state of science, only somewhat vague and unsatisfactory replies 

 are possible," yet they are questions the replies to which, however incom- 

 plete, will always command deep interest. And if the facts and specula- 

 tions accumulated since 1881 have not thrown much light upon these 

 subjects, something has been gained in a clearer comprehension of both 

 the strength and the weakness of prevailing theories. Those are pregnant 

 sentences, for instance, in which, after pointing out the objections to the 

 late Dr. Siemens's theory of the sun, he remarks: "And yet one almost 

 regrets that the theory can not be accepted, for it would remove very 

 serious difficulties that now embarrass the problem of the evolution of our 

 planetary system. The accepted contraction theory of Helmholte certainly 

 appears to allow too little time for the sun's lifetime of radiant activity to 

 be consistent with a reasonable explanation of the process by which the 

 present state of things has come about." 



In briefly summarizing the principal additions noted in the new edition 

 the following may be particularly mentioned : The latest work on the solar 

 parallax, including Newcomb's results and the observations on the minor 

 planets Victoria and Sappho ; accounts of all recent advances and discov- 

 eries in solar spectroscopy and spectro-photography, including the work of 

 Hale, Deslandres, Duner, etc. ; a statement of the latest accredited theories 



