340 



NATURE 



[August 13, 1903 



suggestion as to the relative amounts of this and other 

 elements existing in the sun and earth is misleading. 

 Remembering that nothing is known of the chemical 

 constitution of the earth. a few miles below the surface, 

 it is possible that rare elements in the crust may be 

 abundant nearer the centre. The differences between 

 solar chemistry as manifested *by the solar spectrum, 

 and terrestrial chemistry as represented by minera- 

 logical knowledge, are therefore only apparent, and 

 Rowland was justified in his remark, "were the 

 whole earth heated to the temperature of the sun, its 

 spectrum would probably resemble that of the sun 

 very closely." 



The distinction between spark and arc spectra is 

 not sufficiently recognised, with the result that un- 

 sound judgments are sometimes reached. A case of 

 this kind occurs in connection with the discussion of 

 the chromospheric spectrum. The green line of the 

 chromosphere is coincident with one of the members 

 ; — due to iron — of the triplet known as Kirchhoff 1474 

 in the Fraunhofer spectrum. Miss Gierke says : — 



" Now the chromospheric ray agrees in position with 

 the iron line, which is one of secondary importance ; 

 yet it cannot at present be asserted confidently that it 

 really emanates from glowing iron vapour. If it did 

 it should be ordinarily associated with other iron lines, 

 and none have been ascertained to make part of the 

 fundamental chromospheric spectrum." 



If the spark spectrum of iron had been considered 

 Instead of the arc spectrum, these remarks would, we 

 think, have been modified. The iron line at 1474 K 

 is not of secondary importance in the spark spectrum ; 

 indeed, the fundamental chromospheric spectrum 

 consists largely of iron lines — not the ordinary lines 

 of the arc spectrum, but lines such as those at 

 w 1474 K, 5018, 4924, 4584, and 4233, which are 

 enhanced in relative importance in passing from the 

 arc to the spark. 



In connection with the subject of the temperature 

 of the stars, the behaviour of lines of magnesium at 

 different temperatures is referred to. Other con- 

 ditions being the same, the magnesium line 4352 be- 

 comes finer with increase of temperature, while that 

 at 4481 becomes thicker, and this opposite behaviour 

 provides a test of increasing or decreasing tempera- 

 ture. But it is not pointed out that the test must 

 be applied with caution ; for the line 4352 in the spectra 

 of hot stars is not due to magnesium, but is really an 

 enhanced line of iron. If 4352 in the hot stars were 

 a magnesium line, then other lines of the same series 

 ought to be present, but they are not. 



The chapter on new stars is characteristic of a large 

 part of the book. Details are given of observations 

 of new stars from Nova Aurigae to Nova Persei, but 

 the record can scarcely be described as complete, and 

 the chief lesson taught by Novae is overlooked. Many 

 years ago. Sir Norman Lockyer expressed the view 

 that " new stars, whether seen in connection with 

 nebulae or not, are produced by the clash of meteor 

 swarms." When this conclusion was arrived at, few 

 precise observations of the spectra of Novae were 

 available, but It is not too much to say that visual and 

 photographic inquiries made since then into the 

 phenomena of new stars have substantiated it in a 

 KO. 1763, VOL. 68] 



very remarkable manner. By the meteoritic hypo- 

 thesis, new stars approximate to nebulae as they fade, 

 until their light at the last stage is indistinguishable 

 from that of a nebula. This association of new stars 

 with nebulae is now an accepted fact, but the con- 

 sequences have not been so clearly acknowledged. As 

 a new star reverts to the condition of a nebula when 

 it cools, evidently nebulag are not masses of gas at 

 transcendental temperatures. Just as in biology, the 

 course of evolution is traced in the development of the 

 embryo, so we may consider that in its brief life a 

 new star passes in some respects through the various 

 stages which mark the growth and decay of worlds. 



The spectroscopic history of Nova Aurigae was a 

 surprise to astronomers, who regarded the meteoritic 

 interpretation of the phenomena of new stars as a 

 hypothesis of doubtful validity. For, though there 

 might be a difference of opinion as to the meaning 

 of the displacement of the bright and dark lines in 

 the spectrum, there could be none on the fundamental 

 fact that the Nova became a planetary nebula, both 

 visually and spectroscopically, as It sank into 

 obscurity ; and this course of events was precisely that 

 previously found to have been exhibited by new stars 

 which had been subjected to spectroscopic analysis. 

 Rarely has h3'pothesis received such decided confirm- 

 ation, but Miss Gierke does not even mention the paper 

 in which it is put forward. The history of several 

 new stars is concluded with words to the effect that 

 " the regular cycle had been run through : a planetary 

 nebula replaced the faded star," but there Is no refer- 

 ence to the analysis of spectroscopic records before 

 Nova Aurigae, which showed that the reversion to a 

 nebular type is a common characteristic of new stars. 



The case of Nova Persei is of even greater signi- 

 ficance from the point of view of cosmogony than that 

 of Nova Aurigae, for Its light revealed the existence 

 of vast areas of dark matter in Interstellar space. 

 Miss Gierke describes the vicissitudes through which 

 the object passed, and the apparent expansion of the 

 nebula associated with It. With regard to this 

 phenomenon we read : — • 



" An explanatory hypothesis of considerable plausi- 

 bility was hit off independently by Prof. Kapteyn and 

 Mr. W. E. Wilson.^ It affirms the nebula to have 

 been pre-existent, and to remain unchanged. But 

 since we see it by the unchanged light of the Nova, 

 \X.:i various spires and condensations have come 

 successively Into view as the flare of the explosion 

 travelled outward in widening circles. Hence an 

 Illusory effect of radial expansion was produced, while 

 in point of fact, the temporarily Illuminated cosmic 

 folds were as immovable as aligned snow-peaks, in 

 turn set aglow by the setting sun." 



In other words, cosmic dust, or meteoritic particles, 

 or dark nebular matter — whatever you care to term 

 it — existed In the part of space In which the new star 

 made Its appearance. The fundamental idea of the 

 meteoritic hypothesis is here accepted, and Its applica- 

 tion to the phenomena of new stars acknowledged. 

 Astronomers have, in fact, been driven to the belief 

 in the existence of sheets or streams of non-luminous 

 matter in space; and dark nebulae, as Prof. Turner 

 has termed them in an article in the Fortnightly 



1 Nature, January 30, 1902. 



