12 



KNOWLEDGE 



[Januaby 2, 1893. 



surrouading cluster aud nebulosity, while the structures 

 a, h and ( are seen tbrouG;b a certain depth of nebulosity, 

 so that they appear veiled and less black than the summits 

 of the dark structures which extend in a direction towards 



US. 



If we adopt this explanation of the facts observed, we are 

 forced to conclude that the ejected matter, when it is fii'st 

 shot outwards from the central region, is dark or opaque, 

 and that it afterwards becomes luminous and gives rise to 

 the stellar points which we have spoken of as stars, though 

 they may really correspond to considerable areas of 

 tiocculent nebulous matter, giving much less light, area tor 

 area, than the photosphere of our sun. According to 

 Prof. E. C. Pickering's estimate of the sun's light in stellar 

 magnitudes, our sun, if it were removed to a distance 

 where it would appear to shine like a star of the fourteenth 

 magnitude, would (if there were no abioptiou of light in 

 its passage through space) have an apparent diameter of 

 only the iifty-thousandth part of a second," but the 

 smallest stars shown have in these photographs an apparent 

 diameter of at least fifteen seconds of arc, so that it is ! 

 possible that the bright regions we are referring to, and I 

 which we have hitherto spoken of as stars, may have \ 

 diameters many thousand times greater than that of our 

 sun, and may be many million times less bright, area for ' 

 area, than the solar photosphere. 



The branching character of the dark prominence forms, 

 and the spreading nature of their summits, indicates that 

 they are projected into a resisting medium, and that in 

 expanding they are doing work against resistance ; con- 

 sequently if the dark prominences are composed of gaseous 

 matter, they must cool as they expand. It seems also 

 highly probable that matter shot up from the interior of 

 a cluster or nebula, which is cooling into space, would be 

 hotter than the matter which has been in the exterior 

 parts of the nebula or cluster for a considerable time. 

 We are therefore, no doubt, warranted in concluding that 

 [he ejected matter, as it first issues in a hot condition, is j 

 dark and opaque, aud that as it cools it becomes luminous 

 and comparatively transparent, or transparent in the inter- 

 spacBS between highly luminous regions. 



It seems to me that the phenomena we observe are very 

 much what we might expect to see if a mass of mixed vapour 

 at a high temperature were projected into a region occupied 

 by cooler vapour, or any resisting cooler medium, such as a 

 cloud of dust : the elements having the highest evaporating ' 

 and melting points would first condense into a luminous 

 mist, the particles of which would act like floating bodies 

 on a stream, and be swept aside into the eddies and vortices 

 surrounding the mam current of the outrushing gas. To 

 judge of large things by comparatively small, we see the 

 same tendency to collect into fiocculent masses in the 

 willow-leaf and rice-grain structure of the solar photosphere, 

 and in our own "mackerel" clouds in a gale of wind. 

 According to my theory, the dark structures of the Milky 

 Way and nebulous clusters are the stellar analogues of the 

 solar spots. 



When a large sunspot is near to the centre of the sun's 

 disc, and we are able to look down into it, wo do not see 

 the heated central body of the sun, nor do we see the I 

 bright shell of photosphere which, it is only reasonable to j 

 assume, is shining both upwards and downwards on the 

 opposite side of the sun. We are, no doubt, looking into 

 a deep mass of heated vapour, and the wave-lengths, which 

 are radiated by the hotter and deeper strata, are entirely 

 swallowed up and absorbed by cooler vapour at a higher 

 level, or, to speak with greater caution, such a large 



* Sec The Old and Sew Anli'onoini), ]>agL' 777. 



proportion of the radiated energy is swallowed up that the 

 intensely heated gaseous mass looks comparatively black 

 beside the relatively cooler incandescent mist into which 

 tbe outer gaseous matter condenses where it can radiate 

 freely into space ; aud this is the case even though a 

 portion of the hght of the incandescent mist is absorbed 

 by vapours above it, as is evident from the channels cut 

 out of its continuous spectrum. In the same manner, 

 we should expect the intensely heated gas issuing from the 

 hotter parts of a nebula or cluster to appear black and 

 opaque compared with the relatively cooler matter com- 

 posing the outer parts of the nebulous mass or cluster. 

 In the case of the sun, where there is a rapid fall in the 

 temperature as we pass upward through a few thousand 

 miles, we should expect the incandescent clouds of 

 condensing vapour to form a comparatively thin spherical 

 layer corresponding to the photosphere ; but in the 

 case of vast streams of heated gas which are carried 

 to enormous distances from the heated region from 

 which they are ejected, before they become sufficiently 

 cooled for any part of the constituent vapours to condense 

 into a white-hot mist, the conditions of cooling would be 

 altogether different. The condensing gas is probably much 

 rarer than in the sun at the level of the photosphere, and 

 it seems not improbable that in the nebula the regions of 

 most rapid cooling and condensation would be intimately 

 associated with the vortices formed in the surrounding 

 medium, where the most rapid contact of heated and cooled 

 material would take place, and where the most rapid com- 

 pression and expansion of gases causing changes of tem- 

 perature would also be localized. Such at least seems to 

 me to be the most probable solution of the great riddle 

 presented to us in the complicated phenomena observed. 



It seems certain that the stars of irregular soar-clusters 

 cannot be movmg under the action of gravity so as to form 

 a permanent system with motions about the common 

 centre of gravity of the cluster. It is possible that the 

 individual stars of such a cluster might exist for a hmited 

 time until a collision took place and they were shot forth 

 again on a new orbit ; but it is also possible that the stars 

 of such clusters are not bodies similar to our sun, but that 

 they correspond to regions of rapid condensation where a 

 white-hot mist is formed in a gaseous medium pervading 

 the whole region of the cluster. The curvihnear streams 

 of. stars with forking branches which have been noted in 

 so many clusters (see Miss Gierke's " System of the Stars," 

 p. 243), seem to indicate that they are intimately associated 

 with streams of gaseous matter projected into a resisting 

 medium, and recent photographs also seem to point to the 

 conclusion that all star clusters are nebulous. 



THE LATE MR. MATTIEU WILLIAMS. 



M 



ANY of our readers will hear with sincere regret 

 of the death of Mr. W. Mattieu Williams, who 

 was formerly, during the editorship of Mr. 

 Proctor, a very constant contributor to the pages 

 of Knowledge. Mr. Williams was born in 1820, 

 and was at the time of his death in his seventy-third year. 

 He left school at the early age of eleven, and was appren- 

 ticed to a Mr. Street, an optical instrument maker in 

 London. During this hard-working period of his boyish 

 life he attended a mechanics' institute in the evenings, and 

 taught himself French aud German. On coming of age 

 he inherited a small sum of money, which enabled him to 

 go to Edinburgh to study chemistry, and he subsequently 

 further continued his education by making a tour through 

 the principal countries of Europe on foot. In 1851, on 



