3i6 



NATURE 



\Am^. 15, 1872 



with reference lo ilie more recent work of Dr. Iluggins. In his 

 observations he found that the brightest line of the three liright 

 hnes which constitute the spectrum of the gaseous nebula- was 

 coincident with the brightest of the lines of the spectrum of 

 nitrogen ; but the aperture of his telescope did not permit him 

 to ascertain whether the line in the nebula; was double, as is the 

 case with the line of Nitrogen. With the large telescope placed 

 in his hands by the Royal Society, he has found that the line in 

 the nebula; is not double, and in the case of the great ncljula in 

 Orion it coincides in position with the less refrangible of ilie two 

 lines which make up the corresponding nitrogen-line. He has 

 not yet been able to find a condition of luminous nitrogen in 

 which tlie line of this gas is single and narrow and defined like 

 the nebular line. 



lie has extended the method of detecting a stars motion m 

 the line of sight by a change of refrangibility in the line of a 

 terrestrial substance existing on the star to about 30 stars besides 

 Sirius. The comparisons have been made with lines of hydrogen, 

 magnesium, and sodium. In consequence of the extreme diffi- 

 culty of the investigation, tlie numerical velocities of the stars 

 have been obtained by estimation, and are to be regarded as pro- 

 visional only. It will be observed that, speaking generally, the 

 stars which the spectroscope^shows to be moving from the earth, 

 as Sirius, Betelgeux, Rigel, Procyon, are situated in a part of 

 the heavens opposite to Hercules, towards wliicli the sun is ad- 

 vancing; whilst the stars in the neighbourhood of this region, as 

 Arcturus, Vega, and a Cygni, sho^v a motion of approach. 

 There are, however, in the stars already observed, exceptions to 

 this general statement ; and there are some other considerations, 

 as the relative velocities of the stars, which appear to show that 

 the sun's motion in space is not the only or even in all cases the 

 chief cause of the observed proper motions of the stars. In the 

 observed stellar motions we have to do probably with two other 

 independent motions — namely, a movement common to certain 

 groups of stais and also a motion peculiar to each star. Thus 

 the stars, /3 7 5 e ^, of the Gre&t Bear, which have similar 

 proper motions, have a common motion of recession ; wliile the 

 star a of the same constellation, which has a proper motion in 

 the opposite direction, is shown by the spectroscope to be 

 approaching the earth. From further researches in this direction, 

 and from air investigation of the motions of stars in the line of 

 sight in conjunction with their proper motions at right angles to 

 the visual direction obtained by the ordinary methods, we may 

 hope to gain some definite knowledge of the constitution of the 

 heavens. 



This discovery supports, in a somewhat striking manner, the 

 views which Mr. Proctor has been urging respecting the distri- 

 bution of the stars in space. According to these views there 

 exist within the sidereal system subordinate systems of stars 

 forming distinct aggregations, in which many orders of real mag- 

 nitude exist, while around them is relatively barren space. He 

 had inferred the existence of such systems from the results of 

 processes of equal-surface charting applied successively to stars 

 of gradually diminishing orders of brightness. He found the same 

 regions of aggregation, whether the charts included stars to the 

 sixtli order only or were extended, as in his chart of the nortliern 

 heavens, to the tenth and eleventh orders ; and these regions of 

 aggregation are the very regions where the elder Herschel found 

 the faintest telescopic stars to congregate. Applying a new 

 system of charting to show the proper motions of the stars, he 

 found further evidence in favour of these views. The charts indi- 

 cated the existence of concurrent motions among the members 

 of several groups or sets of stars. Selecting one of the more 

 striking instances as affording what appeared to him a crucial 

 test of the reality of this slar-drift, Mr. Proctor announced his 

 belief that whenever the spectroscopic method of determining 

 stellar motions of recess or approach should be applied to the 

 five stars, yS 7 5 f and (,' Ursa; Majoris, these orbs (which formed 

 a drifting set in the chart of proper motions) would be found to 

 be drifting collectively either towards or from the earth : this has 

 been confirmed. 



The time has now come for more closely investigating the 

 various theories which have been propounded by such profound 

 thinkers as Tyndall, Tait, Reynolds, and others, to account for 

 the phenomena of Comets. I do not propose to enter into a 

 statement of these theories; but I venture to call attention to 

 ZoUner's views, which have recently given rise to a great amount 

 of controversy. In doing so, I am solely influenced by a desire 

 to give information on this subject, without implying thereby 

 that I give my adherence, or even preference, to his theory. * 

 * See Appendix, p. 12. 



The vaporisation of even sulid bodies at low temperatures 

 suggests that a mass of matter in space will ultimately surround 

 itself with its own vapour, the tension of which will depend 

 upon the mass of the body (that is, upon its gravitative energy) 

 and the temperature. If the mass of the body is so small 

 that its attractive force is insuflicient to give to the enveloping 

 vapour its maximum tension for the existing temperature, the 

 evolution of vapour will be continuous until the whole mass is 

 converted into it. It is proved by analysis that such mass of 

 gas or vapour in empty and unlimited space is in condition of 

 unstable equilibrium, and must become dissijiated by continual 

 expansion and consequent decrease of density. It follows that 

 celestial spaces, at least within the limits of the stellar universe, 

 must be filled with matter in the form of gas. 



A fluid mass existing in space at a distance from the sun or 

 other body radiating heat would, if its mass were not too great, 

 be converted entirely into vapour after the lapse of sufficient 

 time. But if the fluid mass approach the sun, solar heat would 

 occasion a more rapid development of vapour on the sunward 

 side ; and the total vaporisation would require an incomparably 

 short time with reference to the interval necessary in the former 

 case ; this time would be shorter the smaller the mass of the 

 body. Prof ZiiUner points to the smaller comets, which often 

 appear as spherical masses of vapour, as examples of such bodies, 

 while the spectra of some of the nebuke and smaller comets 

 render the existence of fluid masses giving out vapour highly 

 probable. 



The self-luminosity and train of comets he refers to other 

 causes. Two causes only are known through the operation of 

 which gases become self luminous — elevation of temperature (as 

 by combustion), or electrical excitement. Setting aside the first 

 as involving theoretical difficulties, the second cause is demon- 

 strated by him to be sufficient to account for the self-luminosity 

 and the formation of the train, provided it be gi-anted that 

 electricity may be developed by the action of solar heat, if not 

 in the process of evaporation, at least in the mechanical and 

 molecular disturbances resulting from it. The production of 

 electricity by such processes within the limits of our experience, 

 must be admitted as a well-known fact. The spectrum of the 

 vaporous envelope of a comet, illuminated in this manner, must 

 necessarily be that produced by the passage of an electrical dis- 

 charge through vapour identical in substance with a portion of 

 the comet's nucleus, from which the envelope is derived. As, 

 according to this supposition, water and liquid hydrocarbons are 

 import?.nt constituents of these bodies, the spectra of the comets 

 should be such as belong to the vapours of these substances ; and 

 in this manner the resemblance and partial coincidence of the 

 observed cometic spectra with those of gaseous hydrocarbons is 

 explained. 



The form and direction of the train indicate undoubtedly the 

 action of a repulsive force ; and Prof ZoUner asserts that the 

 assumption of an electrical action of the sun upon bodies of the 

 solar system is necessary and sufficient to account for all the 

 essential and characteristic phenomena of the vaporous envelope 

 and the train. The direction of tlie train, towards or from the 

 sun, is, according to this theory, to be easily explained by the 

 supposition of a variabiHty in the mutual electrical conditions. 

 This accords perfectly with the phenomena observed in tlie 

 development of electricity by vapour-streams in the hydroelectric 

 machine, where the sign of electricity depends upon the presence 

 or absence of various substances in the boiler or tire tubes. 



The theory acquires an additional interest from Schiaparelli's 

 remarkable discovery of the identity of the paths of certain 

 comets with great meteor-streams, since the meteoric masses 

 must inevitably be conver:ed into vapour on approaching the 

 sun, with the exhibition of the characteristic appearances of the 

 comets. 



The intimate connection of planetary configuration and solar 

 spots, of the latter and terrestrial magnetism and auroral pheno- 

 mena, must tend to establish also a connection between solar 

 spots and solar radiation. It is demonstrated by the researches 

 of Piazzi Smyth, Stone, and Cleveland Abbe, that there is a 

 connection between the amount of heat received from the sun 

 and the prevalence of spots — a result clearly in harmony with 

 those derived from recent investigations into the nature of the solar 

 atmosphere. Further, in a paper by Mr. Meldruui, of Mauritius, 

 which will be read before you during this session, most remark- 

 able evidence is given on the close connection of these pheno- 

 mena. It appears that the cyclones of the Indian Ocean have 

 a periodicity corresponding with the sun-spot periodicity ; so 

 that if an observer in another planet could see and measure the 



