10 KEPORT— 1872. 



■where the elder Herschel found the faintest telescopic stars to congregate. Ap- 

 plying a new system of charting to show the proper motions of the stars, he 

 found fm'ther evidence in favour of these views. Ihe charts indicated the exis- 

 tence of conciuTent 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 star-drift, Mr. Proctor announced his belief 

 that whenever the spectroscopic method of determining stellar motions of recess 

 or approach should be applied to the live stars /3, y, 8, e, and ( Ursse 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 your attention to Zollner'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*. 



The vaporization of even solid bodies at low temperatures suggests-that a mass 

 of matter in space vnW ultimately surround itself with its own vapour, the tension 

 of which wiU 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 insufficient to give to the enveloping vapour its maximum tension for the exist- 

 ing temperature, the evolution of vapour wiU be continuous until the whole mass 

 is converted into it. It is proved by analysis that such a mass of gas or vapour in 

 empty and unlimited space is in a condition of unstable equilibrium, and must be- 

 come dissipated by continual expansion and consequent decrease of density. It 

 follows that celestial spaces, at least within the limits of the stellar imiverse, 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 vaporization 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. Professor Zolhier 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 nebulfe 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 demonstrated 

 by him to be sufficient to account for the self-luminosity and the formation of the 

 train, provided it be granted that electricity may be developed by the action of 

 solar heat, if not in the process of evaporation, at least in the mechanical aud 

 molecular disturbances resulting from it. The production of electricity by such 

 processes within the limits of our experience must be admitted as a well-kno'^Ti 

 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 discharge 

 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 hj'drocarbons are important 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 imdoubtedly the action of a repul- 

 sive force : and Professor Zcillner asserts that the assimiption 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 



* See Appendix, p. 12. 



