310 THE ADDRESSES, LECTURES, ETC., OF 



hope that these occasional visitors will serve to furnish us with 

 positive evidence in my favour. Astronomical physicists tell us 

 that the nucleus of a comet consists of an aggregation of stones 

 similar to meteorites. Adopting this view, and assuming that the 

 stones have absorbed in stellar space gases to the amount of six 

 times their volume, taken at atmospheric pressure, what, it may 

 be asked, will be the effect of such a divided mass advancing 

 towards the sun at a velocity reaching in perihelion the prodigious 

 rate of 866 miles per second (as observed in the comet of 1845), 

 being twenty-three times our orbital rate of motion ? It appears 

 evident that the entry of such a mass into a comparatively dense 

 atmosphere must be accompanied by a rise of temperature by 

 frictional resistance, aided by attractive condensation. At a 

 certain point the increase of temperature must cause ignition, and 

 the heat thus produced must drive out the occluded gases, which 

 in an atmosphere 3,000 times less dense than that of our earth 

 would produce 6 x 3,000 = 18,000 times the volume of the stones 

 themselves. These gases would issue forth in all directions, but 

 would remain unobserved excepting in that of motion, in which 

 they would meet the interplanetary atmosphere with the compound 

 velocity, and form a zone of intense combustion, such as Dr. 

 Huggins has lately observed to surround the one side of the 

 nucleus, evidently the side of forward motion. The nucleus 

 would thus emit original light, whereas the tail may be supposed 

 to consist of stellar' dust rendered luminous by reflex action pro- 

 duced by the light of the sun and comet combined, as foreshadowed 

 already by Tyndall, Tait, and others, starting each from different 

 assumptions. 



Although I cannot pretend to an intimate acquaintance with 

 the more intricate phenomena of solar physics, I have long had a 

 conviction, derived principally from familiarity with some of the 

 terrestrial effects of heat, that the prodigious dissipation of solar 

 heat is unnecessary to satisfy accepted principles regarding the 

 conservation of energy, but that solar heat may be arrested 

 and returned over and over again to the sun, in a manner 

 somewhat analogous to the action of the heat recuperator in 

 the regenerative engine and gas furnace. The fundamental 

 conditions are : 



1. That aqueous vapour and carbon compounds are present in 

 stellar or interplanetary space. 



