28 PROCEEDINGS OF SECTION A. 



Generally speaking the accepted idea is that all the planets are in 

 different stages, not so much of age as of condensation and 

 temperature. Some, we know, are of very low density compared 

 with others. Saturn would float in water, while the earth is six 

 times as dense. Some are at much higher temperatures than 

 others, while one, and perhaps two, are at the stages of their 

 existence which, so far as human knowledge goes, is the only one 

 favourable to animal or vegetable life. The stages of a planet's 

 existence are generally supposed to begin after actual con- 

 densation into a spherical mass, with one of great surface heat, 

 and very low density — hot plastic masses, hardly coherent enough 

 to resist disruption by centrifugal force ; then a stage where 

 cooling has brought about a recombination of dissociated 

 elements ; then a farther cooling and formation of a solid crust to^ 

 the sphere, which later on arrives at the stage of our earth. 

 When, by further cooling, water becomes eternally solid, except 

 in equatorial regions, it arrives at a later stage when all appears 

 dead and unchangeable as seems to be the case with our moon. 



We will now, briefly review our present knowledge of the 

 several bodies included in Laplace's famous nebular hypotheses, 

 the sun and planets. 



As regards the sun, the chief mathematical questions are his 

 distance and dimensions ; the former has now been ascertained 

 within the narrow limits (astronomically speaking) of about 

 1,000,000 miles, that is, all recent determinations place it between 

 92,000,000 and 93,000,000 of miles, and with our present method 

 and means of observation, this almost appears to be the limit of 

 accuracy attainable. Up till about thirty-four years since, the 

 sun's distance had been accepted as 95,000,000 of miles. The 

 sun's real diameter is a point that cannot be spoken of with 

 certainty owing to the vaporous condition of its svirface. The 

 most recent assumption gives a diameter of 867,000 miles. 



With respect to the physical constitution and condition of the 

 sun, I can but take a brief glance at the astounding results which 

 observation and investigation have revealed by means of the 

 spectroscope. Summing up our knowledge, we find this body in 

 all probability to consist of a vaporous mass of a very low density, 

 which, as Professor Young suggests, appears to be of a consistence 

 analogous to tar or honey. The vapour consists of gases at a 

 temperature at which dissociation of elements takes place. The 

 visible surface or photosphere is probably somewhat cooler than 

 the underlying vapours, and the spectroscope reveals therein 

 absorption lines, indicating the presence of most of the elements 

 known on the earth excepting the metalloids. Outside the 

 photosphere is an envelope of incandescent gases called the 

 chromosphere, invisible except by aid of the spectroscope, but 

 wliich is still at an extremely higli temperature. Enormous 

 eruptions of incandescent hydrogen pierce the photospheric and 



