president's address. 27 



attack afresh many of the numerous and baffling problems of the 

 universe whicli come within the province of the Astronomer. 

 Thus we find tliat the revelations of the spectroscope in the 

 laboratory at once made possible the solution of a host of difficult 

 questions and the testing of numerous hypotlieses on the chemistry 

 of the universe ; while the dawning of the ditterent stages in our 

 knowledge of dynamics, heat and light, and their inter-relations, 

 were quickly followed by corresponding advances in physical 

 astronomy, which have thrown new light upon the great question 

 of the history and present constitution of the heavenly bodies, 

 and afford grounds upon which to base new theories and 

 hypotheses. 



I think we may take for granted that the great questions of 

 physical astronomy now may be briefly summed up as follows : — 



What is the pi'esent constitution of the visible universe ; is it 

 uncliangeable, or is it in a state of change 1 



What has been its previous history, and to what is it all 

 tending 1 



The objects for study to which we look for most help in the 

 solution of these questions are of two classes ; first, those tenants 

 of space in our immediate neighbourhood whose distance from us 

 can be counted in millions of miles — the numbers of the 

 solar system — and, second, those bodies which are so far removed 

 from the solar group that light cannot span the intervening space 

 in less than three years. 



The history of the solar group has been hypothetically sketched 

 in the nebular theories of Kant and Laplace, the former 

 imagining in the beginning a cloud-like mass of dissociated 

 elements with a nucleus or condensed centre, occupying the space 

 now tilled by our solar system, and that formation of separate 

 bodies out of this matter took place by mutual gravitation of 

 parts. 



Laplace was led to very similiar conclusions, his assumption 

 being that the planetary space was occupied by the sun, sur- 

 rounded by an immense fiery atmosphere, endowed with a rapid 

 rotatory motion, which eventually broke the hot vaporous matter 

 into rings, these in turn condensing into planets with their 

 appropriate orbital motions. 



This grand conception, despite certain difficulties which present 

 themselves in view of more modern knowledge, is up to the 

 present time the accepted theory of the early evolution of our 

 system, and while advance in knowledge has suggested objections 

 to the full acceptation of the hypothesis, it has, on the other 

 hand, furnished strong evidence of its general correctness. 



There is plenty of evidence that the matter of which the 

 several bodies consist is generally speaking the same. The same 

 constituents are known to exist in all, but under different co)i- 

 ditions, due to difi'erent grades of heat, condensation, and motion. 



