49 
lope this world of science, shrouding it usually with a dull 
watery fog of thick vapour ; but ever and anon, in some wild 
and monstrous hypothesis, streaming off, like the tail of a 
comet, into infinite space and the outer darkness ? The second 
and not the first, I hold to be the true description of modern 
science, in spite of all its progress. This is true both in 
physics, which deal with lifeless matter, and physiology, which 
deals with living creatures. If true in the first, it “must be 
doubly true in the second and higher department, which all 
confess to be more difficult and mysterious. My object in this 
address will be to establish its truth, even in physics, and for 
this end to consider these topics in succession ; the law of gra- 
vitation; the nature of matter; the existence of ether; the 
conservation of energy, with the doctrine of evolution, and the 
nebular theory; the dissipation of energy and the solar percus- 
sion theory; the molten nucleus theory of the earth's forma- 
tion; and the astro-glacial theory of the great ice-period, 
supposed to have lasted for ages before man appeared on the 
earth. 
.!• Tlie Law of Gravitation stands foremost among the doc- 
trines of modern physics. The evidences of its truth have 
gone on increasing for two full centuries, ever since the 
Pnncipia of Newton appeared. That any person of intelli- 
gence should still doubt it, after it has been confirmed by all 
the complex calculations and verified results of astronomy 
through these two hundred years, is to me a matter of wonder 
and amazement. 
But has this truth, however firm and solid, no nebula still 
surrounding it ? In that case, such a paper as the one in your 
fourth volume by your former secretary, on “ Current Physical 
Astronomy, would have been impossible. And that jiaper by 
no means stands alone. Statements of Dr. Tyndall and Mr. 
Spencer, and the hypotheses named by Professor Maxwell in 
his articles on “ Atoms ” and “ Attraction," prove still more 
decisively how much remains debated, uncertain, and obscure, 
even in the most certain of scientific truths. 
And first, what do we mean by a physical law ? Dr, Tyn- 
dall answers boldly, a fatal necessity. Torricelli, Newton, the 
scientific men of the present day, all knew, he says, that the 
succession, besides being permanent, is necessary; that the 
giavitating force must produce the observed course of the sea- 
sons. If the force be permanent, the phenomena are neces- 
xt 1 ^ the y do or do not resemble what has gone before. 
Nothing has occurred to indicate that the operation of the 
laws has ever been suspended, or nature crossed by spontaneous 
VOL. XI. F 
