In one of the chapters towards the end of the volume, the reader will find evidence from the testimony of geology that the age of the sun's heat must be far greater than it could possibly have been had the heat been derived, as is generally supposed, from the condensation of the sun's mass.
I may mention that although the greater part of the work has already appeared in separate articles in the Philosophical Magazine and some other journals, the volume, nevertheless, is not a collection of detached papers; for nearly the whole of the articles were written in connected order with the view of appearing in their present form.
There are many of the topics discussed which I could have wished to consider more at length, but advancing years and declining health constrain me to husband my remaining energies for work in a wholly different field of inquiry-work which has never lost for me its fascination, but which has been laid aside for upwards of a quarter of a century.
EDINBURGH, October, 1885.