vi PREFACE. 



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. 



