ADVERTISEMENT TO THE READER. 
Tue greater part of the substance of the following Essays 
has already been published in the form of Oral Discourses, 
addressed to widely different audiences, during the past three 
years. 
Upon the subject of the second Essay, I delivered six 
Lectures to the Working Men in 1860, and two, to the 
members of the Philosophical Institution of Edinburgh in 
1862. The readiness with which my audience followed 
my arguments, on these occasions, encourages me to hope 
that I have not committed the error, into which working 
men of science so readily fall, of obscuring my meaning by 
unnecessary technicalities: while, the length of the period 
during which the subject, under its various aspects, has been 
present to my mind, may suffice to satisfy the Reader that, 
my conclusions, be they right or be they wrong, have not 
been formed hastily or enunciated crudely. 
Lonvon: January, 1863. 
