ADVERTISEMENT TO THE READER. 



The greater part of the substance of the following 

 Essays has already been published in the form of Oral 

 Discourses, addressed to widely different audiences, dur- 

 ing 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. 



T. H. H. 



London: January, 1863. 



