IntrodiLctory. 39 



exact order in which the topics will be discussed. 

 As the same phenomena appear in different depart- 

 ments of nature the same topics will appear in the 

 discussion whenever the subject in hand aids in 

 their illustration or needs them for its own. In 

 such a field there is scope for the most thorough 

 research and analysis. If we can but call more 

 careful attention to these departments of study we 

 may hope for much advantage to speculative sci- 

 ence and practical life. 



