PREFACE. pa 
phenomenon of Phosphorescence in dead animal 
substances, and the emission of light by living 
animals. 
The fourth division comprises some historical 
notes upon the subject, and the theory by which 
I have endeavoured to account for the various 
phenomena to which the attention of the reader 
is called in the previous chapters. I have pur- 
posely placed my theoretical considerations in 
this section, as they will thus have a better 
chance of fair appreciation than if their elements 
had been dispersed throughout the work, ap- 
pended to every isolated fact or experiment as 
mentioned or described. 
