VI PREFACE. 



shew only the general agency of these properties, and, as a 

 consequence of this general agency, that no single process of 

 an animal body can be fairly and fully investigated, without 

 taking this agency into the account ; nay, without making its 

 exhibition a primary concern in the analysis of relations. 



The work here proposed for the accomplishment of the 

 experimentalists, is by no means new, or foreign to their 

 habits: considerable progress has already been made in it; 

 but if this progress can be facilitated by any additional sug- 

 gestions, or by a more extended connection of the subject, the 

 cause of science will be greatly served, and my present 

 attempt will be amply rewarded, 



The necessity and importance of the living principle in the 

 phenomena of animals, has been remarked from the earliest pe- 

 riods of observation. That the Ancients were aware of the 

 necessity of this principle, is proved by the fable of Prome- 

 theus, who having formed a man of clay, stole fire from 

 Heaven to animate him : in this fable the word " fire" is ob- 

 viously an allegorical allusion to a principle of life. Since 

 this earliest distinction between matter and life, the last has 

 been variously noticed ; and, until latter periods, has been 

 rather a subject for fancy or imagination than one of philoso- 

 phical inquiry, whether by experiment or induction. But, 

 without excepting any period, the operations of this principle 

 have been spoken of generally and vaguely, with but few ex- 

 ceptions, as will appear from a brief sketch of the light in 

 which this principle has been hitherto regarded. 



The first notice of the existence of a living principle, such 

 as that suggested in the fable, assigned to it no precise im- 

 portance and no particular influence. Many crude and 

 fanciful conjectures were broached relative to an agency of life, 

 and they appear to have been indulged in equally by poets 

 and physicians. From one of the hitter order, a theory sub- 

 sequently arose winch might more consistently have origi- 

 nated with the poets : according to this theory, that of Stahl, 

 an intelligent spirit was said to animate bodies, and to direct 



