II OF THE NATURAL HISTORY SfTEXOES 53 



4. Verification, which is the process of ascer- 

 taining whether, in point of fact, our anticipation 

 is a correct one. 



Such are the methods of all science whatsoever ; 

 but perhaps you will permit me to give you an 

 illustration of their employment in the science of 

 Life ; and I will take as a special case the 

 establishment of the doctrine of the Circulation of 

 the Blood. 



In this case, simple observation yields us a 

 knowledge of the existence of the blood from some 

 accidental haemorrhage, we will say ; we may even 

 grant that it informs us of the localisation of this 

 blood in particular vessels, the heart, &c., from 

 some accidental cut or the like. It teaches also 

 the existence of a pulse in various parts of the 

 body, and acquaints us with the structure of the 

 heart and vessels. 



Here, however, simple observation stops, and we 

 must have recourse to experiment. 



You tie a vein, and you find that the blood 

 accumulates on the side of the ligature opposite 

 the heart. You tie an artery, and you find that 

 the blood accumulates on the side near the heart. 

 Open the chest, and you see the heart contracting 

 with great force. Make openings into its principal 

 cavities, and you will find that all the blood flows 

 out, and no more pressure is exerted on either side 

 of the arterial or venous ligature. 



Now all these facts, taken together, constitute 



