8 4 LAY SERMONS, ADDRESSES, AND REVIEWS. [v. 



Here, however, simple observation stops, and we 

 must have recourse to experiment. 



You tie a vein, and you find that the blood accumu- 

 lates 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 open- 

 ings 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 the 

 evidence that the blood is propelled by the heart through 

 the arteries, and returns by the veins — that, in short, the 

 blood circulates. 



Suppose our experiments and observations have been 

 made on horses, then we group and ticket them into a 

 general proposition, thus : — all horses have a circulation 

 of their blood. 



Henceforward a horse is a sort of indication or label, 

 telling us where we shall find a peculiar series of phe- 

 nomena called the circulation of the blood. 



Here is our general proposition, then. 



How, and when, are we justified in making our next 

 step — a deduction from it % 



Suppose our physiologist, whose experience is limited 

 to horses, meets with a zebra for the first time, — will he 

 suppose that this generalization holds good for zebras 

 also ? 



That depends very much on his turn of mind. But 

 Ave will suppose him to be a bold man. He will say, 

 " The zebra is certainly not a horse, but it is very like 

 one, — so like, that it must be the ' ticket ? or mark of a 

 blood-circulation also ; and, I conclude that the zebra 

 has a circulation." 



That is a deduction, a very fair deduction, but by no 



