RESPIllATION, CIRCULATION, DIGESTION DIAGRAM 1. 13 



this reason, when we are about to descend into a well, or mine, 

 or a cistern, where no one has been for a long time, we must 

 let down a lighted candle by a cord ; if it is not extinguished, 

 oxygen is present, and we can descend without fear ; but if 

 the candle goes out, there is no oxygen left, and a man would 

 die there. 



Nitrogen is a gas like oxygen, but it can neither support 

 combustion, nor sustain life. 



Carbonic acid is the gas which causes the froth of beer, 

 seltzer water, cider, or of sparkling wine. Carbonic acid, 

 like nitrogen, is neither fitted to support combustion nor to 

 sustain life. 



Air contains about one part of oxygen to three parts of 

 nitrogen, with a very small quantity of carbonic acid. During 

 respiration, the air which enters the lungs leaves behind a 

 certain quantity of oxygen, and returns charged with a larger 

 proportion of carbonic acid. Therefore, if a man is shut up in 

 a room where the air cannot be renewed, he gradually exhausts 

 all the oxygen, and at last dies. He dies very quickly under 

 water, because the oxygen no longer reaches his lungs, and he 

 can no longer breathe ; and this also happens when the neck 

 is squeezed sufficiently to compress the windpipe. 



The air expelled from the lungs during respiration, contains 

 lome aqueous vapour as well as a large quantity of carbonic 

 acid : this forms the moisture of the breath, and we can thus 

 perceive if a patient still breathes, by holding a glass to hia 

 mouth. 



CIRCULATION. The body contains a great number of vessels 

 which proceed from the heart and return to the heart. The 

 first are the arteries, and the second are the veins. These 

 vessels, which grow finer and finer the further they extend from 

 the heart, and larger and larger according as they approach it, 

 are all filled with blood. But it is not the same colour in the 

 veins and in the arteries ; and it has no longer the same 

 quality. It is often believed that venous blood is blue, and it 



