34 THE REASON WHY. 



"Lord, make me know mine end, and the measure of my days, that I may know 

 how frail I am." PSALM xxxix. 



a powerful stimulant, gives an unnatural activity to the nervous 

 centres and the brain. 



39. In what proportions are the atmospheric gases found in 

 the Hood? 



The mean quantity of the gases contained in the human blood has 

 been found to be equal to l-10th of its whole volume. In venous 

 blood, the average quantity of carbonic add is about 1-1 8th, that of 

 oxygen about l-85th, and that of nitrogen about l-100th of the 

 volume of the blood. In arterial blood their quantities have been 

 found to be carbonic acid about l-14th, oxygen about l-3Sth, 

 and nitrogen about l-72nd. 



40. Then is nitrogen taJcen into the Hood from the air ? 

 Such a supposition is highly improbable. It is probably derived 



from nitrogenisedfood, just as carbonic acid is derived from car- 

 lonisedfood. 



41. W hat is venous blood? 



Venous blood is that which is returning through the veins of the 

 body from the organs to which it has been circulated. 



42. What is arterial blood ? 



Arterial blood is that which is flowing from the heart through 

 the arteries to nourish the parts where those arteries are distri- 

 buted. 



43. What is the difference between venous and arterial 

 blood? 



Venous blood contains more carbonic acid, and less oxygen and 

 nitrogen than arterial blood. 



44. Will nitrogen burn ? 



It will not burn, nor will it support combustion. 



45. What is the difference between " burning" and " sup- 

 porting combustion?" 



Oxygen gas will not burn of itself, but it aids the decomposition 

 by fire of bodies that are combustible. It is therefore called a sup- 

 porter of combustion. But hydrogen gas, though it burns of itself 



