RESPIRATION AND THE VOICE. 135 



when it contains much carbonic-acid gas as when it con- 

 tains little. 



4, The blood is the common carrier of the nutriment 

 that we get from food and air ; and it takes away, not only 

 carbonic-acid gas, but all other waste matters. Some it 

 discharges by the kidneys, some by the skin; the car- 

 bonic-acid gas, chiefly by the lungs. 



WASTE MATTERS GIVEN OFF BY THE LUNGS. -CHANGES 

 IN THE AIR. -VENTILATION. 



SECTION VI. 1, The lungs, therefore, serve not only 

 to take in oxygen, but also to discharge carbonic-acid gas, 

 which is one of the chief waste products of the body. 



Another product of the waste of the body, which is dis- 

 charged by the lungs, is water. This ordinarily passes 

 off in the breath as invisible vapor. But in very cold 

 weather it is condensed as it comes out, and we can see 

 it. It lodges and freezes on the beards of men, and on 

 the hair of animals. If the vapor in the breath of a man 

 is collected for twenty-four hours, and condensed to water, 

 it measures as much as a pint. 



2, The breath of man, and of every animal, when 

 breathed out, contains, besides carbonic-acid gas and 

 water, a very little of a certain substance which gives to 

 each its peculiar odor. The breath of a cow, for ex- 

 ample, has a smell peculiar to the animal. In pure, 

 fresh human breath, we do not recognize any odor; but 

 the' substance is present: and in ill-ventilated rooms, 

 where many people have been breathing, it becomes 

 changed, and makes the air offensive and unhealthy. 

 These three substances oxygen, carbonic-acid gas, and 

 an unnamed animal substance are always present in 



