114 FIRST YEAR SCIENCE 



Animals smother in carbon dioxide. It is known to 

 coal miners as choke damp, because frequently after they 

 have escaped from an explosion they are smothered by it. 

 It occurs at a few localities, as at the Dog Grotto near 

 Naples and in Death Gulch, Yellowstone National Park, 

 in sufficient quantities to be fatal to animals passing 

 through these places. 



As a rule, however, the proportion of oxygen, nitrogen 

 and carbon dioxide is the same for all places on the sur- 

 face of the earth and it is only where for some peculiar 

 cause carbon dioxide is emitted from the ground in a 

 place sheltered from the wind, that it can accumulate. As 

 animals and men breathe it out, rooms where they stay 

 must have proper ventilation. 



The nitrogen is needed to dilute the oxygen. If oxygen 

 were undiluted, animals could not live, and a fire once 

 started would burn up iron as readily as it now does wood. 

 As has already been stated, certain bacteria take nitrogen 

 from the air arid prepare it so that plants can use it. 



Plants and animals both need water vapor. Were it 

 not for this form of moisture there would be no rain, and 

 without rain life could not exist. Thus the air which 

 contains oxygen and water vapor for both plants and an- 

 imals, carbon dioxide for the plants, and nitrogen to dilute 

 the oxygen, is one of the greatest factors in 



rt IB the life history of the earth. 



52. Weight of Air. Experiment 60. Into a five- 

 pint bottle insert a tightly fitting rubber stopper through 

 which a glass tube extends. To the outer end of the 

 glass tube tightly fit a thick -walled rubber tube of suf- 

 ficient length for the attachment of an air pump. Put 

 Fig. 51. a Hoffman's screw upon the rubber tube. See that all 1 

 connections are air-tight. Weigh carefully the apparatus 

 as thus arranged. Now attach the rubber tube to the air-pump 

 and extract the air from the bottle. When all the air that can be 



