110 INTRODUCTION TO GENERAL SCIENCE 



e. 1305 : 55-58. Composition and Uses of Atmosphere. 



/. 1307 : 214-216. The Atmosphere and its Properties. 



g. 1701 : 83-91. The Atmosphere and its Properties. 



h. 1707 : 140-146. The Air and its Constituents. 



i. 1709 : 170-176. Composition of the Atmosphere. 



Experiment 40. Composition of Air. 



Apparatus: Beaker 150 c.c., crystallization dish 4" in 

 diameter, flat cork 1" in diameter. 



Materials : Piece of yellow phosphorus size of a pea. 



a. Fill crystallization dish half full of water, float the piece 

 of phosphorus on the cork, and invert the beaker over it. If 

 beaker does not stay in place, put a small weight upon it. 

 Leave for twenty-four hours. How high does the water rise ? 

 Phosphorus combines with the oxygen of the air and leaves 

 the nitrogen and other constituents of the atmosphere. 



81. WEIGHT OF THE AIR 



We learned in Section 2 that all matter is composed of 

 molecules, and in Section 3 that the difference between solids, 

 liquids, and gases is chiefly that of the relative velocity of the 

 molecules. We would expect, therefore, that matter, in any 

 of its states, would have weight on account of gravitation. 

 See Section 39, Universal Gravitation. 



The total weight of the air, or atmosphere, is not of special 

 interest. The number of tons means nothing to us, for we 

 cannot comprehend its vastness. What does interest us is the 

 weight of the atmosphere on each square inch of surface, 

 wherever we may be. The weight per unit area is called 

 pressure. At the surface of the ocean the pressure of the at- 

 mosphere is 14.7 pounds per square inch, or, approximately, a 

 long ton per square foot. Sea level is taken as the real level 

 of the earth's surface. 



