n8 GENERAL SCIENCE 



first leaving an almost pure liquid oxygen. This oxygen as 

 a much compressed gas in strong steel cylinders can be trans- 

 ported for use anywhere. 



SUMMARY 



Differences in atmospheric pressure are due to differences in density. 

 This change in density results from changes in temperature and from 

 variations in the amount of water vapor present in the air. The higher 

 the temperature and the greater the amount of water vapor the less 

 the density. 



The rise and fall of the mercury column in the barometer measures 

 the varying pressure of the atmosphere whatever the occasion of its 

 change in density. Any considerable fall of the barometer within a 

 short time usually indicates a marked increase in the amount of water 

 vapor in the atmosphere, making more probable the precipitation of 

 some of it as rain or snow. 



The volume of any gas is inversely proportional to the pressure upon 

 it. 



Gases may be changed to liquids by subjecting them to high pressure 

 and low temperature. The heat resulting from their compression is 

 easily disposed of by allowing cold water to flow over the pipes in 

 which the gas is confined. 



Compressed gases when allowed to expand abstract from surrounding 

 bodies an amount of heat equal to what was freed in compressing them. 

 The cooling effect of compressed gases when allowed to expand is ex- 

 tensively used in maintaining low temperatures in cold storage houses, 

 in the manufacture of artificial ice, and in keeping large buildings cool 

 in summer. 



Ammonia gas is a by-product of the plants where illuminating gas 

 is made. This gas is easily liquefied, and it can be purchased in the 

 liquid form confined in heavy steel cylinders. It is to be remembered 

 that the ammonia water of the laboratory and the laundry is merely 

 water containing ammonia gas in solution. 



The temperatures at which different gases change to liquids when 

 under the same pressure vary widely. For the same gas it is always 

 the same. 



APPLICATIONS OF ATMOSPHERIC PRESSURE 

 That man can live and move about far below the surface 

 of water, that he can build foundations for bridges out in 



