i6 CHE M 1ST R Y FOR A GRICUL TURAL STUDENTS 



At the level of the sea, the average height of the barometer is 30 inches 

 or 760 millimeters, and this is taken as the normal or standard pressure. 

 The old-fashioned weather glass consists of a j, -shaped barometer tube, in 

 which a fall of the mercury in the long limb is accompanied by a rise of 

 mercury in the short and open limb. Upon the surface of the mercury in 

 the short limb is a metallic float connected by a cord with a wheel to 

 which a pointer working against a dial is connected. The aneroid barometer 

 consists of a partially exhausted box having a flexible side, and which 

 expands or contracts with changes of pressure, the variation being exhibited 

 by means of a dial and pointer. Such an instrument, being portable, is 

 especially useful in determining altitudes by means of difference of atmos- 

 pheric pressure. The action of the suction pump also depends upon the 

 atmospheric pressure. Mercury being 13^ times as heavy as water, it is 

 theoretically possible to draw water from a depth 13^ times as great as the 

 barometric column. Owing, however, to dissolved gases and imperfect 

 valves, 28 feet is the greatest depth from which it is practically possible to 

 draw water by this means. 



III. COMPOSITION OF AIR 



To learn what air is, a careful study must be made of some 

 of those changes which are observed constantly taking place in 

 air, e.g. rusting and burning. 



When iron rusts, the change which occurs might be 

 due to absorption of air or of a constituent of it. If so, 

 the weight of the rust would be greater than that of the 

 ^^^ original iron, since air has weight. Place some iron 

 filings in a capsule and weigh. Now expose the 

 capsule to the air, and, when the filings have begun 

 to rust, weigh again. Do the filings weigh more after 

 rusting ? 



A further experiment is needed to determine whether 



air has actually disappeared. Shake some fine iron 



—J filings in a wide graduated tube, previously wetted 



^ 



with water so that the filings adhere to the side, 

 Fig. 4. and quickly invert the tube into a vessel of water. 



