14 CHEMISTRY FOR AGRICULTURAL STUDENTS 



flame. Seal up the ends of a tube by melting in the Bunsen flame till the 

 glass runs together ; on now blowing through the open end the sealed end 

 will expand into a little bulb. 



By employing a stout glass flask which can be completely 

 exhausted of air by means of an air-pump, the weight of a 

 volume of air can be determined. Under normal conditions 

 (see p. 39) a liter of dry air is found to weigh i. 226 grams. The 

 atmosphere is known to extend in appreciable quantities to the 

 height of one hundred miles above the earth, and its total 

 weight and consequent pressure upon the earth's surface must 

 be very great. What this amounts to can be ascertained by 

 the following experiment : — 



Fill a stout glass tube, at least 4 millimeters in bore, 8 deci- 

 meters long and closed at one end, with pure dry mercury. 

 P Remove all bubbles of air by tapping the tube or 

 other means, then close the open end with the finger 

 and invert the tube into a capsule or trough con- 

 taining mercury. Now remove the finger, and note 

 that the mercury remains supported in the tube to a 

 certain height. By means of a three-foot rule or 

 meter measure, accurately determine the height of 

 the column from the surface of the mercury in the 

 trough. Compare the height with that in a tube of 

 _, different length used by another student, and note 

 / that it is independent of the length of the tube 

 Pjq 2 Convey the apparatus to the top of the building, and 

 note that, if 40 feet above the laboratory, the column 

 of mercury now supported is at least one millimeter shorter. 

 This must be due to the smaller atmospheric pressure at the 

 greater altitude ; and the experiment shows that the mercury 

 in the tube is balanced and supported by the atmosphere, 

 and that its height is an accurate measure of the atmospheric 

 pressure. 



Whenever mercury is used, the experiment must be made on a tray or 

 special table designed to catch falling particles. Without this precaution 



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