PROPERTIES OF GASES 



67 



that the mercury stands at the same level in the closed and open tubes. 

 The imprisoned air is then at the same pressure as the air outside. 



If the closed tube is uniform in bore, and the inside of the sealed end is 

 nearly flat, the volume of the imprisoned air is proportional to the length 

 of tube occupied by the air, so that if the air is made to occupy one-half 

 the original length of tube its volume is one-half the original volume. The 

 pressure upon the enclosed air is equal to the pressure due to the column 

 of mercury between the level of mercury in the closed tube and that in the 

 open tube, plus the pressure of the atmosphere. Observe the height of 

 the barometer, and make the difference of level of the mercury in the two 

 tubes equal to it by lifting the open tube. Then record as follows : 



Height of the barometer, 



Length of air column when the mercury is at the 

 same level in both tubes, that is, when the im- 

 prisoned air is at atmospheric pressure, 



Height of barometer cm. + equal height in tube, 



Length of air column under pressure of two 



atmospheres, cm. 



The pressure to which the enclosed air is subjected in the second case is 

 double that in the first case ; find the amount by which the volume of the 

 air, represented by the length of the air column, has been diminished. 



(6) Lower the open tube until the air in the closed tube almost reaches the 

 india-rubber junction. Measure the length of the air column and the 

 difference of level of the mercury in the two tubes. Repeat the experiment 

 by reading the volume of air and the head of mercury at every few centi- 

 metres up to the highest point you can raise the open tube. Record your 

 results as indicated below : 



.... cm. 

 .... cm. 



ii. A simple form of Boyle's Law apparatus. Procure a length of thermo- 

 meter tubing, AB (Fig. 54), about 75 cm. long and 1 mm. bore. Seal it 

 at B and expand the end A somewhat. Clamp AB in a vertical position 

 by the side of a metre scale, and connect a small funnel to A by means of 

 a short piece of rubber tubing. Pour a little pure, clean mercury into the 

 funnel and induce it to run down the bore of the tube by inserting a thin, 

 clean, steel wire. In this way any desired volume of air can be enclosed. 



The length of the column of enclosed air may be taken to represent its 

 volume (V). If #=the height of the barometer, and & = the length of the 

 mercury thread (both expressed in the same units), then the total pressure 

 on the enclosed a,ir = (H +h). 



Introduce more mercury in the same manner, and in this way alter the 

 values of V and h. The volume of the air under the pressure of the atmo- 

 sphere alone can be observed by laying the glass tube flat on the table. 



