214 



Domestic Science 



Fig. 67. 



cylinder by noting the reading at the water-level inside 

 the jar. Allow the apparatus 

 to stand for a day and again 

 determine the length of the 

 air-column. A further reading 

 after another interval of five 

 or six hours may be made, 

 when it will probably be found 

 to agree fairly closely with the 

 one last obtained. To ascer- 

 tain the nature of the gas still 

 contained in the gas- jar remove 

 the bullet and any remain- 

 ing phosphorus by raising the 

 jar and carefully withdrawing 

 them, keeping the mouth of the jar under water. Slip 

 a glass plate under the mouth of the jar without removing 

 the mouth from the water, press the plate firmly on the 

 j ar, remove from the trough and invert. Test the gas left 

 in the jar by inserting a lighted taper and a piece of 

 burning phosphorus (as in Experiment 78). It will be 

 found that it does not support combustion. 



If we assume, as may be usually done without 

 serious error, that the area of cross section of the 

 cylinder is the same throughout its length, then the 

 volumes of gas enclosed in the cylinder are directly 

 proportional to the lengths of the portions of the 

 cylinder occupied by them. 



Our first reading, then, is a measure of the volume of 

 the air in the cylinder, and our second of that part of 

 the air which does not support combustion. By sub- 

 tracting the latter from the total volume of air, we 

 obtain a measure of the volume of the part which dis- 

 appears during combustion. It will be found that the 



