ANALYSIS OF COMPLEX MATTER 133 



87. Measurement of the Mass of a Litre of Air. A round- 

 bottomed flask, of about half a litre capacity, has a caoutchouc 

 cork carefully fitted, and its position marked on the neck of 

 the flask. This cork is provided with a short glass tube, pass- 

 ing just bo the bottom of the cork, and continued above in a 

 short caoutchouc tube on which is fixed a clip. The capacity 

 of the flask to the mark, together with that of the short tube, 

 is taken by measuring in a graduated vessel the water which 

 fills them. A small quantity of water is then poured into the . 

 flask, and the cork carefully fitted to the mark ; the clip is 

 opened, and the water carefully boiled for a few minutes, until 

 the steam formed has swept out all the air. The clip is closed, 

 and the flame simultaneously removed. The flask with water, 

 &c., is now weighed. Then the clip is opened, and the flask 

 allowed to cool to the temperature of the room, and then again 

 weighed. The increase of weight shows the mass of air enter- 

 ing. The volume of air entering is given by subtracting the 

 volume of water still remaining in the flask from the total 

 volume originally ascertained. 



The mass of a given volume of air may also be measured by 

 the following process : A round- bottomed flask, fitted as in 

 the previous determination, is weighed together with the air it 

 contains. It is then exhausted as completely as possible by an 

 air-pump, filter-pump, or the mouth, the clip closed, and the 

 mass again taken. The mass now found represents the flask 

 and a small remnant of air. By opening the clip under water, 

 allowing the water to enter, adjusting the water-level inside 

 to that outside, and closing the clip, the volume of the ex- 

 hausted air is easily determined. 



Neither of the above determinations, however, do more 

 than give the mass of a certain volume of air, at a certain 

 temperature and pressure, mixed with more or less water- 

 vapour. The density of the air varies with temperature and 

 pressure in a manner which has been shown. The thermo- 

 meter and barometer readings will indicate the corrections to 

 be made. The quantity of water-vapour present has been 

 shown to depend upon the temperature. By consulting the table 

 on pagellO, the proportion of water- vapour present at the time 



