Minor Constituents of Air 239 



Experiment 99, plot the values obtained showing the 

 relation between these average differences of tempera- 

 ture and the time, using the scale of time adopted in 

 the previous experiment. The sort of weather experi- 

 enced between each pair of consecutive readings should 

 also be noted, i.e. whether fine, cloudy, wet, foggy, or 

 otherwise. 



Exercise for Student. 



Try to trace, and to account for, any relation between the average 

 temperature differences shown by the hygrometer and the loss of 

 weight sustained by the water during the same interval. 



162. Tables have been constructed by reference to 

 which it is possible to obtain, for any given pair of 

 simultaneous readings of the wet and dry bulb ther- 

 mometers, an idea of the actual percentage of water- 

 vapour present in the atmosphere in the neighbourhood 

 of the instrument at the time of observation. As it 

 is unlikely that the student will have access to such a 

 set of tables, we will next describe a chemical method 

 of determining the weight of water-vapour present in a 

 given quantity of air. 



EXPERIMENT 101. Fit up the apparatus shown in 

 Fig. 77. Before commencing the experiment, weigh 

 each U-tube with its contents separately, and then 

 quickty fix them in position. See that all joints and 

 corks fit closely. 



Turn the stopcock so that a slow stream of water 

 is produced. Continue this till at least 1 litres of water 

 have run from the aspirator. Catch the water as it 

 leaves the tap in a large measuring vessel. When the 

 latter is filled to its topmost graduation, turn off the 

 tap, quickly pour out the water from the measuring 



