CHAPTER IV 



THE MEASUREMENT OF GASES. THE MOLECULAR 

 HYPOTHESIS 



AFTER the discussion in regard to the proportions of oxygen 

 in the air and the measurement of the volume of oxygen removed 

 (Fig. 15, p. 26), it will readily be imagined that measuring the 

 volume of a sample of gas is a common operation in chemistry. 

 Indeed, it is much easier to measure quantities of gas by noting 

 their volumes than by weighing them. Some facts have to be 

 taken account of, however, in order that the measurements of the 

 volume may be of value. 



A sample of gas diminishes in volume when the pressure in- 

 creases, and it increases in volume when the temperature rises. 

 Hence, when the volume of the gas is measured, the pressure 

 and the temperature must also be stated. 



Measurement of Pressure. In order that the pressure may 

 be easily measured, we arrange the sample of gas so that its pres- 

 sure is the same as that of the atmosphere at the 

 moment. To do this, if, for example, the gas is con- 

 tained in the narrow tube (Fig. 23), we bring the water 

 inside the tube to the same level as that outside by lower- 

 ing the tube. Then we read the volume by means of the 

 graduation (not shown) on the tube and at the same time 

 we ascertain the atmospheric pressure from the height of 

 the barometer. 



FIG. 23 A smi pl e form of barometer is shown in Fig. 33, p. 61 

 (tube on left of diagram only). A glass tube, about 1 

 meter long and closed at one end, is completely filled with mercury 

 and carefully inverted, with its open end dipping into a mercury 

 reservoir, The mercury falls inside the tube, leaving a vacuum 



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