CHANGE OF STATE LIQUID VAPOUR. 



177 



the weight of the bulb empty. Subtracting this from the weight in the 

 second weighing, we have the weight of the contained vapour at the 

 temperature of the bath and the atmospheric pressure. Finding the ratio 

 of this to the weight of the same volume of air at the same temperature 

 and pressure, we have the vapour-density. 



In Gay-Lussac's method as modified by Hoffmann, a graduated 

 barometer is enclosed in a wider tube (Fig. 101) through which circulates 

 steam or other suitable vapour from a boiler, the barometer thus being 

 maintained at a known temperature. A very small stoppered bottle is 

 then quite filled with the liquid, and 

 weighed, so that the weight of the liquid 

 used is known. This is floated up through 

 the mercury column, and on arriving in 

 the Torricellian vacuum, the stopper is 

 forced out, and the liquid evaporates. 

 Such a quantity of liquid is taken, and 

 such a temperature is maintained, that 

 the whole of the liquid evaporates, its 

 pressure, of course, depressing the mercury 

 column. The volume occupied by the 

 vapour is observed, and its pressure is 

 given by the difference between the 

 original and the new height of the baro- 

 meter column. Hence we have the data 

 for determining the vapour-density. 



Two methods have been devised by 

 Victor Meyer. In that most commonly 

 used, which we shall describe, a known 

 weight of liquid is introduced in a small 

 stoppered bottle into a heated flask 

 containing air. The vapour formed dis- 

 places its own volume of air, and this is 

 collected and measured. The method is 

 applicable to vapours denser than air. 

 One form is shown in Fig. 102. 



The flask A contains air, and is heated 

 in a constant -temperature bath. The 

 neck rises above the bath, and a narrow 

 side-tube leads down to the trough B, where the expelled air may be 

 collected. The neck is closed by the slanting hollow stopper 0, in which 

 is placed the small bottle containing the liquid. When the temperature of 

 the different parts of the apparatus is steady, C is turned round and the 

 bottle falls down to the bottom of the flask on to some asbestos packing, 

 put to prevent breakage. The stopper is forced out, and evaporation 

 takes place, the vapour driving the air upwards. If the displaced air is 

 raised sufficiently slowly, as each layer rises it takes the temperature and, 

 therefore, the volume of that which it displaces. Hence the weight of 

 the air forced out and collected at B is equal to that of the air displaced 

 by the vapour in A. The vapour-density, as compared with that of air 

 at the same temperature and pressure is, therefore, the weight of liquid 

 introduced divided by the weight of air expelled. 



3. 102. Victor Meyer's Method 

 for Vapour-Density Determi- 

 nation. 



