1893.] the Paraffins and their Monohalogen Derivatives. 103 



The vapour densities of propane and all the halogen compounds 

 were determined at various pressures for the material as it was used 

 in the velocity of sound experiment, thus avoiding to a great extent 

 any error arising from impurity. 



FIG. 1. 



-"D 



The apparatus used for this purpose is shown in fig. 1. Before 

 joining the parts together, the tube CD is calibrated, and after it haw 

 been attached, but before the side tubes are fixed on, the volume of 

 the whole is determined by filling with water and weighing. Then, 

 from the calibration of CD, the volume is known between A and a 

 file mark at K. 



A weighed quantity of the liquefied gas whose vapour density is 

 required is sealed up in a small tube with capillary ends, and intro- 

 duced at D, and, by inclining the apparatus, is made to slide over the 

 bend and rest at M. The end D is then closed with a cork, and the 

 apparatus exhausted through the three-way tap Or, on the completion 

 of which operation mercury is allowed to flow in from the reservoir 

 H, and the tap E is opened. The difference of the levels of the 

 mercury in the two tubes is read with a cathetomcter, and, subtracted 

 from the height of the barometer, gives the pressure of the residual 

 air. On tilting the apparatus, the tube slides over into the wider 

 part, and the end breaks off, allowing the liquid to evaporate. By 

 reading the levels again we get the pressure of the gas, and knowing 

 its weight and volume, we have all the materials required for calcu- 

 lating its specific gravity. 



