MOLECULES AND ATOMS 



303 



tion of definite chemical compounds, shows that the volumes of the react- 

 ing substances in a gaseous or vaporous state are either equal or are in 



FlG. 54. Hofmann's apparatus for 

 determining vapour densities. The 

 internal tube, about one metre long, 

 which is calibrated and graduated, 

 is filled with mercury and inverted 

 in a mercury bath. A. small bottle 

 (depicted in its natural size on the 

 left) containing a weighed quantity 

 of the liquid whose vapour density is 

 to be determined, is introduced into 

 the Torricellian vacuum. Steam, 

 or the vapour of amyl alcohol, &c., 

 is passed through the outer tube, 

 and heats the internal tube to the 

 temperature (, at which the volume 

 of vapour is measured. 





; -A3SESTW 



FiO. 55. Victor Meyer's apparatus fop 

 determining vapour densities. The 

 tube b is heated in the vapour of 

 a liquid of constant boiling point. 

 A glass tube, containing the liquid 

 to be experimented upon, is caused 

 to fall from d. The air displaced is 

 collected in the cylinder e, in the 

 trough / 



weighing, and, if quite full of the liquid, breaks when heated m a vacuum) is intro- 

 duced into a graduated cylinder heated to t, or simply into a Torricellian vacuum, as 

 shown in fig. 54, and the number of volumes occupied by the vapour noted when the 

 space holding it is heated to the desired temperature t . 



The method of displacement (c) proposed by Victor Meyer is based on the fact that a 

 space 6 is heated to a constant temperature t (by the surrounding vapours of a liquid of 

 constant boiling point), and the air (or other gas enclosed in this space) is allowed to 

 attain this temperature, and when it has done so a glass bulb containing a weighed quan- 



