302 



PRINCIPLES OF CHEMISTRY 



Such an investigation (either direct, or by calculation from the densi- 

 ties and composition) of every chemical reaction, resulting in the forma- 



with f). In certain cases the separation of one form of deviation from the other maybe 

 explained by special hypotheses. 



With respect to the means of determining p and v, with a view to finding the vapour 

 density, we may distinguish three chief methods : (a) by weight, by ascertaining the 

 weight of a definite volume of vapour ; (b) by volume, by measuring the volume occupied 

 by the vapour of a definite weight of a substance ; and (c) by displacement. The last* 



PIG. 52. Apparatus for determining the vapour density 

 by Dumas' method. A small quantity of the liquid 

 whose vapour density is to be determined is placed in 

 the glass globe, and headed in a water or oil bath to a 

 temperature above the boiling point of the liquid. 

 When all the liquid has been converted into vapour 

 and has displaced all the air from the glone, the latter 

 is sealed up and weighed. The capacity of the globe is 

 then measured, and in this manner the volume occu- 

 pied by a known weight of vapour at a known tem- 

 perature is determined. 



FlG, 53. Devil!e and Troost's apparatus 

 for determining the vapour densities, 

 according to Dumas' method, of sub- 

 stances which boil at. high temperatures. 

 A porcelain globe containing the sub. 

 stance whose vapour density is to be 

 determined is heated in the vapour of 

 mercury (350), sulphur (410), cad- 

 mium (850), or zinc (1,040). The globe 

 Is sealed up in an o.tyhydrogen flame. 



mentioned is essentially volumetric, because a known weight of a substance is taken, 

 and the volume of the air displaced by the vapour at a given t and h is determined. 



The method by weight (a) is the most trustworthy and historically important. Dumas' 

 method is typical. An ordinary spherical glass or porcelain vessel, like those shown 

 respectively in figs. 62 and 68, is taken, and an excess of the substance to be experimented 

 upon is introduced into it. The vessel is heated to a temperature t higher than the boil- 

 ing point of the liquid : this gives a vapour which displaces the air, and fills the sphe- 

 rical space. When the air and vapour cease escaping from the sphere, it is fused up or 

 closed by some means ; and when cool, the weight of the vapour remaining in the sphere 

 is determined (either by direct weighing of the vessel with the vapour and introducing 

 the necessary corrections for" the weight of the air and of the vapour itself, or the 

 weight of the volatilised substance is determined by chemical methods), and the volume 

 of the vapour at t and the barometric pressure h are then calculated. 



The volumetric method (6) originally employed by Gay-Lussac and then modified by 

 Hofmann and others is based on the principle that a weighed quantity of the liquid to 

 be experimented with (placed in o. small closed vessel, which is sometimes fused up before 



