MOLECULES AND ATOMS 



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

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



the fact may servo as a guiding string for the investigation of the chemical changes 

 which are undergone by the substance in a state of vapour, or, at least, as an indi- 

 cation of a deviation from the laws of Boyle, Mariotte, and Gay-Lussac (for the expansion 

 of gases from t). In certain cases the separation of one form of deviation from the other 

 may be 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 



FIG. 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 heated 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 globe, 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. 



FIG. 53. Deville and Troost's apparatus for 

 determining the vapour densities, accord- 

 in g to Dumas' method, of substances 

 which boil at high temperatures. A porce- 

 lain globe containing the substance whose 

 vapour density is to be determined is 

 heated in the vapour of mercury (350), 

 sulphur (410), cadmium (850), or zinc 

 (1040). The globe is sealed up in an 

 oxyhydrogen flame. 



weight of a definite volume of vapour ; (6) by volume, by measuring the volume occupied 

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

 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. 52 and 53, 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 witk 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 is then calculated. 



