8 OUTLINES OF PHYSICAL CHEMISTRY
trough. The end of this evolution of gas inchoates the moment when a stationary temperature has been established in the whole apparatus. A measuring cylinder filled with water is then inverted over the end of the capillary tube. The tube b is now opened, a known weight of the sub- stance is dropped in, and b at once closed again. The substance which has fallen to the bottom of a volatilises, and displaces the air. The superimposed layers of air are gradually expelled 1 and collected in the measuring cylinder, where finally a volume of air will have collected equal to the volume which would be occupied by the weight of the substance taken in the vapour state at the temperature of the water in the trough. The displacement of the air, as a rule, ceases sufficiently suddenly to indicate the end of the operation. The measuring tube is then transferred to a deep vessel filled with water, and so adjusted that the levels of the water inside and outside the tube are the same, then the volume is read off and the temperature and barometric pressure noted. We are then in possession of all the data (g, v, t f and p) necessary for the calculation. To obtain exact results it is necessary to reduce the barometric pressure to 0° and to deduct from p the tension of aqueous vapour at t°.
Complementary Details and Bemarks
1. To facilitate the introduction of the substance and avoid opening the tube b, the top part of the apparatus is generally somewhat modified.
2. The weight of substance to be taken must be regulated by the consideration that its vapour must not fill more than a third of the bulb a ; that is, it must only occupy about thirty cubic centimetres.
8. According to the degree of volatility of the substance, it is weighed in small open or closed bulbs (of glass or
1 The displaced air, all along its path, is at the same temperature as those parts of the apparatus with which it is momentarily in contact.
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