The Gay-Lussac Method of Vapor Density Measurement
Measurement of the Weights of Vapors at a Given Volume and at Determined
Pressure and Temperature.
J. B. Biot, Traite de Physique Experimentale et Mathematiqu,e Vol. I, pg. 291, (1816)
(translated by Amelie Berube)
From the experiments reported in the previous chapter we were able to determine that a very
small amount of liquid was sufficient to give a considerable volume of vapor. Several
physical and chemical studies required the knowledge of the volume of this expansion; in
other words, that we could determine the volume of vapor that could be given by a given
weight or by a given volume of each liquid. But, this determination seemed quite difficult
because the expansion of the vapor was substantial.
Unfortunately, it was not possible to recondense in one mass the liquid used to generate it.
Fortunately, M. Gay-Lussac eluded this difficulty by reversing it, in other word, by
determining the volume of vapor that can be produced by a given volume of liquid. you first
have to determine the exact volume of liquid used, which is the real difficuly. M. Gay-Lussac
blows small glass bubbles (BB, Fig. 83) [Note: Fig. 83 nor the bubbles B were designated in
Appendix III. They appear in the upper righthand corner of Fig. 84 to the right.] They are
nearly spherical , but one side has an elongated spout. We first weigh one of the little
bubbles when it is filled with air; we then introduce the liquid the same way we would for a
thermometer by emerging it in the liquid after warming it up to release most of the air. When
the small bubble is mostly full, the spout is sealed with a torch. This procedure does not
change the composition of the glass but simply the shape of the bubble. We then weigh the
filled bubble again and by subtracting the weight of the glass shell previously weighed, we
obtain the weight of the liquid that it contains. We will soon see how we can deduce its
volume. To now transform all this liquid into vapor, M. Gay-Lussac uses an apparatus similar
to the one M. Dalton used to observe the tension of vapors under vacuum. He uses a long
narrow glass dome (VV, Fig. 84)., which is divided in sections of equal volume and the total
volume is about 1.5 liters. He fills it with mercury and puts it upside-down and then he
introduces the small bubble filled with liquid (B). The bubble reaches the top of the tube. We
now vaporize it. To do so, M. Gay-Lussac covers the dome with a glass envelope (MM)
longer than the tube so its lower portion is plunged into the mercury. He fills the cylinder with
water and the dome is then covered. The apparatus is placed on on a stove (FF) and a fire is
lighted up. as the water and mercury are warmed, the liquid in the bubble is also warmmed.
The liquid in the the bubble expands and breaks the glass shell, spreads at the top of the dome
and soon becomes vapor. The temperature is increased until the water in the cylinder is
boiling. [Note: Water is applicable for liquids that boil below 100 oC.] We then measure the
height of the mercury column that remains in the dome above the exterior level. this is how m.
Gay-Lussac operates to be as accurate as possible. The sides of the shallow basin (vv) are
made of cast-iron are straight and leveled horizontally. He sets on the side a ruler made of
copper (cc) with a garaduated rod (T) set vertically so its tip barely touches the surface of the
mercury outside. A marker (H) that goes up and down along the vertical rod is set at the same
level as the mercury inside the dome. The elevation of mercury can the be measured precisely.
The difference (between the height of the mercury column and the atmospheric pressure
measured in the same units) represents the elastic force of the vapor contained in the dome. In
other words, the pressure that it sustains. We know the volume of this vapor by the number of
sections that it occupies in the dome. With these values we can calculate the ratios of volumes
for the liquid and the vapor at a given temperature and pressure.