THERMO Spoken Here! ~ J. Pohl © ~ 2017 (B6800-107)

Victor Regnault

This careful and meticulous scientist (1810 - 1878) was one of the first to measure the density of atmospheric air. A master glass blower, Victor crafted two near identical flasks; one being a test flask, the other being a "counterpoise." Both flasks were filled with dry (standard) atmospheric air. Next, an air pump was used to extract air from the test flask until the inside air pressure became 560 mm Hg barometric. Both flasks were placed on a beam balance. They came to balance once 1.24 grams was added to the test flask pan.

Use the information of the sketch. Repeat Regnault's calculation of the standard density of air.

Precisely what system Regnault used is made clear by what he placed on the left pan and right pan for each step of his procedure.

(1) mair,1 + mflask = mair,1 + mflask
The Left flask is removed from the pan, air is extracted from it and it is placed again on the left pan. Next mass is added to the Left pan to bring the system in balance. The resulting balance is (1).
(2) mair,2 + mflask + 1.24g = mair,1 + mflask

Next we write the difference (1) - (2) of the steps. Then change the masses to density times volume.

(1) 1

Rearrange to get ρatm alone:

(2) 2

By use of the Ideal Gas equation we have two facts and a conclusion from those facts:

(3) 3

Applying conclusion (3) to equation (2) yields:

(4) 4

Of course, Regnault performed a great many measurements. Convenient numbers for the standard density of atmospheric air are: ρatm = 1.2 kg/m3 (or 0.075 lbm/ft3).

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