Dalton's Experiment on the Pressure of Mixed Oases.

To show that the Maximum Pressure produced by a Vapour in a given Space depends on the Temperature and not on the Presence of Air or other Vapours in that Space.

The apparatus and experiment are described in Garnett's 'Heat', §144.

A, B, G, fig. 21, are three barometer tubes. A and B are to be filled with mercury and inverted over the cistern of mercury D E. G contains some air above the mercury.

We require, first, to explain how to fill the tubes with mercury.

They must first be cleaned by washing out with dilute acid, and then dried by being repeatedly exhausted with the air-pump and filled with air that has passed through chloride of calcium tubes. This can be done by means of a three-way cock, as already described (§16). Having cleaned and dried a tube, we may proceed to fill it.

For this purpose it is connected with a double-necked receiver which contains enough mercury to fill the tube, the other neck of the receiver being connected with the air-pump, and the tube and receiver are exhausted by working the air-pump. Then by raising the end of the tube to which the receiver is attached and tilting the receiver the mercury is allowed to flow into the empty tube from the receiver. We are thus able to fill the tube with mercury free from air without its being necessary to boil the mercury.

The three tubes should be filled in this way and inverted over the mercury cistern. A convenient arrangement for the latter is a hemispherical iron basin screwed on to the end of a piece of iron tubing, the lower end of the tubing being closed.

Connect the open end of G by means of a bent piece of small-sized glass tubing with the drying tubes, and allow a small quantity of dry air to flow in. The amount of air introduced should be such as to cause the mercury in G to rise to about half the height that it reaches in A and B. The quantity can be regulated by pinching the india-rubber tube which connects G with the drying tubes.

Adjust in a vertical position behind the three tubes a scale of millimetres, and hang up close to them a thermometer. Place a telescope at some distance off, so as to read on the millimetre scale the height at which the mercury columns stand and also the thermometer. The tube G should be so placed that it can be depressed into the iron tubing below the cistern.

Mark the height at which the mercury stands in G by means of a piece of gummed paper fastened on round the tube.

Read on the millimetre scale the heights of A, B, and G, above the level of the mercury in the cistern.

Suppose the readings are

A = 765
B = 765
G = 524

The mercury in B will fall. The amount of fall will depend on the temperature. Let us suppose that the new reading in B is 354 mm, then the mercury has fallen through 765-354 mm; thus the ether exerts a pressure equivalent to that of 411 mm. of mercury.

The mercury in G will fall also, but not by so much as that in B, for the pressure in G is the pressure of the ether vapour together with that of the contained air; and as the mercury falls, the volume of the contained air increases and its pressure consequently decreases.⁽¹⁾

Now lower the tube G in the cistern until the level of the mercury in G just comes back again to the paper mark. The volume of the contained air is now the same as before, therefore so also is its pressure. The depression of the mercury column in G below its original height is due therefore to the pressure of the ether vapour. Now read the height of G on the scale; it will be found to be about 113 mm. The column in G, therefore, has been depressed through 524-113 mm, or 411 mm. Thus B and G are depressed through equal amounts provided that the volume of air in G is allowed to remain the same.

The assumption has been made that the temperature remains constant during the experiment. This will not be far from the truth in the laboratory, provided that the readings are taken from a distance so as to avoid the heating effects of the body; if necessary, a correction must be applied for a change in temperature.

Having made these measurements, depress B into the iron tube; it will be found that the consequence is simply to increase the amount of condensed liquid above the surface of B without altering the height of that surface.

The difference between the heights of the columns in A and B gives in millimetres of mercury the maximum pressure which can be exerted by ether vapour at the temperature of the laboratory.

Experiment. Determine the maximum pressure exerted by the vapour of ether at the temperature of the laboratory, and show that it is independent of the presence of air.

Enter results thus:

Height of mercury in A: 765 mm
Height of mercury in B:
     initially                    765 mm.
     after introduction of ether: 354 mm
Pressure of ether vapour: 411 mm
Height of mercury in G:
     initially                    524 mm
     after introduction of ether  113 mm
Pressure of ether vapour: 411 mm
Temperature 15.5°C throughout











(1)
The presence of the air in G retards the evaporation of the ether; considerable time must therefore be allowed for the mercury to arrive at its final level.