Page:Outlines of Physical Chemistry - 1899.djvu/69

 ��determined. The liquid is separated from the paraffin oil of the reservoir by a column of mercury about 10 centi- metres long. The glass tube is fixed with sealing-wax into the metallic junction-piece p which is screwed on to the copper tube t.

By means of the screw v the pressure is raised to a little below the expected critical pressure, and the large glass tube m, which has been slipped over the experiment tube, is heated by means of a Bunsen burner. After some time vapour begins to form, and consequently also the remaining liquid is sepa- rated from the vapour by a meniscus. The heating is continued and in such a way that the upper part (t) of the tube is raised above the critical temperature whilst the lower part (n) remains cool. Under these conditions there is always a part of the tube where, ' the critical temperature obtains. The screw v is then v turned so as to gradually increase the pressure. At the moment when the critical pressure is reached the level of the liquid surface will have moved to that part of the tube which is at the critical temperature ; but at this spot the critical conditions will then be completely established ; there will be confusion between the liquid and vapour, and the meniscus will have disappeared. The pressure at which this disappearance takes place is noted as the critical pressure.

There are several indirect methods of ascertaining the

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