48 OUTLINES OF PHYSICAL CHEMISTRY
apparatus from 4 to 6 centimetres long. 1 The liquid is then boiled and, while the vapour is still issuing, the capillary tube is sealed.
The tube may be filled to two-thirds with the liquid to be examined, leaving the remaining third filled with the vapour of the substance. When the critical temperature is approached, the meniscus of the liquid should be about the middle of the tube.
To determine the critical temperature, the tube is placed alongside a thermometer in a bath of paraffin or in an air-bath with treble metallic walls, and glass or mica observation-windows. By means of a Bunsen burner the temperature is gradually raised. As the critical point is approached, the meniscus of the liquid is seen to lose its sharpness ; at the critical point, the meniscus disappears completely and peculiar iridescent flashes generally become visible in the tube. If the temperature be raised slightly above the critical temperature, there remains only gas in the tube.
If the tube is now allowed to cool, the iridescent flashes again make their appearance at the critical point, and at a certain moment one can distinguish in the lower part of the tube some liquid with a sharply defined meniscus.
A very simple method for the determination of the obitical pbessure has been devised by Altschul in Ostwald's laboratory at Leipsic.
The cylinder c is filled with paraffin oil, and by means of the screw v the pressure within may be varied. Two capillary copper tubes branch off from this reservoir ; one of these communicates with a spring manometer filled with oil, and the other with a strong glass tube about 40 centimetres long and 1 to 2 millimetres internal diameter. In Fig. 4 the tube is shown as containing in its upper part the liquid whose critical pressure is to be
1 If the liquid be very volatile it is preferable to construct the apparatus completely, then fill it.
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