of Edinburgh, Session 1879 - 80 . 
535 
fluid less volatile than sulphurous acid, proceed as follows : — Apply 
heat at T and in the bend next T until the liquid leaves that part 
of the enclosure and stands nearly at a level in the short and long 
vertical branch, the instrument being held with A down. Apply a 
freezing mixture to T, taking care not to cool it to quite as low a 
temperature as -11° C. ; so that the pressure of the sulphurous 
acid liquid and steam may remain something above the external 
atmospheric pressure. Occasionally open the stop-cock L very 
slightly to prevent the liquid from being drawn up the short vertical 
branch through preponderance of temperature in the long vertical 
branch. Continue this until about a centimetre df liquid has been 
distilled over into the bulb T. Then open the stop-cock L very 
carefully until Jill the liquid in the two vertical branches is blown 
out, leaving that which has been distilled over into the bulb T, and 
then close L again. 
(6.) Then dip the end E under pure mercury, and by opening L 
very gently add warming the free, surface of the liquid sulphurous 
acid, let gas escape bubbling up through the mercury. Close L 
again before or when the quantity of liquid in the bulb at T begins 
to be perceptibly diminished. Then apply a freezing mixture to T 
until mercury is drawn in. Incline the instrument with A up and 
L down, and watch until the mercury is drawn up to A, then 
incline with A down and let a little mote mercury come in. Then 
close L. Lastly, keeping T still in the freezing mixture, melt the 
glass below L till it collapses and blows the nlercury down, leaving 
Torricellian vacuum at the sealed end. The instrument is now 
complete and ready for use. 
Sulphurous Acid Stedm- Pressure Differential Thermometer. 
This consists of a XT tube, with its ends bent down, as shown in 
the drawing, containing mercury in the main bend and in the 
lower parts of the straight vertical branches, and sulphurous acid 
gas, steam, and liquid in the rest of the enclosure. Every other 
part of the enclosure must be kept somewhat warmer than the 
warmer of the two ends, T, T'. 
The infinitesimal quantities of matter in the transitional layers, 
between liquid and steam, at T and T', constitute the thermometric 
