

Temperatures above the Boiling-point of Mercury. 427 



support A A, in order to bring the column to a definite state of 

 equilibrium. When all oscillation has ceased, the number of 

 millimetres and fractions of millimetres corresponding to the 

 height is read off upon the graduated scale. This is the zero- 

 point under the barometric pressure at the time of the experiment. 



The point 100 is then similarly determined by placing the 

 reservoir B in a flask in which distilled water is made to boil. 



The point 440 is next determined. The air- reservoir B is 

 placed in a tubulated retort containing sulphur, which has pre- 

 viously been fused and allowed to solidify ; the tubulure is closed 

 by a cork pierced to admit the stem 1 1, and cut down the middle 

 so as to fit closely round it. The retort is firmly fixed upon a 

 stand and the air-reservoir placed within it, the capillary stem 

 being held in the centre of the tubulure. The two halves of the 

 cork are then carefully pushed down between the stem and the 

 tubulure in such a manner as to close the opening. The beak 

 of the retort is next placed in a receiver, and the sulphur heated 

 by a large gas-lamp until it perfectly boils and distils in a conti- 

 nuous stream. The air-reservoir ought to be fixed partly in the 

 boiling sulphur and partly in its vapour (see fig. 1). 



The point 350 is then determined by means of boiling mer- 

 cury. This operation is performed exactly as in the determina- 

 tion of the boiling-point of sulphur, but with the following extra 

 precautions. The air-reservoir B dips partly under the surface 

 of the mercury : it must of course be firmly fixed, in order that 

 it may not be jerked out of position by the upward pressure of 

 the metal. The tubulure of the retort must also be more care- 

 fully closed, to avoid the introduction of mercurial fumes into the 

 atmosphere of the laboratory. The upper surface of the cork is 

 therefore to be covered with a luting of pipeclay. Above all, the 

 ebullition must be very carefully conducted, — a needless caution, 

 perhaps, to all who have had occasion to distil mercury. 



After this operation it is absolutely necessary to verify the 

 capacity of the air-reservoir B, in order to ascertain if it has un- 

 dergone any variation by exposure to the temperatures of 350° 

 and 440°. This is easily done by redetermining the zero- and 

 100°-points : the variation ought to be nil, or very slight. If, 

 however, their position has appreciably changed, it will be ne- 

 cessary to repeat the experiments. 



The four fixed points 0, 100, 350, 440 have thus been deter- 

 mined. It may appear at first sight that these points answer 

 only to a certain atmospheric pressure, since the height of the 

 mercury in our instrument will evidently be affected by varia- 

 tions in atmospheric pressure in the same way as the baro- 

 meter. But it will easily be seen that these oscillations do not 

 affect the length of the spaces between the fixed points when 



