68 THE NATURE AND [SECT. n. 



At the commencement of the experiment, after the liquid, well freed from air, 

 has been let up, the quicksilver is made to coincide with the edge of the ring /, 

 by cautiously pouring mercury in a slender stream into the open leg of the syphon 

 D. The level ring at L is then carefully adjusted. 



From the mode of conducting the experiments, there remained always a quantity 

 of liquid in contact with the vapour, a circumstance essential to accuracy in this 

 research. Suppose the temperature of the water, or the oil in A (fig. 11.) to be 

 32, as denoted by a delicate thermometer, or by the liquefaction of ice ; and let 

 L D be a column equal to the atmospheric pressure ; communicate heat to the 

 cylinder A, by means of two Argand flames, playing gently against its shoulder 

 at each side. When the thermometer indicates 42, modify the flames, or remove 

 them, so as to maintain an uniform temperature for a few minutes. Then the 

 elasticity will be faithfully represented and measured, by the mercurial column 

 which must be added over D, in order to return the quicksilver to the line /, its 

 zero or initial level. 



At E a piece of cork is fixed, between the parallel legs of the syphon, to sustain 

 it, and to serve as a point by which the whole is steadily suspended. 



For temperatures above the boiling point, the part of the syphon under E is evi- 

 dently superfluous, merely containing in its two legs a useless weight of equipoised 

 mercury. Accordingly for high heats, either the apparatus fig. 12 or 13 is 

 employed, and the same method of procedure is adopted. The aperture at 

 (fig. 13) admits the bulb of the thermometer, which rests against the tube. The 

 recurved part of the tube is filled with mercury, and then a little liquid is passed 

 through it to the sealed end. Heat is applied by an Argand flame to the bottom 

 of c, which is filled with oil or water, and the temperature is kept steadily at 212 

 for some minutes. Then a few drops of quicksilver may require to be added at 

 D, till L and / be in the same horizontal plane. The further conduct of the 

 experiment differs in no respect from what has already been described. The liquid 

 in c is progressively heated, and at each stage mercury is progressively added 

 over L to restore the initial level, or volume at /, by equipoising the progressive 

 elasticity. The column above L being the accession of elastic force. When this 

 column is desired to be extended very high, the vertical tube requires to be placed 

 for support in the groove of a long wooden prism. 



