Young — Vapour-Pressures, 6fc., of Thirty Pure Substances. 395 



Liquid is then allowed to enter until it nearly fills the tube ; and it is then 

 carefully boiled away from above downwards until the required amount is 

 left. The bulb-tube is then tilted up so that mercury enters and completely 

 fills the remainder of the experimental tube. The mercury should give a 

 clear metallic sound on meeting the liquid, and condensation of vapour 

 should be instantaneous; otherwise air has not been completely removed. 

 The experimental tube, wlien cold, is inserted in the iron pressure-apparatus, 

 great care being taken to prevent the entry of air at the moment when the 

 open end of the tube is plunged under the mercury. 



The experimental tube is heated by means of the jacketing-tube H, fig. 3. 

 A perforated india-rubber stopper is first passed over the experimental tube ; 

 and the jacketing-tube is placed in position. A little mercury is then poured 

 tlirough a long funnel so as to cover the upper surface of the india-rubber 

 and protect it from the action of the pure liquid, the vapour of which is 

 to serve as tlie heating agent. This liquid is introduced through the same 

 long funnel ; and it is afterwards boiled in the bulb wliicli forms part of the 

 jacketing apparatus. If the liquid does not boil at a higher temperature than 

 160°, a water-jacket may be employed, as shown in the diagram, to condense 

 the vapour ; if the boiling-point is higher, it is not difficult to regulate the 

 supply of heat — from the burner below the bulb — so that the vapour shall 

 condense completely in the narrow tube. In this case it is advisable to 

 protect the jacketing-tube from draughts, by placing outside it a wider cylin- 

 drical glass tube. The top of this tube may be loosely closed by asbestos 

 cardboard. 



The jacketing-tube is connected with an air-pump to reduce the pressure, 

 a large air-reservoir to keep the pressure steady, and a gauge to measure it. 

 The temperature of the vapour depends on the pressure under which the 

 liquid is boiling ; it may be kept constant for hours at a time, or it may be 

 rapidly altered to any desired extent within the available limits. If the 

 pressure is greatly reduced, the liquid is liable to boil with bumping, 

 and it is therefore advisable to have a series of pure liquids at hand. 

 For temperatures up to 280° the liquids recommended are carbon 

 disulphide, ethyl alcohol, chlorobenzene, bromobenzene, aniline, quinoline, 

 and bromonaphthalene. 



The temperatures are ascertained by reference to the tables of vapour- 

 pressures,' which are given for intervals of one degree for each liquid within 

 workable limits. 



When the vapour in the jacketing-tube is at the required temperature, 



Ramaay and Young, Trans. Chem. Soc, xlvii., p. 640, 1885 ; Young, ibid., Iv., p. 483, 1889. 



