ELASTIC FORCES OF AQUEOUS VAPOUR. 585 
freely in air; and this temperature can be kept stationary as long 
as is required. The apparatus which I arranged to suit these 
conditions is represented in fig. 6. It consists of a retort of 
copper, A, closed by a cover screwed to it. This cover carries 
four tubes closed at bottom ; two of these tubes descend nearly to 
the bottom of the retort, the other two only reach to the middle. 
These tubes, which are 7 millimetres internal diameter, and 
about 1 millimetre thick, are surrounded by a very thin hollow 
copper cylinder, attached to the lid and pierced in the upper part 
by the holes 0,0,0. The neck of the retort is connected with 
a tube TT’, about 1 metre in length, enclosed within a hollow 
cylinder of copper, through which a constant stream of cold 
water is kept up; this tube is in communication with a copper 
globe B, having a capacity of 24 litres, and enclosed in a vessel, 
VV’, filled with water at the surrounding temperature. The 
globe at its upper part is supplied with a connecting piece having 
two arms; into one of these arms 2s, the tube egh of the ap- 
paratus fig. 2, is cemented when experiments are to be made at 
lower pressures than those of the atmosphere, or the tube pg of 
the apparatus fig. 8, when the experiments are to be conducted at 
higher pressures. The second arm communicates by means of 
the leaden tube ¢?', either with an air-pump for rarefying the 
air in the apparatus, or with a forcing-pump, if the air is to be 
condensed. 
The four iron tubes are filled with mercury within a few cen- 
timetres of their upper end ; mercurial thermometers are placed 
in these tubes; the bulbs of these thermometers descend to the 
bottom of the tubes, two of these thermometers are thus im- 
mersed in water, and two in vapour. 
The air is exhausted in the apparatus, so as to leave but a 
very slight pressure. The water is heated to boiling; the va- 
pour soon condenses in the condensing-tube TT’, and falls again 
into the retort. The pressure at which ebullition takes place is 
measured by the difference in the height of the mercury in the 
tube and the level of the mercury in the barometer. The column 
of mercury sustained in the tube which communicates with the 
globe is never absolutely stationary ; it has nearly always a slight 
oscillatory motion, but this motion only occurs within an ex- 
tremely minute space, not exceeding one-tenth of a millimetre, 
when the fire is properly regulated under the retort. The ob- 
server following the meniscus under the thread of the telescope 
of the cathetometer can easily observe the meniscus when it is 
