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MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
produce a particular phase, and in the course of the experiment the exhaustion has 
been carried beyond that degree which permits of the production of that phase, one or 
more charges of gas may be successively admitted into the tube by filling the calibrated 
chamber with gas at any particular pressure, and then opening the stop-cock 
communicating with the tube ; the lost phase is thus reproduced. 
The apparatus which we have found it advantageous to adopt for the exhaustion of 
our tubes is shown in fig. 35 ; it comprises three means of exhaustion which are 
successively employed as the vacuum becomes more perfect. The first is an Alvergniat 
Fig. 35. 
high-pressure water trompe in connexion with the high-pressure water-main of the 
West Middlesex Water Company, the head of water being 106 feet; it produces 
a vacuum to within half-an-inch (0’47 in. = 12 millims.) of the height of the barometer. 
The pipe leading to it is so marked in the drawing ; it is attached, through a cock, 
to a four- way-junction-piece F, provided with three more cocks, communicating : — 
one to one end of the tube T, one to the last drying bottle of the gas generator G G/ 
and one to a mercurial gauge. The other end of the vacuum tube T communicates by 
means of a Y-piece to both, an Alvergniat mercurial pump, on the right of the figure, 
and a Sprengel pump, on the left. After the trompe has done its work, the Alvergniat 
is used for rapid exhaustion, and then shut off by means of the glass cock C, 
