THE MECHANICAL EQUIVALENT OF HEAT. 
375 
As it was advisable to free that space from air bubbles, the screw plug P was removed 
before the insertion of the mercury, and replaced by a tube leading to a Geissler’s 
mercury pump. A tube from D dipped into a basin of hot mercury, and in this tube 
was a tap which was at first closed. The air in the annular space was withdrawn 
until the pressure was a small fraction of a millimeter, and the hot mercury was then 
slowly admitted—the exhaust pump being worked at intervals. When the mercury 
became visible within the tube at P, the tube was cautiously removed, and the air¬ 
tight screw plug rej)laced. The mercury necessary to fill the space weighed 70 lbs. 
So true were the surfaces of the two* parts of the metal chamber, which were 
screwed together by the bolts at A and C, that, although no packing except the finest 
possible coating of grease had been placed upon them, they proved absolutely air¬ 
tight. These twm portions of the chamber were each formed from a single block of 
steel and, when placed together, somewhat resembled a double hat-box. The inner 
and outer surfaces were turned smooth and true, and then were nickel-plated. Two 
holes on opposite sides were bored down between the mercury chamber and the inner 
surface, as shown at E and F. These holes were filled with mercury and, their inner 
walls being very thin, the thermometer j^laced in them would accurately indicate the 
temperature of the inner surface of the chamber. This inner surface, as also the base 
of the chamber, was, in our more recent experiments, covered with highly-polished 
speculum metal. A ring of lead wire was placed round the top edge of the chamber, 
on which was laid that portion of the lid shown in section at G and H, and in plan 
at G' and H', and the twelve bolts securing it were screwed home. Many as have 
been our difficulties with regard to air-tight joints, those made in this manner, viz., 
by the crushing of a lead ring between two surfaces, have never given us any anxiety. 
The second lid, from which the calorimeter was suspended, and whose section is shown 
at K L, and plan at K' L, was secured in a similar manner. 
The necessity for this double lid is not, at first sight, obvious, but it must be 
remembered that it was impossible to separate the calorimeter from the portion of the 
lid to which it was attached, and, therefore, a heavy steel plate would have been 
inconvenient, and have rendered any direct weighings impossible. On the other 
hand, the total pressure on this lid, when the chamber was exhausted, was very great, 
and, had the whole of it consisted of one thin piece, it is probable that it would not 
have withstood the strain v/ithout “ panting,” and, as it was necessary on account of 
the stirring rod, &c., that the whole apparatus should be absolutely rigid, it was 
essential to avoid any phenomenon of this kind. The first lid, therefore, is of con¬ 
siderable thickness, and the orifice in it only just sufficient to allow for the intro¬ 
duction of the calorimeter. Again, if the lid had been of one piece, it would have 
been impossible to remove it without disturbing the tubes M and N. 
The tube M communicated with a five-way Sprengel pump and a McLeod gauge; 
the tube N with a Geissler pump and a water pump. The arrangements for drying 
and exhausting by means of these tubes is described in Section V. 
