In order to diminish radiation as much as possible, the surface of the apparatus should be polished. The instru- ment is set in a polished metallic vessel, and this is supported on badly conducting material in a vessel, which last is set in a bath of water. The outermost vessel is covered with felt. In order to prevent as much as possible loss of beat by evaporation of the water in the calorimeter, the instrument is provided with a cover with holes for the in- sertion of the thermometer and of a stirrer. This is the Berthelot apparatus.
Julius Thomaen uses a sim- pler arrangement. His calori- ^ metric vessel is surrounded by .. only a single shield made of cardboard or of vulcanite. i However, he takes extraor- i tf dinary precautions to keep the
l", as near 18°C. as possible. ' The calorimeter is preferably made of platinum. This ^ metal resists the action of most chemical reagents, and ■ < besides it possesses the advantage of having a very low 1 y specific heat. Calorimeters made of nickel, brass, or glass cA are also much used. Glass is more or less attacked by liquids, and above all has the disadvantage of a high specific heat. Specific heats :
'a. Calorimeters are usually provided with a mechanical jirtirrer, which serves to intimately mix the substances ' ^taking part in the action.
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