Page:Outlines of Physical Chemistry - 1899.djvu/152

 ��OUTLINES OF PHYSICAL CHEMISTEY

��THIRD PAET

��I. THERMO-CHEMISTRY

��The Calorie

In physics, the calorie is that quantity of heat which raises the temperature of one gram of water from 0° to 1°.

In practice, it is better to take the calorie as that quantity of heat which is necessary to raise the tempera- ture of one gram of water, at the ordinary laboratory temperature (15° to 18°C), through one degree. As the specific heat of water is not absolutely constant, the new unit differs slightly from that first defined.

Specific heat of water (Bartoli and Stracciati) :

��at 0°

�• • •

�at 20°

�• • •

�„ 15°

�• • •

�,,25°

�• • •

�„ 18°

�* • •

�,, 30°

�• • •

��In works on thermo-chemistry our unit is denoted by the letter c : this is the small calorie. In many oases, use is made of a unit 1,000 times greater — the large calorie (heat necessary to raise 1 kilogram of water through one degree), which is denoted by the capital C. The large calorie agrees better with the exactitude of calorimetric observations than the small calorie. Ostwald, in his work on theoretical chemistry, denotes by k the quantity of heat required to raise 1 gram of water from 0° to 100°. This unit is approximately equal to 100 small calories.

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