Page:Elementary Text-book of Physics (Anthony, 1897).djvu/209

§ 168] Bunsen's ice calorimeter (Fig. 66) is used for determining the specific heats of substances of which only a small quantity is at hand. The apparatus is entirely of glass. The tube $$B$$ is filled with water and mercury, the latter extending into the graduated capillary tube $$C.$$ To use the apparatus, alcohol which has been artificially cooled to a temperature below zero is passed through the tube $$A.$$ A layer of ice forms around the outside of this tube. As water freezes, it expands. This causes the mercury to advance in the capillary tube $$C.$$ When a sufficient quantity of ice has been formed, the alcohol is removed from $$A,$$ the apparatus is surrounded by melting snow or ice, and a small quantity of water is introduced, which soon falls in temperature to zero. The position of the mercury in $$C$$ is now noted; and the substance the specific heat of which is to be determined, at the temperature of the surrounding air, is dropped into the water in $$A.$$ Its temperature quickly falls to zero, and the heat which it loses is entirely employed in melting the ice which surrounds the tube $$A.$$ As the ice melts, the mercury in the tube $$C$$ retreats. The change of position is an indication of the quantity of ice melted, and the quantity of ice melted measures the heat given up by the substance. The number of divisions of the tube $$C$$ corresponding to one calorie can be determined by direct experiment.

168. Method of Mixtures.—The method of mixtures consists in bringing together, at different temperatures, the substance of which the specific heat is desired and another of which the specific heat is known, and noting the change of temperature which each undergoes.

The water calorimeter consists of a vessel of very thin copper or brass, highly polished, and placed within another vessel upon non-conducting supports. A mass $$P$$ of the substance of which the specific heat is to be determined is brought to a temperature $$t'$$ in a suitable bath, then plunged in water at a temperature $$t,$$