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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



sists of two concentric boxes of sheet metal which are fastened together so that 

 there is space of about one and a half inches between them filled with water 

 (Fig. 142). The outer box is fifteen inches in height and width, and eighteen 



CT 



FIG. 142. Reichert's water calorimeter. 



inches in length. An opening (A) nine inches in diameter is made in one end 

 for the entrance and exit of the animal. It is also perforated with three small 

 holes in the top corners, and a slit-like opening in the top on one side. Two of 

 the holes are for the tubes for the entrance and exit of air (EN, EX), the entrance 

 tube being carried close to the bottom, while the exit tube extends only to 

 the top of the box, and is placed in the opposite diagonal corner, thus ensuring 

 adequate ventilation.. In the third hole a thermometer (C T) is inserted, 

 by means of which the temperature of the calorimeter (jacket of metal and 

 water) is obtained. The opening in the side is for the insertion of a stirrer (8), 

 which is for the purpose of thoroughly mixing the water and thus equalizing 

 the temperature of both water and metal in other words, of the calorimeter. 

 Before using the apparatus the calorimetric equivalent must be determined, 

 that is, the amount of heat required to raise the temperature of the instrument 1. 

 This may be obtained indirectly by knowing the different substances used in 

 the construction of the instrument, their weights, and their specific heats, and 

 estimating from these data. It is better, however, to make the determination 

 by burning a definite amount of absolute alcohol or hydrogen within the instru- 

 ment, or by using a sealed vessel of hot water of a known temperature and 

 allowing it to cool to a definite extent. The process is simple ; for instance, 

 each gram of alcohol or each liter of hydrogen completely oxidized yields a 

 definite number of calories ; similarly, a definite weight of water cooled a 



