178 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



latter is the indirect determination based upon estimates of the quantities of O 

 absorbed and CO a eliminated, or upon the amount of potential energy ingested 

 in the food and probably transformed into kinetie energy within the body 

 (p. 474). 



Calorimeters of various forms have been employed, some of which have 

 been devised to study the body as a whole, while others are adapted only for 

 studying parts, such as a leu or arm. They may be classified as ice, air, and 

 wafer calorimeters in accordance with the chief medium employed to absorb 

 the heat. They consisl essentially of an insulated jacket of ice, air, or water, 

 which encloses the animal and serves to absorb the heat. The ice calorimeter 

 is impracticable for physiological uses ; the air calorimeter until very recent 

 years lias found but little acceptance, hut is deservedly fast gaining in popular- 

 ity; the water calorimeter is the form of apparatus usually employed, having been 

 first used by Crawford in 1788; it has been materially modified by Despretz 

 and Dulong and subsequent investigators. The now classical instrument of 

 Dulong consists of two concentric cases. The animal is placed within the 

 smaller case, which is submerged in the water contained in the larger ease, 

 this in turn being placed within a large box, between which and the calorime- 

 ter some non-conducting material such as feathers or w r ool is packed. Suit- 

 able openings are made for the proper supply of fresh air and for the agitation 

 of the water in the calorimeter so that an equalization of the temperature of 

 the instrument can be obtained. This apparatus has certain serious defects, 

 however, which render it troublesome for expeditious and accurate work. An 

 improved form devised by the author 1 which is now in general use meets 

 every essential re(|nircment for a satisfactory instrument. The apparatus con- 

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

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

 ( Fig. 80). The outer box is fifteen inches in height and width, and eighteen 

 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 tube- for the entrance and exit of air (AW, 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 (S), 

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

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



Before using the apparatus the cahHmetric equivalent must be determined, 

 that is, the amouutof 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 

 1 Reichert: University Medical Magazine, L890, vol. 2, p. 173. 



