246 PRINCIPLES OF ANIMAL NUTRITION. 
part is expended in the evaporation of water from skin and lungs 
and takes the form of the latent heat of water vapor. 
ANIMAL CALORIMETERS.—The direct determination of the heat 
produced by an animal, especially a large animal, is not an easy 
task. It requires in the first place a calorimeter large enough to 
contain the animal and in the second place, for experiments of any 
length, the maintenance of a sufficient ventilating current of air 
under such conditions as shall not affect the accuracy of the calori- 
metric determination, while the latent heat of the water vapor 
carried out in the air-current must also be taken account of. In 
other words, such an apparatus must be at once a respiration appa- 
ratus and a calorimeter, and hence the name respiration-calorimeter 
has come to be applied to it. Various forms of animal calorimeters 
have been devised, some of which may be briefly mentioned. 
Lavoisier & Laplace,* in their investigations upon the origin of 
animal heat, employed an ice-calorimeter, in which the heat is 
measured by: the amount of ice melted. Crawford ¢ investigated 
the same subject using a water-calorimeter, as did, later, Dulong t 
and Despretz,§ while more recently Wood,|. and still later 
Reichert, have also employed the water-calorimeter. 
The ice-calorimeter, however, necessarily subjects the animal 
to an abnormally low temperature, while with the water-calorim- 
eter it has been found very difficult to secure a uniform heating of 
the different strata of water. These facts led to the employment 
of air as the calorimetric substance, the heat being measured either 
by the increase in the volume of a confined body of air at a constant 
pressure or the increase in the pressure at constant volume, and 
until quite recently the most exact methods have been based on 
this principle. ' 
Scharling,** Vogel,t+ and Hirn,t}{ between 1849 and 1864, used 
* Hist. Acad. Roy. d. Sc., 1780, 355. 
t Experiments and Observations on Animal Heat. London, 1788. 
t Ann. de Chim. et de Phys. (3), 1, 440. 
§ Ibid., (2), 26, 337. 
|| Smithsonian Contributions, 1880. 
q Univ. Med. Mag., Phila., 2, 173. 
#* Jour. pr. Chem., 48, 435. 
tt Arch. d. Ver. f. Wiss. Heilk., 1864, p. 442.. 
tt Recherches sur l’équivalent méchanique de la chaleur. Paris, 1858, 
