of the material in question. The unif of measurement of this heat is 

 termed a calorie. A calorie is the amount of heat necessary to raise one 

 kilogram of water 1*^ centigrade, or one pound of water 4° Fahr. The 

 Units of heat, or niimber of calories, contained in a given weight of the 

 different food ingredients, have been very accurately ascertained by means 

 of the calorimeter. 



It would take up too miich space to describe in detail the calorimeters 

 which have been used in the analyses of foods. In general, the calor- 

 imeter consists of a vessel, bomb shaped according to the latest design, 

 about 10 cm. by 13 cm.; this bomb is immersed in a vessel containing 

 two liters of water, which vessel is surrounded by an empty cylinder, 

 enveloped in its turn by another empty vessel. As a protection to these, 

 there is still another cylinder containing water; the outside of this 

 cylinder is lined with a thick layer of feit. Hence, there is a layer of 

 feit, one of water, and two of air between the calorimetric apparatus 

 proper and the external air. The material to be burned is placed in a 

 platinum capsule in the "bomb," and ignited in presence of compressed 

 oxygen by means of an electric spark. A very finely graduated ther- 

 mometer is connected with the vessel containing the two liters of water, 

 and the heat imparted to the water and indicated by the thermometer, 

 measures the fuel value of the food-material burned. 



It has been found that, taking the ordinary foods as they are at our 



command — 



One pound of protein yields 1,860 calories. 



One pound of carbohydrates yields 1,860 calories. 



One pound of fat yields 4,220 calories. 



That is, the fuel value of fat is about 2.25 times that of either the 

 starchy or the nitrogenous substances, as was stated when discussing 

 the nutritive ratio. 



Now, if a pound of fat is consumed by an animal it becomes a heat- 

 producer; in other words, a slow combustion takes place, and the result 

 is identical, as far as heat units are concerned, with that obtained by 

 the calorimeter. This has been proved by means of respiration calor- 

 imeters, devised by some German investigators in this work, notably 

 Drs. Rubner, Rosenthal, and Pettenkofer, of Munich, and they have 

 obtained the same figures for the heat units in the nutrients of the food 

 as those just given, viz.: 1,860 calories for protein and carbohydrates, 

 and 4,220 calories for fat. Several different forms of respiration calor- 

 imeters have been tried with varying success. The best forms are those 

 designed by Rubner and Rosenthal. 



Rubner's respiration calorimeter consists of a metal box with double 

 walls. In the interior are arrangements for keeping an animal, as in 

 the ordinary respiration apparatus. Provision is made for conducting 

 a current of air through this Chamber and for measuring its amount 

 and determining its composition. 



In Rubner's apparatus the air in the double walls is allowed to expand 

 and contract; and these changes, as shown by a manometer (pressure 

 gauge), are the measure of the heat radiated from the body. In this 

 case it is the increase or decrease of volume under constant pressure. 

 In Rosenthal's apparatus, it is the Variation of pressure from a constant 

 volume. 



