26 CALORIMETRY 



4. Cooling Constant. 



Another correction to be made in the final calculation is 

 that of the cooling constant of the apparatus. The chief source 

 of error in calorimetric experiments lies in heat exchange with 

 external objects by conduction and radiation. To reduce this 

 error to a minimum (a) the chemical action must go on as fast 

 as possible, hence the use of oxygen under pressure ; (b) the 

 temperature of the calorimeter water is kept as nearly as possible 

 at the same value as the temperature of the room. We have 

 already stated how the grosser errors of conduction and radiation 

 are avoided by the structure of the insulating chamber. In spite 

 of this there is a certain loss, which is measured as part of the 

 regular routine of an experiment and is allowed for. 



In order to calculate the energy of any material one must know 

 what the end-products of the combustion are. We have seen 

 that C and H are always under these circumstances completely 

 oxidised to CO 2 and H 2 O, which undergo little or no further 

 energy changes. S and N, on the other hand, are converted into 

 sulphuric and nitric oxides, which in turn dissolve in and combine 

 with water forming H 2 SO 4 and HNO 3 . A correction has to be 

 applied for their heats of solution and combination. For very 

 fine work, corrections may be applied for the latent heat of 

 evaporation of water and for the heat of solution of CO 2 . 



Determinations of the energy-value of the proximate principles 

 of food have shown that only minute differences exist between 

 the various members of any class. For example, one gram of 

 each of the following carbohydrates has the accompanying value 

 in calories : 



TABLE II. 



Starch =4-191 large calories. 



Cane Sugar =3-955 



Maltose =3-94 ,, 



Milk Sugar (Lactose) =3-95 

 Glucose =3-74 



Fruit Sugar (Fructose) =3 -75 ,, 



Average values have therefore been adopted and accepted as 

 standard. E.g. 



Carbohydrate 4-1 Calories. 



Fat - - 9-3 

 Protein 5-3 



