24 



CALORIMETRY 



S^ 



of the foodstuff of C, H and O requires a complicated chemical 

 technique. This method of calculating energy- value is little used 

 in biology. Pure substances are seldom used as food material, 

 except in certain kinds of experiments. 



B. Measurement of E.V. of foods by calorimetric combustion. 



The principle underlying this method is the combustion of a 



known amount of the material in an apparatus so devised that 



practically all the heat evolved is ab- 

 sorbed by a known amount of water 

 and by the apparatus itself (which is 

 of known heat capacity). Some form 

 of bomb calorimeter is now universally 

 employed for this purpose. The in- 

 strument (Fig. 2) consists of three 

 main parts. 



1. The bomb itself is constructed of 

 steel, nickel-plated, with a cover to 

 be screwed on firmly against a lead 

 washer. Its capacity is about 300 c.cs. 

 Through the cover the entrance and 

 exit gas channels pass ; K2 with its 

 continuation platinum tube, R, is for 

 the introduction of oxygen, and Kl 

 for the withdrawal of the gaseous pro- 

 ducts of combustion. Both channels 

 are closed by means of the screw 

 spindles V\ and V2, running in stuffing 

 boxes. Si and S2 are screws to stop 

 the lateral communication with Kl 

 and K2. Into these may be screwed 

 nickel - plated tubes. Through the 

 centre of the cover passes a strong 

 platinum wire, D, and this, as well 

 as R, is fitted with short pegs, 



FIG. 3. Section through a Kroeker 

 bomb (see text). 



or, on 



which hangs the crucible 

 T. A short collar, just above these pegs, is for the attachment 

 of the ignition wire. Pi and P2 are two small screw-clamps for 

 attaching to the electric wires for ignition (Fig. 3). 



2. The insulating chamber is a double- walled copper vessel 

 of about 11 litres capacity, and the space between the walls is 

 to be filled with water at room temperature. It is lined with 



