508 ILLUSTRATIVE EXPERIMENTS 



tion is that of the coohng 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 («) the 

 chemical action must go on as fast as possible, hence the use of oxygen under 

 pressure ; {h) 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 in 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 we 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 CO2 and H^O. which 

 undergo little or no further energy changes. N and S, on the other hand, are 

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

 with water forming H2SO4 and HNO3. 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^. 



Preparation of the Bomb. Unscrew the cover of the bomb and remove 

 the small bottle or other vessel containing soda lime (which is left in the 

 bomb after each experiment for absorbing moisture). Press the material 

 under investigation into cylindrical shape ; weigh accurately (0-5-1 gram) 

 and place it in the quartz (or platinum) crucible (Fig. 102). Fix the crucible 

 T firmly in place with the two little arms of the holder, a^ and a^, passing 

 through the two holes in its side. Attach a piece of the ignition wire {q.v) to 

 the conducting rods so that it passes straight across and above the crucible. 

 To the centre of this wire, attach the fuse of cotton thread {q.v.). The other 

 end of this thread may be incorporated in the substance whose calorie value 

 is being estimated. Examine the lead washer in the cover to ensure the 

 absence of any grit or burr which would interfere with its function as a seal. 



After these arrangements are completed, place the bomb in the cast-iron 

 holder (as in A, Fig. 3, p. 24), and put the cover on as far as possible by hand. 

 Finish the process with the spanner C provided for this purpose. Note that 

 the part of the spanner coming in contact with the bomb is furnished with 

 card gaskets to prevent damage to the plating. 



By means of the cone and nut G connect the bomb through the pressure 

 gauge and union to a cylinder of compressed oxygen, (i.) Open the inlet 

 valve F2 (Fig. 102) of the bomb, (ii.) close the fine adjustment release valve 

 (needle valve) on the cylinder, and then (iii.) open the cylinder (niain) valve. 

 Gmdually open the release valve of the cylinder. If this valve is opened too 

 rapidly, the inrush of oxygen will tend to scatter the contents of the crucible. 



When the gauge indicates a pressure of 15-20 atmospheres, close (i.) release 

 valve, (ii.) inlet valve of bomb, and (iii.) cylinder main valve. Disconnect 

 and place bomb in inner calorimeter vessel. 



Preparation op Calorimeter. The calorimeter — a heavily nickel-plated 

 polished cylinder of copper should have placed in it about 2,000 grams of 

 water (accurately weighed). The calorimeter vessel and its charge of water 

 are best kept in an outer room with a temperature about 1° C. below that of 

 the room in which the calorimetric combustion is to be done. When the 

 bomb is placed in it, the water should cover the oblong excrescence, but not 

 the terminals P^ and P, ^^or the valve spindles V^ and Fg. One can now see 

 whether the bomb is gas tight or not. Small leaks do not render the experi- 



