24 CALORIFIC POWER OF FUELS. 



The coal for the experiment must be in pieces; if in 

 powder, the combustion is more difficult, unburnt gases 

 escaping in considerable quantities, so that it is rare to obtain 

 a complete combustion, and the cinders almost invariably 

 contain small quantities of coke. To determine these, the 

 capsule and tube are withdrawn from the combustion-cham- 

 ber, dried, and weighed. The coke and the little soot on the 

 sides of the capsule are burnt off by calcination in the air and 

 a new weighing made, giving the weight of the carbon and 

 cinder elements which must be considered in the corrections. 

 From half a gram to a gram of coal may be used. 



When the combustion-chamber containing the weighed 

 substance is put into the calorimeter all the parts of the 

 apparatus are connected by caoutchouc joints and tested. 

 A slow current of oxygen * from a gas-holder is passed 

 through the apparatus. The combustible is ignited by a few 

 milligrams of burning charcoal, the joint r in the tube being 

 broken for the moment, and immediately reconnected without 

 stopping the flow of oxygen. The little glass M allows inspec- 

 tion of the combustion, the intensity of which can be regulated 

 by the flow of oxygen from the gas-holder. The temperature 

 shown by the thermometer is recorded each minute to obtain 

 the data necessary for the correction spoken of above (pages 

 1 6 et seq.). 



To calculate the heat-units developed by the combustion 

 the following elements are needed: 



1. Weight of the combustible used; 



2. Weight of the carbon remaining in the cinders unburnt 

 or as black ; 



3. Weight of the cinders; 



4. Weight of hydrogen escaped unburnt; 



* To prepare the oxygen a copper flask of one litre capacity is used, in 

 which is placed some chlorate of potash, which is then heated by a gas 

 flame. The gaseous current is very regular, except towards the end, when 

 it may become tumultuous. The addition of a small percentage of black 

 oxide of manganese promotes the regularity of the gas generation. 



