METHODS OF DETERMINING HEAT OF COMBUSTION. II 



in which N is the heat of combustion sought, m is the coeffi- 

 cient previously determined, and P is the weight of oxygen 

 necessary for the combustion of one kilogram of the substance. 

 Giving P the value resulting from the use of the equiva- 

 lents 1 6 for oxygen to burn 6 of carbon, and 8 for oxygen 

 to burn I of hydrogen we have 



and the general formula becomes 



N = Sm (- + H) = 26880 (- + H). 



J 



To use this method the combustible is mixed with an 

 excess of litharge and heated in a crucible. The button of 

 lead formed shows the amount of oxygen consumed, and from 

 this is deduced the heat by means of the formula. The heat 

 should be increased very slowly. Mitchell substituted white 

 lead for litharge and claimed to obtain uniform results. 



This formula was recommended by Berthier, and has been 

 used since by a few others. It is faulty, as was shown by 

 some of Berthier's own determinations in which contradictory 

 results were obtained. Dr. Ure showed that no uniform re- 

 sults could be obtained using the same materials. Scheurer- 

 Kestner in 1892 showed that the formula not only gave erro- 

 neous results, but actually reversed the relation of combus- 

 tibles. In one case cited the heats actually obtained by a 

 calorimeter were 8813 and 8750, while by the litharge test 

 they were 7547 and 79/7. The results were not only low, 

 but reversed the ratio. 



This method is allowable only in cases where the crudest 

 approximations are desired and where no analyses or calori- 

 metric tests can possibly be made. 



