COMPOSITION OF A CHARCOAL. 489 



this thermal excess, that is, a portion of this excess of energy, 

 remains in the residual carbon. 1 



Further this latter carbon sometimes retains an excess of 

 hydrogen which yields, weight for weight, four times as much 

 heat as carbon. 



4. It is hardly possible accurately to estimate the influence of 

 these complex circumstances, unless by very special analysis 

 and calorimetric determinations made on the charcoal employed 

 in the manufacture of a given powder. But it is clear that they 

 tend to reduce the error committed by assuming in the calori- 

 metric calculations the weight of the charcoal employed as 

 equal to the weight of pure carbon, than which it is really lower 

 by about a fifth. This compensation extends itself even to the 

 volume of the gases ; since the deficiency in volume of carbonic 

 acid produced is almost entirely replaced at the moment of the 

 explosion by the volume of water vapour, resulting from the 

 hydrogen and oxygen contained in charcoal. 



5. With a view to rendering these notions clearer, we shall 

 give some observations made on the composition of a charcoal 

 derived from pure lignite. Having had occasion to see, in the 

 powder factory at Toulouse, some spindle-tree charcoal, pre- 

 pared with the ordinary precautions, that is to say protected 

 from the air and at a relatively low temperature, from young 

 branches containing a considerable quantity of pith, it was 

 deemed of interest to examine the carbonaceous portion derived 

 from this pith, a pure and homogeneous substance. 



Further, the central position allows of the decomposition of 

 the substance by heat taking place outside the influence of the 

 the air and the gases formed by secondary reaction in the dis- 

 tilling apparatus. Some of the carbonised branches were 

 obtained, and the charcoal contained in the medullary channel 

 extracted and examined. 



It retained exactly the appearance and structure of the 

 original pith, except, of course, its colour. In order to analyse 

 it, it was dried in an oven, and burnt in a current of oxygen, 

 completing the combustion of the gases by a column of cupric 

 oxide. Eesults : 



(1) Loss at 100 9-0 



This loss is due to water, which can be absorbed by sulphuric 

 acid. However, there is also produced a trace of carbonic acid, 

 as was proved, which is doubtless produced by oxidation on 

 contact with the air, which is worthy of notice, from the low 

 temperature of the experiment (100). But the weight is less 

 than the one-thousandth part, from direct measurements. 



(2) Ash 3-5 



1 See also the works of M. Scheurer-Kestner, who has found an analogous 

 excess in the combustion of certain kinds of coal. 



