COMPLETE COMBUSTION BY AN OXIDISING AGENT. 267 



quantity to convert all the potash into potassium carbonate. 

 The five equivalents of oxygen supplied by the potassium nitrate 

 will give off, then, 27 Cal. less than if they were free, and 

 generated with the carbon, free carbonic acid; or 54 Cal. 

 for each equivalent of oxygen. We may even add that this 

 estimate is not quite exact whenever cooling takes place in an 

 atmosphere of carbonic acid and aqueous vapour, because these 

 convert the neutral carbonate, K 2 0, C0 2 , into the bicarbonate ; 

 K 2 0, H 2 0, 2C0 2 , causing a complementary disengagement of 

 248 Cal. (beginning from liquid water). Consequently, the excess 

 of heat developed during the combustion of a hydrocarbon 

 compound, in free oxygen, over that developed by the same 

 combustion by means of potassium nitrate, is reduced to 14*6 

 Cal. only for KN0 3 (= 101 grins.), i.e. to 2*9 Cal. for each 

 equivalent of oxygen employed. 



9. We will add that, in cases where the combustion of the 

 explosive body is rendered complete by the addition of an 

 oxidising agent, we must not forget that the weight of the latter 

 is added to that of the explosive substance ; so that a gramme 

 of the mixture, subjected to total combustion, may give off less 

 heat than a gramme of the explosive body decomposing 

 separately in pursuance of a less complete oxidation. Various 

 compensations may be made with respect to this. For instance, 

 when it is required to make up one kilogramme of an explosive 

 mixture, copper oxide is the most efficacious of the oxides in 

 use, on account of the smallness of its equivalent (79*2 grms.) 

 and the comparatively low value (38*4 Cal.) of its heat of 

 formation. Lead oxide presents the double inconvenience of an 

 equivalent three times as high (223 grms.) and a greater heat 

 of formation (51*0 Cal.), which diminishes in a corresponding 

 degree the heat given off in combustion in which it is the 

 agent. 



The oxides of mercury and silver present, on the contrary, 

 smaller heats of formation (31-0 Cal. and 7'0 Cal.). But the 

 thermal increase resulting from this is counterbalanced with 

 the unit of weight, by the magnitude of their equivalents (216 

 grms. and 232 grms.). 



The oxides of tin and antimony, which are, for a given 

 weight, somewhat richer in oxygen than copper oxide, have 

 heats of formation that are, for each equivalent of oxygen, 

 almost double that of the latter. 



It has been thought advisable to give these numbers, because 

 they render definite and correct many of the current ideas on 

 combustion by means of metallic oxides. We see that prefer- 

 ence should be given to copper oxide on account of the smallness 

 of its equivalent If lead oxide, and particularly the oxides of 

 mercury and silver, seem to be more powerful, it is no doubt 

 because they react and decompose at a lower temperature ; a 



