8 CHEMICAL COMPOSITION. 



The mode of expansion, the nature of the work accomplished, 

 and the more or less complete transformation of the heat into 

 work at the moment of explosion must necessarily play an 

 important part in this connection. 



This diversity in the products helps to explain the very varied 

 effects which the explosion of one and the same body may 

 produce, according to the method of inflammation. 



2. DISSOCIATION. 



1. In order to have a clearer idea of the effects produced by 

 explosive substances, it is necessary to examine not only the 

 products obtained after cooling, but also those which are pro- 

 duced during the explosion, and starting from the moment when 

 the system reaches the maximum temperature. Now, these 

 first products are sometimes simpler than those which are 

 observed after cooling ; they result partly from the formation of 

 a lower compound. For instance, from a polysulphide splitting 

 up into sulphur and monosulphide, and partly from incomplete 

 combination, as in the case of a mixture of water vapour with 

 its elements, hydrogen and oxygen. 



In the above connection it is indispensable to take account of 

 the phenomena of dissociation. 



The quantities of heat and the gaseous volumes under dis- 

 cussion are calculated at C. and 760 mm. This calculation 

 is admissible for explosive compounds which can be resolved 

 into their elements, such as nitrogen sulphide, or for those 

 which give simple and stable products, such as mercury fulmi- 

 nate, which can be completely decomposed into mercury, 

 nitrogen and carbonic acid. But it is inadmissible when car- 

 bonic acid, water vapour, potassium polysulphide, sulphate, 

 or carbonate, etc., are formed. In these cases the compounds 

 probably do not exist as such. At the high temperature developed 

 during the reaction they are, no doubt, replaced either wholly 

 or in part by simple combinations, perhaps even by their 

 elements. Consequently the quantity of heat corresponding to 

 the real reactions is less than the quantity measured or calcu- 

 lated from the products observed after cooling, and lowers the 

 maximum temperature, as well as the corresponding pressure. 

 This last point is worthy of closer examination. 



2. The pressure of a gaseous system is always diminished by 

 the fact of dissociation. 



At first sight, this would seem to be a paradox, as dissocia- 

 tion has the effect of increasing the volume of gases reduced to 

 and 760 mm., when there has been condensation in the act 

 of combination, as in the formation of water vapour or carbonic 

 acid. But, on closer examination, it will be found that in all 

 known cases of combination accompanied by condensation, the 



