IMPORTANCE OF THEORETICAL CONSIDERATIONS. 205 



same alkaline base with two distinct acids, these salts being 

 considered under the solid and anhydrous form, we find a 

 nearly constant thermal difference, whatever be the base, when 

 we reckon the quantities of heat liberated from the elements up 

 to the anhydrous salts. For example, the formation of the 

 anhydrous potassium, sodium, ammonia, calcium, strontium, 

 lead and silver sulphates, liberates a mean value of 54 Cal. 

 more than the formation of the corresponding nitrates. 



A similar difference exists between the nitrates and the 

 majority of the oxygen salts. It exists even between the 

 alkaline chlorides, bromides, and iodides, without, however, 

 extending itself to the anhydrous metallic chlorides. 



13. These numbers permit, as will be shown later, of estima- 

 ting the heat liberated by any decomposition or definite com- 

 bustion of service powder or other powders, inflammable 

 materials or explosive mixtures constituted by the nitrates. 

 It is with the aid of analogous data, derived from the heat of 

 formation of nitric acid, that we can calculate the heat of 

 formation of nitroglycerin, and of organic compounds derived 

 from nitric acid. The figures thus calculated agree moreover 

 with the experiments of Sarrau and Vieille, as far as can be 

 expected in verifications of this nature. 



14. If this agreement is dwelt upon, it is because, in the 

 author's opinion, the applications of explosive substances, as 

 well as the applications of human industry, need to be guided 

 by theoretical notions. We must raise ourselves above em- 

 piricism if we wish to obtain the most favourable results. It 

 is thus that blasting powder, so long exclusively employed in 

 practical applications, tends to-day to be replaced by dynamite 

 in the majority of its uses. Now this substitution is encouraged 

 and regulated by theory. Indeed, the latter teaches us that 

 blasting powder, as well as service powder, is far from utilising 

 in the best manner the combustive energy of nitric acid. 



In the combustion of ordinary powder, the products formed 

 are neither the most oxidised, nor those which would liberate 

 the most heat for a suitable proportion of the various ingredients, 

 seeing that the maximum of heat which would be developed by 

 a known weight of saltpetre acting on the sulphur and the 

 carbon does not correspond to the maximum volume of the 

 gases liberated. Between these two data of the problem, 

 empiricism has led to a sort of compromise being adopted, 

 which is our traditional powder. But it would be far preferable 

 to arrange in such a manner that the maximum of the two 

 effects should occur in it for the same proportions. 



This is not all. The formation of potassium nitrate itself, 

 reckoned starting either from nitric acid or the elements, corre- 

 sponds to very powerful affinities and gives rise to a greater 

 liberation of heat, and consequently to a greater expenditure of 



