HEAT OF COMBUSTION OP GASES. 149 



the oxygen yielding gases such as nitrogen monoxide and 

 dioxide, the apparatus employed not having been described in 

 the work previously quoted. 



2. The method consists in mixing in a suitable vessel the gas 

 or vapour with the proportion of oxygen strictly necessary to 

 burn it completely, or even with a slight excess of oxygen when 

 this excess is not detrimental ; then, in causing the explosion of 

 the mixture in a closed vessel, and at constant volume. The 

 detonator having been previously placed in a calorimeter, the 

 heat produced is measured. By proceeding in this manner, 

 the combustion lasts only a fraction of a second, and is always 

 total, at least for gases properly so called ; in short, the calori- 

 metric measurement is effected in the shortest possible time 

 that is to say, under the conditions of the greatest accuracy. 



3. From this measurement is deduced, by calculation, the heat 

 liberated by the total combustion of the gas, simple or com- 

 pound. If, further, the sum of the quantities of heat liberated 

 by the combustion of the elements, when the gas is compound, 

 be known, it is sufficient to deduct from this sum the heat of 

 combustion of the said compound gas to obtain the heat of 

 formation of this gas, by means of its elements. 



For example, marsh gas, CH 4 , taken at the weight of 16 grms., 

 liberates, when burning at constant pressure, 213*5 Cal. Now 

 its elements liberate respectively, for C = 12 grms., taken in the 

 diamond form, 94 Cal., and for H 4 = 4 grms., 138 Cal. ; hence 

 we conclude that the formation of marsh gas from its elements 

 C (diamond) + H 4 = CH 4 ; liberates + 94 + 138 - 213-5 = 

 + 18-5 Cal. 



4. The same method has enabled the author to measure in an 

 inverse sense the heat of formation of nitric oxide employed 

 as oxygen yielding gas. This gas, mixed with hydrogen, does 

 not detonate under the influence of the electric spark ; but it 

 explodes violently when mixed with ethylene or cyanogen. 

 Such a mixture has, therefore, been made in the proportions 

 strictly necessary for total combustion, exploded in the apparatus, 

 and the heat liberated measured. 



The same experiment has been made with the same com- 

 bustible gases and pure oxygen. 



This having been done, it is sufficient to deduct the heat 

 liberated in the first case from that produced in the second, in 

 order to obtain the heat of formation of nitric oxide by its 

 elements without any other data than these two intervening in 

 this calculation. In this way we find a negative number, viz. 

 - 21-6 Cal. for N + = NO (30 grms.), which means that the 

 combustion of an oxidisable body, effected by nitric oxide, 

 liberates more heat than the same combustion effected by pure 

 oxygen. 



Thus nitric oxide is formed from its elements with absorp- 



