46 CALORIFIC POWER OF FUELS. 



To pass from this to the heat given off under constant 

 pressure, observe that CO occupies a unit of volume and O a 

 half unit. Then 



N = if 

 CO a occupies a unit of volume and 



N' = i. 

 Hence N N' = f , 



At there would be, then, for the difference between the 

 heat of combustion at constant pressure and that at constant 

 volume, 



+ 0.542 X | = +0.271 calories. 



At -f- 15 add to this -|- 0.015, which increases the cor- 

 rection then to 0.286. The heat of combustion of carbonic 

 oxide at constant pressure and 15 is then -f- 68.29 calories. 



With a solid or liquid, this volume in relation to those 

 of the gases formed may be practically neglected, the same 

 as with the water; all reduce then to. the contraction and 

 expansion of the gases. Thus, for naphthalin, this correc- 

 tion does not exceed 8. 8 in 9692 calories less than o. I per 

 cent. 



In case of solids -or liquids with unknown molecular 

 weight, as with fuels generally, this difference irtey still be 

 approximately calculated, as it is sufficient to know the volume 

 of oxygen used in the combustion and that of the gases pro- 

 duced. 



The first calorimeter of constant volume in date is that of 

 Thomas Andrews, who in 1848 published results obtained 

 with a closed calorimeter. The calorimeter was not applicable 

 to solids or liquids ; the combustion of the gases was con- 

 ducted as in a eudiometer, but he did not take all the 

 precautions necessary to be certain of complete combustion. 



