250 Professor Arthur Smithells on 



without undergoing combustion, in order that there may he 

 some on the outer surface to maintain the combustion there 

 taking place ; and oxygen must be passing through from the 

 outside to maintain the combustion of the hydrogen on the 

 inner surface. If, then, we can get a thermometric instrument 

 small enough to be wholly immersed in the region of com- 

 bustion, the temperature indicated will not be that of the steam 

 just formed but the average temperature of the hydrogen, 

 oxygen, and steam within the sheath. 



From this I think it is plain that no thermometric instru- 

 ment can possibly indicate the temperature of the product of 

 combustion in a flame at the moment of its formation. The 

 temperature of 1500° ascribed to the hottest part of a Bunsen 

 flame is therefore not the temperature of the products of com- 

 bustion, and experiments that show these products to be non- 

 luminous at a temperature even of 3000° C. afford no proof 

 that the luminosity of flame is due to some other cause than 

 the mere high temperature of the products. 



How, then, are we to ascertain the temperature of the pro- 

 ducts of combustion ? The only remaining method is to 

 calculate it theoretically. This may be done if we make the 

 assumption (which must be considered as doubtful in view of 

 recent experiments) that the specific heat of gases does not 

 alter greatly with increase of temperature. 



In the production of 18 grammes of steam from 2 grammes 

 of hydrogen and 16 grammes of oxygen there are evolved 

 57,560 calories. The specific heat of steam being *4805 we 

 have the rise of temperature : — 



<2 "* 1= 18?Xo5 = 66550 - 



This temperature then (with the proviso as to the constancy 

 of specific heat of steam) must be the temperature of the 

 steam-molecules formed in a flame of hydrogen, assuming the 

 whole heat liberated during the reaction to be stored in the 

 newly-formed molecules. It is well known that steam begins 

 to dissociate far below this temperature, but the question of 

 dissociation only enters into our present consideration in this 

 respect, that it forbids us to assume a hydrogen-flame as con- 

 sisting of molecules all in the act of combining. Whatever 

 the degree of dissociation there is plainly some combination 

 taking place, and if the heat liberated is stored in the molecules 

 formed their temperature must rise to the theoretical 6600°. 

 Even the average thermometric temperature of a hydrogen- 

 flame is above the temperature at which dissociation of steam 

 begins, and there must be therefore a certain number of un- 



