384 THE SCIENTIFIC PAPERS OF 



as the case may be, the gas will take up the heat to the extent of 

 nearly 2000, and since another 1000 is again produced in com- 

 bustion, the temperature of the furnace will this time attain 3000, 

 and in this way it might be argued that, unless work is done in 

 the furnace, the heat developed in combustion will, step by step, 

 increase the temperature of the furnace 1000, or something less, 

 each time the reversal of the valves takes place, till we arrive at 

 the practical limit imposed by the melting point of the most 

 refractory substance we can find (pure silica, in the form of Dinas 

 brick), of which the melting chamber is usually formed. This 

 high temperature is obtained by a gradual process of accumulation, 

 and without any such current as would be likely to destroy, by 

 oxidation, the metal in the bath, or cut away the sides and roof of 

 the melting chamber. 



There is, however, a theoretical as well as a practical limit to 

 the degree of heat obtainable in combustion, which was first 

 pointed out by M. H. St. Claire Deville, namely, the point of 

 dissociation at which carbonic acid would be converted back into 

 its constituents, carbon and oxygen. If carbonic oxide or any 

 other combustible gas and air enter the furnace at a temperature 

 very nearly equal to the point of dissociation, it is evident that 

 association or combustion cannot take place, and thus nature 

 fortunately steps in to restrict the increase of heat by accumula- 

 tion, within comparatively safe limits. In a furnace fully heated 

 up to the melting point of iron, this action of dissociation can be 

 very clearly observed. At first, when the gas and air are com- 

 paratively cold, combustion takes place sluggishly, the gases will 

 flow through the furnace and produce only a dark-red flame ; the 

 next time the valves are reversed a whitish flame is produced ; the 

 next time a short white flame ; and after having reached a full 

 white heat, exceeding the welding point of iron, the flame will 

 again become a long one, but this time not red, and of little 

 apparent power, but bluish white, and flowing in clouds. This 

 indicates the near attainment of the point of dissociation ; com- 

 bustion can no longer take place, except in the measure of the 

 heat being dispersed to surrounding objects, or to the metal in 

 the furnace, and that is about the degree of heat required for the 

 process of making steel on the open hearth. 



Before I leave the question of the furnace, I must refer back to 



