326 HEPORTS ON THE STATE OF SOTENCE. 



with that required to reach maximum pressure — the transformation of the 

 chemical energy into thermal form is everywhere complete except in a 

 thin surface layer where this transformation is retarded by the cooling 

 action of the walls. 



If this view be accepted the correction of the results for incomplete 

 combustion is of the nature of a surface correction, and can be determined 

 by comparing the pressures reached by the same mixture when exploded 

 in vessels of different sizes. 



In the discussion of this important matter the Committee have 

 derived great assistance from the experience of Professors Dixon and 

 Bone, who have made a special study of the velocity of chemical action in 

 gases. These gentlemen are of opinion that though such action may be of 

 great complexity, involving in many cases several successive molecular 

 operations, yet, if it is not retarded by the presence of cold foreign bodieS; 

 it will generally be completed within a period which, for the purposes of 

 gas-engine theory, may be regarded as negligibly small. In the simple 

 case of the explosioji of hydrogen and oxygen they consider that the 

 complete transformation of the mixed gases into steam at any given point 

 is complete within a time measured by the interval between molecular 

 collisions. When the action is more complicated, as in the explosion of 

 carbon monoxide and oxygen in the presence of water, or in the combus- 

 tion of hydrocarbons, the period will be larger, but will still be measured 

 by thousandths of a second. 



Some direct evidence that incomplete combustion in an explosion is 

 mainly, if not entirely, a surface phenomenon is to be found in Hopkin- 

 son's measurements of the temperature at points within a large explosion 

 vessel by means of a platinum thermometer. A photographic record of 

 the resistance of a line platinum wire immersed in the gas showed that 

 when the flame reached it the temperature rose in less than ^^jth of a 

 second from 20° C, which was the temperature of the unburnt gas, to about 

 1250° C, which was that of the burnt gas, and that it remained at the 

 latter figure quite steadily except in so far as the increase of pressure iu 

 the vessel caused it to rise. In other words, there was no increase of 

 thermal energy except that due to work done upon the gas from out- 

 side. ' The mixture was one part of coal gas to nine parts of air — a slow- 

 burning mixture — and the time taken to reach maximum pressure was 

 about a quarter of a second or at least ten times that required for 

 combination of the gases at any one point. It is true that the vessel 

 was of rather large size— about six cubic feet capacity — but, on the other 

 hand, owing to the fact that the platinum wire extended over about 

 1 cm., so that the flame took an appreciable time to completely envelop 

 it, it is probable that the period of ^J^th of a second, given above, is 

 a superior limit which greatly exceeds the actual time taken to effect the 

 combination at any one point. 



On the other hand, it cannot be doubted that combustion must be 

 greatly retarded in the neighbourhood of the cold metal walls ; and there 

 is nothing to show that this surface retardation is not sufficient to 

 account for all the phenomena of delayed combustion. A simple cal- 

 culation based upon the rate of flow of heat per square foot into the 

 metal of a gas-engine cylinder (which is roughly known from measure- 

 ments of the lieat carried away by the jacket water) shows that the 



» Proe. R.S., A, vol. Ixxvii. p, 387 



