1922] on The Internal Combustion Engine 591 



velocity of the flying' molecules. The actual velocity of the products 

 of combustion in the vessel depends on the mean temperature. 

 Direct measurement of the temperatures of the working charge of a 

 gas engine gives 2570° abs. as a reasonable temperature from which 

 to calculate molecular velocities. At this temperature the carbon 

 dioxide molecules are moving at 8950 ft. per second, the steam 

 molecules at 6166 ft. per second, and the nitrogen molecules at 

 4950 ft. per second, these numbers being the square roots of the 

 mean squares of the actual velocities. 



The next point for consideration is the time taken to effect this 

 change. The time-interval taken by oxygen to combine with carbon 

 and hydrogen lies along a time scale beginning with a detonation and 

 ending with slow burning. In a mixture of air and pentane the 

 oxygen molecules are a long way, on the average, from the carbon 

 and hydrogen of the pentane molecule, and also the freedom of 

 action of the oxygen molecules is clogged by the inert nitrogen 

 present, but the rapidity with which oxygen can combine when the 

 circumstances are favourable is shown by nitro-glycerine. 



Chemists have discovered how to produce this nitro-glycerine 

 molecule so that oxygen lies side by side with the carbon and the 

 hydrogen. Its action is unclogged by any other substance, and the 

 molecular distances have been annihilated, or perhaps it would be 

 better to say that they have become atomic distances. Moreover, 

 the molecule contains almost the exact quantity of carbon and hydro- 

 gen required to satisfy the oxygen present. As expressed by Lord 

 Moulton, it is a case of the lion and the lamb lying down together. 

 A mechanical shock causes an immediate transformation — the lion 

 devours the lamb ; and the time-interval for the meal is so short 

 that it is not measurable. This is called a detonation. Chemists have 

 shown by their researches how to combine nitro-glycerine with other 

 substances in order to control the rate of combustion. Engineers 

 are also trying to get control of the rate of combustion of some of 

 the mixtures used in the internal combustion engine. Thus the 

 chemist and the engineer are working in different parts of the same 

 wide field of research. 



Experiments initiated by Sir Dugald Clerk are now proceeding at 

 the National Physical Laboratory under the general supervision of 

 the Aeronautical Research Committee for the Air Ministry. Appa- 

 ratus of the most refined nature has been devised, and the research 

 is being carried out by Mr. Fenning. Various combustible mixtures 

 are made up in a bomb. These are exploded and the time taken 

 for the chemical combination to take place is recorded. Two 

 results may be mentioned : a mixture of one part by volume of 

 hydrogen, 2 \ parts by volume of air, was compressed to 64 lbs. per 

 sq. inch and then exploded. Between the passage of the spark and 

 the beginning of the rise of pressure about four-thousandths of a 

 second elapsed. The combination was complete in about the same 



