Intra-Molecular Energy during Combustion. 869 



inflammation occurs, but no rise o£ pressure is registered by 

 an indicator or pressure-gauge. In the gaseous mixtures 

 experimented upon, the number of molecules alter combustion 

 is only about 3 per cent, less than that before combustion. 

 Consequently the translational energy of the freshly formed 

 molecules during this period is not greater than that of 

 the molecules before ignition took place. There is also no 

 appreciable radiation emitted in this interval, and it follows 

 that no appreciable energy has been acquired by those 

 vibratory degrees of freedom of the freshly formed molecules 

 which give rise to radiation of the kind possessing consider- 

 able energy. Assuming, then, that the chemical energy of 

 the inflamed portion of: the gaseous mixture has been trans- 

 formed into thermal energy of the freshly formed molecules, 

 it would appear that the intra-molecnlar energy acquired 

 during this interval of gentle burning must be in the form of 

 rotational energy and possibly of low-frequency vibrations, 

 such, e. g. y as those corresponding to the 15 /x radiation of 

 C0 2 and the much slower vibrations which steam molecules 

 are believed to possess. 



Early stage of the explosion period. — In the early stage of 

 the explosion period the pressure at first rises very slowly, 

 indicating that combustion is still proceeding gently. During 

 this time a small, but appreciable, amount of radiation is 

 emitted, as may be seen from curve F in fig. 1 *. This 

 curve shows the radiation from the gaseous mixture which is 

 transmitted through a plate of fluorite — a substance which 

 is highly transparent to all kinds of infra-red radiation down 

 to about 11 /ub. The curve Q in the same figure shows the 

 radiation transmitted through a plate of quartz which is 

 transparent to infra-red radiation down to about 3'5yu only f. 

 It will be noted that the radiation of the kind transmitted by 

 quartz does not begin to be emitted as early as that of longer 

 wave-length, which is transmitted by fluorite. The theory 

 which has been advanced accounts satisfactorily for this. In 

 the initial stage of the pressure-rise it is to be supposed that 

 the combustion proceeds so gently that the vibrations corre- 

 sponding to the radiation which is transmitted by quartz arc 

 not excited, but it is sufficiently vigorous to excite those 

 corresponding to radiation of longer wave-length. A frac- 

 tion of a second later, combustion is proceeding sufficiently 



* The curves in fig-. 1 relate to a 10-per-cent. mixture of coal-gas 

 and air. 



t The quartz plate transmits about 70 per cent. o( the radiation of 

 wave-length between about 11 jx and S-o^i ; it is opaque to radiation of 

 greater wave-length, but is more transparent to radiation o( shorter 

 wave-length. 



