206 REPORTS ON THE STATE OP SCIENCE. 



by the volume of air mixed with the gas, and this is very different for 

 the three non-luminous flames. On this account it cannot be asserted 

 that this agreement is not accidental. Moreover, the number of obser- 

 vations is much too small. Nevertheless, the experiment seems worthy 

 of record and will be followed up further.' 



With regard to the last remarks, it is to be noted that the fact that 

 the flame was just rendered non-luminous shows that the air was in 

 each case in approximately the proportion required for complete com- 

 bustion. The heating value of such a mixture is much the same for 

 all the gases in the above table, and the temperatures of the flames 

 would be still more nearly the same, the higher heating value of a 

 CO mixture being partly neutralised by the high specific heat of the 

 products. The agreement is certainly more than a coincidence. W. T. 

 David, from a comparison of the radiation emitted in the steam and CO 

 bands respectively in a coal-gas and air explosion, infers that C0 2 

 radiates about 2J times as much as steam per unit of volume. This 

 result, which was obtained in ignorance of Helmholtz's estimate, agrees 

 with it almost exactly. 



Cold C0 2 shows a strong absorption band at the same point of 

 the spectrum as the emission band given by a flame in which C0 2 is 

 produced, and water-vapour powerfully absorbs the radiation from a 

 hydrogen flame. 



As stated above, it is most probable that the radiation in an explosion 

 also consists almost entirely of the same two bands as are emitted by 

 the Bunsen flame. A complete analysis of the radiation from an ex- 

 plosion has not been made, but Hopkinson and David found, using a 

 recording bolometer, that the radiation is almost completely stopped 

 by a water-cell, and that it is largely stopped by a glass plate. It 

 follows that the luminosity of- the flame in an explosion or in a gas- 

 engine accounts for but little of the energy which it radiates. 



Molecular Theory of Radiation from Gases. 



Much difference of opinion exists as to the physical interpretation 

 of the facts described in the preceding sections. The issues in this 

 controversy can conveniently be staled in terms of the molecular theory, 

 and it is, therefore, desirable to give a short account of this theory. 

 But it will be apparent that the issues are not merely of theoretical 

 interest, but arc in large measure issues of fact capable of being tested 

 by experiment, and that the answers to important practical questions 

 may depend on the manner in which they are settled. 



According to the kinetic theory, the energy of a gas must be 

 referred partly to translational motion of the molecules as a whole and 

 partly to motions of some sort internal to the molecules. The transla- 

 tional motion is that which causes the pressure of the gas, and in the 



case of gases for which ^ is constant (with which alone we are 



concernedin this discussion), the translational energy per unit of volume 

 is equal in absolute measure to If time the pressure. This part 

 of the energy may conveniently be called 'pressure energy.' It 



